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<title>EXP362 – JOIDES Resolution</title>
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<link>https://joidesresolution.org</link>
<description>Science in Search of Earth's Secrets</description>
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<title>EXP362 – JOIDES Resolution</title>
<link>https://joidesresolution.org</link>
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<item>
<title>Live science: first results of ocean drilling 362 expedition</title>
<link>https://joidesresolution.org/live-science-first-results-of-ocean-drilling-362-expedition/?utm_source=rss&utm_medium=rss&utm_campaign=live-science-first-results-of-ocean-drilling-362-expedition</link>
<comments>https://joidesresolution.org/live-science-first-results-of-ocean-drilling-362-expedition/#respond</comments>
<dc:creator><![CDATA[Agnes Pointu]]></dc:creator>
<pubDate>Thu, 06 Jul 2017 14:10:52 +0000</pubDate>
<category><![CDATA[Earthquakes]]></category>
<category><![CDATA[Education]]></category>
<category><![CDATA[Geological time]]></category>
<category><![CDATA[Plate Tectonics]]></category>
<category><![CDATA[Scientific Outreach]]></category>
<category><![CDATA[Tsunami]]></category>
<category><![CDATA[Volcanoes]]></category>
<category><![CDATA[accretionary wedge]]></category>
<category><![CDATA[diagenesis]]></category>
<category><![CDATA[earthquake]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[subduction zone]]></category>
<category><![CDATA[sumatra margin]]></category>
<category><![CDATA[tsunami]]></category>
<guid isPermaLink="false">https://joidesresolution.org//?p=22688</guid>
<description><![CDATA[When earthquakes happen in the ocean, they can displace huge volumes of water and cause tsunamis. This was the case... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/live-science-first-results-of-ocean-drilling-362-expedition/" title="Continue reading Live science: first results of ocean drilling 362 expedition">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p><img decoding="async" class="alignleft wp-image-22692" src="https://joidesresolution.org//wp-content/uploads/2017/07/science.jpg" alt="" width="137" height="175" />When earthquakes happen in the ocean, they can displace huge volumes of water and cause tsunamis. This was the case in 2004 when a magnitude >9 earthquake struck North Sumatra and the Andaman-Nicobar Islands leading to a huge tsunami with coastal waves reaching 15 meters or more. Although earthquakes are expected in subduction zones, the 2004 earthquake ruptured to much shallower depths and closer to trench than most other subduction earthquakes and ended beneath the accretionary prism. This earthquake, as well as several others in the past 15 years (especially the Japan Tohoku-Oki earthquake in 2011), surprises earth scientists in terms of their size and the amount and location of the fault slip.</p>
<p>See the geological setting of the Sumatra subduction zone:</p>
<p><img fetchpriority="high" decoding="async" class=" wp-image-22689 aligncenter" src="https://joidesresolution.org//wp-content/uploads/2017/07/TectonicSetting.png" alt="" width="660" height="310" srcset="https://joidesresolution.org/wp-content/uploads/2017/07/TectonicSetting.png 720w, https://joidesresolution.org/wp-content/uploads/2017/07/TectonicSetting-300x141.png 300w" sizes="(max-width: 660px) 100vw, 660px" /></p>
<p>The North Sumatra margin is distinctive from other accretionary prism because of the thickness of the sediments stacked on the sea-floor of the Indian plate – 1,5 km thick increasing to 5 km at the deformation front. A significant part of the sediments come from the Himalayan mountains, triggered by sediment gravity flows, nearly 2000 km away from our drilling sites! Scientists think that these very thick sediments may explain the unusual type of earthquake of 2004. One goal of Expedition 362 was “<em>to find out more about how specific sediments control the size and type of earthquakes in this kind of environnement</em>” explained Lisa McNeill of the University of Southampton.</p>
<p>During August and September 2016, we spent two months drilling the boreholes U1480 (1432 m depth) and U1481 (1500 m depth) on a section of the seafloor ~200 km west of Sumatra in order to sample the thick sedimentary sequence before it reaches the subduction zone. The sediments were deposited on the seafloor over the last ~70 million years, but the majority were deposited in the ~ last 9 million years.</p>
<p>Current models of subduction predict that rupture should occur at depth within the subduction zone. The mechanical and hydrogeological conditions (i.e. fluid generation) of the fault interface control when and where megathrust (subduction plate boundary) earthquakes occur. And these conditions are closely linked to the properties of the materials being sudducted. Fluids released by compaction of the sediments and by mineral dehydration reactions as the temperatures of the sediments increase as they are buried have a major role in how the fault behaves. Yet, the 2004 Sumatra-Adaman and the 2011 Tohoku-Oki earthquakes do not fit these models because the slip extented much more farther and closer to the seaward limits of the subduction forearc than predicted.</p>
<p>Studies of samples from the boreholes helped the 362 scientists to reappraise the current models of subduction whic failed, until now, to explain the unusual earthquakes.</p>
<p>And the first results just got published in one of the famous science journals! Have a look <a href="http://science.sciencemag.org/content/356/6340/841" class="broken_link">here!</a></p>
<p><strong><em>What does this article explain? How does it improve our comprehension of subduction zones around the world?</em></strong></p>
<p>Firts, you have to understand that earthquakes are closely linked to the capability of the rock to break under stress which means that it has to be hard enough. Sediment deposits turn into rocks during a phase called <strong>diagenesis</strong>. This occurs due to burial beneath more sediments which leads to increased temperature and pressure: sediments lose bounded water under pressure and chemical reactions occur and change their physical properties. Part of the diagenesis process is dehydration of the minerals – this produces fresh water which is released but also changes the composition and properties of the changed minerals. These reactions are very important for the behavior of the sediments because they ultimately increase their strength and their ability to cause an earthquake.</p>
<p>The seismic line shows a particular seismic horizon (labeled in the figure below “high amplitude negative polarity reflector”) which was thought to be a weak, fluid-rich layer before the subduction zone. This horizon could be the locus for the plate boundary fault initiation which means that earthquakes would occur along this horizon but we didn’t know its properties or the cause of the fluid.</p>
<figure id="attachment_22690" aria-describedby="caption-attachment-22690" style="width: 861px" class="wp-caption aligncenter"><img decoding="async" class="size-full wp-image-22690" src="https://joidesresolution.org//wp-content/uploads/2017/07/Huppers2017.jpg" alt="" width="861" height="271" srcset="https://joidesresolution.org/wp-content/uploads/2017/07/Huppers2017.jpg 861w, https://joidesresolution.org/wp-content/uploads/2017/07/Huppers2017-300x94.jpg 300w, https://joidesresolution.org/wp-content/uploads/2017/07/Huppers2017-768x242.jpg 768w" sizes="(max-width: 861px) 100vw, 861px" /><figcaption id="caption-attachment-22690" class="wp-caption-text"><em>Overview of study area and sampling locations (Hüppers et al., 2017)</em></figcaption></figure>
<p>Our geochemist team spent 2 months squeezing the sediments of hole U1480 and U1481 in order to analyze composition of the water from the sediments pores. Production of freshwater is a sign of dehydration reactions. Geochemical analyses have confirmed that fresh water is present and that diagenesis is happening. As these sediments get closer to the subduction zone and more fresh water is generated, the seismic horizon properties indicate it is an important reservoir of water released from dehydrating minerals. Slides of the sediments onboard showed that part of the source of the water was originally volcanic glass from volcanic eruptions.</p>
<p>Dehydration simulations were used to estimate fluid production as temperature increased (as sediment load and burial increased) toward the subduction deformation front. Model results demonstrate that fluid produced by dehydration reactions exceeds fluids produced by compaction of sediments. Sampling of the incoming material at the North Sumatran subduction zone provides direct evidence that the diagenesis reactions that release water and make the sediments and rocks stronger and more likely to rupture as an earthquake have happened before subduction. This is consistent with the shallow slip which was observed during the 2004 earthquake.</p>
<p>Scientists of expedition 362 have shown that there is a varied set of conditions that control earthquake slip and that if we want to better understand how earthquakes happen, we have to take into account the nature of sediments in subduction models, particulary in places were very thick sediments have built up.</p>
<p>I am very proud to have been a tiny part of this great expedition. Congrats to the 362 scientists!</p>
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</item>
<item>
<title>The Sunda/Sumatra subduction zone- how does it compare to other subduction zones around the world?</title>
<link>https://joidesresolution.org/the-sundasumatra-subduction-zone-how-does-it-compare-to-other-subduction-zones-around-the-world/?utm_source=rss&utm_medium=rss&utm_campaign=the-sundasumatra-subduction-zone-how-does-it-compare-to-other-subduction-zones-around-the-world</link>
<comments>https://joidesresolution.org/the-sundasumatra-subduction-zone-how-does-it-compare-to-other-subduction-zones-around-the-world/#respond</comments>
<dc:creator><![CDATA[Agnes Pointu]]></dc:creator>
<pubDate>Fri, 30 Sep 2016 03:36:55 +0000</pubDate>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[subduction zone prism plate lithosphere asthenosphere margin]]></category>
<guid isPermaLink="false">https://joidesresolution.org//the-sundasumatra-subduction-zone-how-does-it-compare-to-other-subduction-zones-around-the-world</guid>
<description><![CDATA[What is a subduction zone? The Earth’s surface is divided into plates which have their own movement. A convergent boundary... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/the-sundasumatra-subduction-zone-how-does-it-compare-to-other-subduction-zones-around-the-world/" title="Continue reading The Sunda/Sumatra subduction zone- how does it compare to other subduction zones around the world?">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p><strong>What is a subduction zone?</strong></p>
<p align="justify">The Earth’s surface is divided into plates which have their own movement. A convergent boundary is when two plates are going towards each other. At the place where they met, one plate moves beneath another and is forced to go back into the mantle (or what geologists call the “asthenosphere” below the plates). It is what geologists called a “subduction zone”.</p>
<p align="justify">During subduction, an oceanic plate is thrust below another tectonic plate, which may be oceanic or continental. It’s always the plate which has the higher density which goes back to the asthenosphere. It is called the “subducting plate”. Because of their different composition in rocks, the oceanic plate is denser that the continental plate. And when two oceanic plates converge, the older is also the colder one which also makes it denser.</p>
<p align="justify"><strong>Subduction zones-are they the same?</strong></p>
<p align="justify">The Sumatra subduction zone is an “accretionary margin”. The margin currently grows by adding sediments that have been scraped off from the oceanic plate. This builds the accretionary prism. Normally this occurs through a series of landward-dipping thrust faults with the youngest structure forming furthest away from the arc- see the diagram below. But in the North Sumatran accretionary prism, many of the faults actually dip in the opposite direction. This different style of faulting is thought to be caused by the properties of the sediments themselves and the same properties might affect how the big plate boundary fault moves and generates earthquakes. These are all questions being examined during this expedition.</p>
<p align="center"><img decoding="async" style="border: 0px solid; margin: 0px;" src="/sites/default/files/u376/Accretionary-margin.JPG" alt="Accretionary-margin" /></p>
<p align="center">Schematic carton of an accretionary prism (modified from Clift and Vannucchi, 2004)</p>
<p align="justify">What also makes the subduction zone offshore Northern Sumatra quite unusual is the amount of sediment on the subducting oceanic plate. Offshore North Sumatra it is up to 5 km thick just before subduction and accretion starts.</p>
<p align="justify"><strong>Why so much sediments on the Indian Plate?</strong></p>
<p align="justify">Geologists have determined that the sedimentary materials being incorporated into the North Sumatra subduction zone are related to the Bengal-Nicobar Fan system, which originates more than 3000 km away from our drilling site! This fan is the largest submarine fan currently on the planet. This enormous sedimentary system originates from erosion of the Himalayan mountains. Rivers carry the eroded material to the coast. Most of the sediment (~80%) is deposited onshore and offshore quite close to the coastline. But a huge amount still makes its way along deep-sea canyons to the deep sea portion of the Indian and even Australian plate. A portion of that is then incorporated into the Sunda subduction zone margin and is transferred to a different tectonic plate (see the post “In the mud for Love”).</p>
<p align="justify">In many other subduction zones, including those that have been studied by drilling, the sediment at the trench is thinner. For example, it’s about 1 km offshore Japan at the Nankai Trough. But in some other subduction zones, particulary those were there is a large submarine fan like the Bengal-Nicobar fan, sediments can be as thick and even thicker. A subduction zone similar to Sumatra is the Makran located offshore Iran and Pakistan, where the sediments of the western Himalaya and Tibetan Plateau feed rivers including the Indus which feeds the Indus submarine fan. Sediments entering the Makran subduction zone are up to 7-8 km thick and this forms probably the widest and largest accretionary prism in the world today!</p>
<p align="justify">Not all subduction zones are fed by large amounts of sediments. During the late 1980s and 1990s, seismic surveys and drilling by the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) revealed that sediment accretion was not a ubiquitous feature of subduction margins. In some cases, oceanic and trench sediments are subducted with fragments of rocks which are tectonically removed from the overriding plate. This is called an “erosional subduction zone”. Erosional subduction zones are common where there is topography on the subducting plate, e.g. seamounts, faults and ridges. These margins commonly have a small or no accretionary prism and the properties of the plate boundary fault could be quite different.</p>
<p align="center"><img decoding="async" style="border: 0px solid; margin: 0px;" src="/sites/default/files/u376/erosive-margin.JPG" alt="erosive-margin" /></p>
<p align="center">Schematic cartoon of an erosional prism (modified from Clift and Vannuchi, 2004)</p>
<p align="justify">The Tonga and Mariana subduction zones are good examples of erosive margins.</p>
<p align="justify"><strong>What makes the differences between accretionary and erosive margins?</strong></p>
<p align="justify">The sediment supply into a subduction zone is one of the major factors that determines the type of margin (Clift and Vannuchi, 2004). Tectonic erosion is favored in regions where the sedimentary cover is < 1 km whereas accretion preferentially occurs in regions where the trench sediment thickness > 1 km which means it’s more often ocean/continent subduction since the most of the sediment is coming from erosion of continental rocks.</p>
<p align="justify">It’s important to say that the same subduction zone can transition between a part which is accretionary and a part which is erosional. Two examples of this are the Chilean subduction zone (Ranero et al. , 2006) and the New Zealand one (Collot et al., 1996). And over time a subduction zone may change from accretionary to erosional or vice-versa- so what we see today may not represent what that happened in the past.</p>
<p align="justify">Due to tectonic stresses as one tectonic plate rides over another, subduction zones are sources for great thrust earthquakes. The 2004 Mw 9.2 earthquake in the northern Sunda subduction zone as well as the Tohoku-Oki Mw 9.0 in 2011 in Japan showed unexpectedly shallow megathrust slip. In the case of North Sumatra, this slip was focused beneath the accretionary prism and the rupture extented a long distance from the island of Sumatra making the rupture very wide. It is quite unusual and not well explained by existing models.</p>
<p align="justify">Problems and questions like this are what have motivated our project. Expedition 362 aims to find out what happens when very thick and sand-rich sediments originating from the land are fed into a subduction zone-does it change the properties of the materials that then control how earthquakes are generated for example? And can we take what we learn here in Sumatra and then apply it to other subduction zones that may have the same type of input materials but with few historic earthquakes, suh as the Makran, to learn more about their hazard potential?</p>
<p align="justify">For my blog in French, it’s<a href="https://expedition362joides.wordpress.com/"> here</a>!</p>
<p align="justify">
<p align="justify">
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<title>The Ninety East Ridge</title>
<link>https://joidesresolution.org/the-ninety-east-ridge/?utm_source=rss&utm_medium=rss&utm_campaign=the-ninety-east-ridge</link>
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<dc:creator><![CDATA[Agnes Pointu]]></dc:creator>
<pubDate>Tue, 27 Sep 2016 04:43:51 +0000</pubDate>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[hotspot]]></category>
<category><![CDATA[ridge]]></category>
<category><![CDATA[volcano]]></category>
<guid isPermaLink="false">https://joidesresolution.org//the-ninety-east-ridge</guid>
<description><![CDATA[The Ninety-East Ridge (NER) is 5000 km long linear bathymetric feature striking more or less paralle to the 90°E meridian... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/the-ninety-east-ridge/" title="Continue reading The Ninety East Ridge">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p>The Ninety-East Ridge (NER) is 5000 km long linear bathymetric feature striking more or less paralle to the 90°E meridian from south of Broken Ridge (at ~31°S), southwest of Australia, north to the Bay of Bengal, east of India. The ridge is about 2-3 km above the adjacent seafloor and its width varies from 150 to 250 km. It is the most prominent feature in the eastern Indian Ocean and separates the central Indian basin on the west from the Wharton basin on the east. The depths of the NER near our sites are about 2000 m below sea level, compared to over 4000 m below sea level at the Expedition 362 sites. Our drill sites are just to the east of this ridge.</p>
<p> </p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23594" src="https://joidesresolution.org//wp-content/uploads/2016/09/p37.jpg" alt="" width="640" height="1099" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/p37.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/p37-175x300.jpg 175w, https://joidesresolution.org/wp-content/uploads/2016/09/p37-596x1024.jpg 596w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p>General view of the NER (modified from Delescluse, 2007)</p>
<p> </p>
<p align="justify">The NER morphologically separates the western Bengal Fan from the eastern Nicobar Fan. At the coring sites of Expedition 362, the NER lies at 1000-2000 m above the present day seafloor on either site of the ridge. Volcanic materials that constitue the ridge are overlain by 100 to 300 m of sediment that have accumulated since the ridge formed. As opposed to the Bengal-Nicobar fan sediments that accumulate in the deepest part of the seafloor, the sediment recorded on the ridge are mostly pelagic (sediments from the water column rather than sediments transported by deeepwater flows) and include many ash layers from volcanic eruptions. The rate of sediment build up here are much lower than where we are drilling on the deeper ocean floor. The ridge has been drilled on several previsous IODP Expeditions in 1972, 1988 and 2014 (source : DSDP Leg 22, ODP Leg 121 and IODP Exp. 353).</p>
<p align="justify"><em>A common question about the ridge is: how was the NER formed?</em></p>
<p align="justify">Several models have been put forward for the origin of the NER. However, it is widely belevied that the NER is a volcanic trace due to a hotpsot. It is commonly assumed that <a class="glossary-term" href="http://archive.joidesresolution.org/glossary/9#term1010"><dfn title="Look up the definition of Hotspot.">hotspot</dfn></a>s are fed by a hot mantle plume rising from deep within the Earth, maybe close to the core-mantle boundary, although there is a debate how deep these plumes may originate and maybe other potential sources of these volcanoes. Hotspots can generate volcanoes in the middle of plates, such as the Hawaiian volcano-<a class="glossary-term" href="http://archive.joidesresolution.org/glossary/9#term646"><dfn title="Look up the definition of Seamount.">seamount</dfn></a> chain, or plate boundaries, such as Iceland forming at the Mid Atlantic Ridge of the North Atlantic. The hotspot model envisages that the plume is fixed relative to the tectonic plates moving overhead producing a trail of volcanic islands and older submerged volcanoes.</p>
<p align="justify"><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23592" src="https://joidesresolution.org//wp-content/uploads/2016/09/hotspot_full.jpg" alt="" width="640" height="316" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/hotspot_full.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/hotspot_full-300x148.jpg 300w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p align="justify">How hotspot made volcanoes</p>
<p align="justify">Just above the hotspot, a volcanic island is created. As the plate drifts over the hotspot, the first volcano is left and a new island appears with a new volcano. The age of the islands therefore increases away from the hotspot as the plate moves. As the island age, they sink by little as they gradually cool after volcanic formation and because they are no longer fed by the hot plume.</p>
<p align="justify">For the NER, it has been proposed that the ridge represents a trace of the Kerguelen hotspot. This hotspot has formed a large plateau on the seafloor in the southern Indian Ocean, north of Antartica. The drilling results on the NER have shown that the rocks of the ridge get gradually younger from north to south. The ridge is about 40 million years old in the south and about 80 million years old in the north. Close to the drill sites of Expedition 362, the ridge is about 60-80 million years old. Furthermore, the ridge has indeed been sinking since it formed-some of the evidence for this is shells that lived in shallow dater depths (less than about 150 m) found in the sediments on the ridge from the ocean drilling expeditions that are now located where the ridge is more than 2000 m below sea level (deep!).</p>
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<item>
<title>Incoming! Oblique Subduction at the Sunda Subduction Zone</title>
<link>https://joidesresolution.org/incoming-oblique-subduction-at-the-sunda-subduction-zone/?utm_source=rss&utm_medium=rss&utm_campaign=incoming-oblique-subduction-at-the-sunda-subduction-zone</link>
<comments>https://joidesresolution.org/incoming-oblique-subduction-at-the-sunda-subduction-zone/#respond</comments>
<dc:creator><![CDATA[Naomi Barshi]]></dc:creator>
<pubDate>Sun, 25 Sep 2016 20:05:30 +0000</pubDate>
<category><![CDATA[earthquakes_659]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[Expedition 362 Sumatra Seismogenic Zone]]></category>
<category><![CDATA[Plate-Tectonics]]></category>
<category><![CDATA[strike-slip fault]]></category>
<category><![CDATA[Subduction]]></category>
<category><![CDATA[subduction zone]]></category>
<category><![CDATA[subduction zones]]></category>
<category><![CDATA[tectonics]]></category>
<guid isPermaLink="false">https://joidesresolution.org//incoming-oblique-subduction-at-the-sunda-subduction-zone</guid>
<description><![CDATA[The Indian and Australian Plates plow northeast into the Sumatra subduction zone, part of the larger Sunda subduction zone, at a... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/incoming-oblique-subduction-at-the-sunda-subduction-zone/" title="Continue reading Incoming! Oblique Subduction at the Sunda Subduction Zone">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p class="p1">The Indian and Australian Plates plow northeast into the Sumatra subduction zone, part of the larger Sunda subduction zone, at a speed of 45 mm/yr. The angle between the direction these two plates move relative to each other is not always at a right angle (90°) to the subduction zone itself–here it is about 50°. Sliding under the Sunda Plate at an angle is not easy, so several large strike-slip fault systems help to accommodate some of this movement. If you thought learning vectors in high school was pointless—think again. This is a perfect vector component problem!</p>
<p>Before we get to big words about big faults, let’s review vectors. If you want to walk from one corner of an intersection to the corner diagonally across from you, you typically go along one side of the intersection, wait for the pedestrian light, and then walk the other side. You’ve walked two sides of a triangle to get to your destination. The third side of the triangle is the line that connects your start and end points:</p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23600" src="https://joidesresolution.org//wp-content/uploads/2016/09/roadCross.png" alt="" width="306" height="244" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/roadCross.png 306w, https://joidesresolution.org/wp-content/uploads/2016/09/roadCross-300x239.png 300w" sizes="auto, (max-width: 306px) 100vw, 306px" /></p>
<p> </p>
<p class="p1">Each side of the triangle is a vector, and we can think of the two pedestrian crossings as components (purple lines/arrows) of the vector that connects your start and end points (orange line/arrow). A vector is a quantity that has a magnitude (in this case length, the distance across the intersection = 20m) and a direction (NW). Any vector can be broken down into component lengths and directions at right angles, like the two sides of the intersection that you just walked.</p>
<p class="p1">The motion of the Indian Plate coming into the North Sumatran subduction zone is also a vector. We can break it down into two components: one parallel to the subduction zone (yellow arrow on map below) and one perpendicular to the subduction zone (orange red arrow). Because the Indian/Australian plate comes in at an angle other than 90º to the subduction zone, we call it “oblique subduction”. (Plate motion vectors and fault locations adapted from Meltzner et al., 2012.)</p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23602" src="https://joidesresolution.org//wp-content/uploads/2016/09/subductvectors.jpg" alt="" width="640" height="462" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/subductvectors.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/subductvectors-300x217.jpg 300w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p>The main subduction zone thrust fault takes up primarily the NE-directed motion of the Indian Plate, perpendicular to the subduction zone. That is, the orange red component of the plate motion vector is taken up by earthquakes on the subduction zone that allow the plates to move past each other. Here are two models that show the motion of the plates during the 2004 M 9.2 Great Sumatra-Andaman Islands Earthquake. During the earthquake the plate overlying the subduction zone actually rebounded towards the SW, over the down-going Indian Plate. Notice that the arrows (from GPS measurements of ground motion change before and after the earthquake) point almost directly across the subduction zone at 90º (models from Chlieh et al., 2007, and Rhie et al., 2007).</p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23601" src="https://joidesresolution.org//wp-content/uploads/2016/09/RuptureGPS.jpg" alt="" width="640" height="599" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/RuptureGPS.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/RuptureGPS-300x281.jpg 300w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p class="p2">But there is still plate motion to account for in the system: the yellow component of the overall plate motion. Faults parallel to the subduction zone take up the slack – This is fairly common in subduction zones because plates rarely meet exactly head on. These long, subduction zone-parallel faults are strike-slip faults. They’re nearly vertical in the subsurface, and motion along them can be seen looking straight down, as if looking at a map. The San Andreas Fault is a well-known example of a strike-slip fault that lets the Pacific Plate move north along the North American Plate. In Sumatra, the primary fault taking up this motion is the Great Sumatran Fault which runs along the center of the island of Sumatra. In fact, the fault runs very close to the volcanic arc of the subduction zone. The southwestern side of the fault moves to the northwest, the same direction as the plate motion not accounted for by subduction. There are also other strike-slip faults offshore, also parallel to the subduction zone, which help to take up some of this plate motion. This figure shows different historic and recent earthquakes in the Sumatra region, including along the Great Sumatran Fault, parallel to the Sunda Trench (Fig. 10 in McCaffrey, 2009).</p>
<p> </p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23598" src="https://joidesresolution.org//wp-content/uploads/2016/09/McCaffrey_SumatraFig.jpg" alt="" width="640" height="276" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/McCaffrey_SumatraFig.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/McCaffrey_SumatraFig-300x129.jpg 300w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p> </p>
<p class="p2">The Great Sumatran Fault carries its own seismic hazard that adds to the risk posed by subduction zone earthquakes and tsunami. On top of the seismic hazard, the southwestern and southern coast of Sumatra and Java host many active volcanoes. The volcanoes are also part of the subduction zone system: they are fed by molten material from the mantle because of the ocean crust subducting below. (That’s another post for another time!)</p>
<p class="p2">Here’s a map of the seismic hazard of Indonesia and Malaysia that shows the maximum amount the ground is likely to move during earthquake shaking in the next 50 years. “How much” is expressed as peak ground acceleration, in percent of the normal acceleration caused by gravity. “Likely” is a 10% chance that the ground will move that much during the 50 year time period. (Map created by the USGS based on data from a variety of sources. See Further Reading for full map poster with citations.)</p>
<p> </p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23603" src="https://joidesresolution.org//wp-content/uploads/2016/09/SumSeismHazUSGS.jpg" alt="" width="640" height="565" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/SumSeismHazUSGS.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/SumSeismHazUSGS-300x265.jpg 300w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p> </p>
<p class="p1">This type of plate boundary geometry is not unique to the Sumatra region. In many plate boundaries where plates converge at an angle rather moving straight toward each other, we see that long strike-slip fault systems help to take up the overall plate motion. The Liquine-Ofqui Fault and Atacama Fault serve a similar purpose in the Andean subduction zone in Chile. The Median Tectonic Line is the largest strike-slip fault in Japan and accounts for the oblique motion at the Nankai subduction zone. Learning about processes at one site on Earth can help us understand what’s going on elsewhere in the world!</p>
<p class="p1"><strong>Sources and Further Reading</strong></p>
<p class="p1"><u>General information</u></p>
<p class="p1"><u>Journal articles</u></p>
<p class="p1">Baroux et al., 1998. Slip-partitioning and fore-arc deformation at the Sunda Trench, Indonesia. Terra Nova, 10 (3), 139-144.</p>
<p class="p1">Bellier and Sébrier, 1995. Is the slip rate variation on the Great Sumatran Fault accommodated by fore-arc stretching? Geophysical Research Letters, 22 (15), 1969-1972.</p>
<p class="p1">Berglar et al., 2010. Structural evolution and strike-slip tectonics off north-western Sumatra. Tectonophysics 480, 119-132.</p>
<p class="p1">Chlieh, M., Avouac, J.-P., Hjorleifsdottir, V., Song, T.-R.A., Ji, C., Sieh, K., Sladen, A., Hebert, H., Prawirodirdjo, L., Bock, Y., and Galetzka, J., 2007. Coseismic slip and afterslip of the great (Mw 9.15) Sumatra-Andaman earthquake of 2004. Bulletin of the Seismological Society of America, 97(1A):S152–S173. <a href="http://dx.doi.org/10.1785/0120050631">http://dx.doi.org/10.1785/0120050631<</a></p>
<p class="p1">Fitch, T.J., 1972. Plate convergence, transcurrent faults, and internal deformation adjacent to Southeast Asia and the Western Pacific. Journal of Geophysical Research 77 (23), 4432–4460. <a href="http://onlinelibrary.wiley.com/doi/10.1029/JB077i023p04432/epdf" class="broken_link">doi:10.1029/JB077i023p04432<</a></p>
<p class="p1">McCaffrey, R., 2009. The Tectonic Framework of the Sumatran Subduction Zone, Annual Reviews in Earth and Planetary Sciences, 37, 345-366.</p>
<p class="p1">Meltzner, A. J., K. Sieh, H.-W. Chiang, C.-C. Shen, B. W. Suwargadi, D. H. Natawidjaja, B. Philibosian, and R. W. Briggs, 2012. Persistent termini of 2004- and 2005-like ruptures of the Sunda megathrust, Journal of Geophysical Research, 117, B04405,</p>
<p class="p1">Rhie, J., Dreger, D., Bürgmann, R., and Romanowicz, B., 2007. Slip of the 2004 Sumatra-Andaman earthquake from <dfn title="Look up the definition of Joint.">joint</dfn> inversion of long-period global seismic waveforms and GPS static offsets. Bulletin of the Seismological Society of America, 97(1A):S115–S127. <a href="http://dx.doi.org/10.1785/0120050620">http://dx.doi.org/10.1785/0120050620<</a></p>
<p> </p>
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<item>
<title>The Earthquake that Triggered Expedition 362</title>
<link>https://joidesresolution.org/the-earthquake-that-triggered-expedition-362/?utm_source=rss&utm_medium=rss&utm_campaign=the-earthquake-that-triggered-expedition-362</link>
<comments>https://joidesresolution.org/the-earthquake-that-triggered-expedition-362/#respond</comments>
<dc:creator><![CDATA[Naomi Barshi]]></dc:creator>
<pubDate>Wed, 21 Sep 2016 22:01:47 +0000</pubDate>
<category><![CDATA[earthquakes]]></category>
<category><![CDATA[earthquakes_659]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[Expedition 362 Sumatra Seismogenic Zone]]></category>
<category><![CDATA[Sumatra Seismogenic Zone]]></category>
<category><![CDATA[tsunami]]></category>
<guid isPermaLink="false">https://joidesresolution.org//the-earthquake-that-triggered-expedition-362</guid>
<description><![CDATA[In 2004, a magnitude 9.2 earthquake struck the northern Sumatra region and triggered a tsunami that inundated the Indian Ocean... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/the-earthquake-that-triggered-expedition-362/" title="Continue reading The Earthquake that Triggered Expedition 362">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p>In 2004, a magnitude 9.2 earthquake struck the northern Sumatra region and triggered a tsunami that inundated the Indian Ocean coast. The disaster was an important reminder to earth scientists that we must better understand the processes at work in subduction zones so that we can help mitigate future disasters. The earthquake was extremely powerful and surprising to geologists in that it was able to break through the plate boundary to relatively shallow depths (5-7 km) below the seafloor. This poster explains some of the details about the events of 26 December 2004, which spurred the scientists on board Expedition 362 to drill into the seafloor and study the rocks and sediments that host major earthquakes once they reach the subduction plate boundary.<br />
<!--break--></p>
<p> </p>
<p>For further reading, check out our pages about earthquakes and subduction zones, two of the main topics under study by <a href="https://joidesresolution.org//expedition/362/">Expedition 362: Sumatra Seismogenic Zone</a>.</p>
<p>This poster is printable on 11×17 in paper.</p>
<p> </p>
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<item>
<title>Family portrait</title>
<link>https://joidesresolution.org/family-portrait/?utm_source=rss&utm_medium=rss&utm_campaign=family-portrait</link>
<comments>https://joidesresolution.org/family-portrait/#respond</comments>
<dc:creator><![CDATA[Agnes Pointu]]></dc:creator>
<pubDate>Tue, 20 Sep 2016 08:38:04 +0000</pubDate>
<category><![CDATA[diatoms]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[foraminifers]]></category>
<category><![CDATA[microfossils]]></category>
<category><![CDATA[nannofossils]]></category>
<category><![CDATA[radiolarians]]></category>
<guid isPermaLink="false">https://joidesresolution.org//family-portrait</guid>
<description><![CDATA[We have 4 micropaleontologists on board during Expedition 362 who are working together to track the age of the core.... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/family-portrait/" title="Continue reading Family portrait">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p>We have 4 micropaleontologists on board during Expedition 362 who are working together to track the age of the core.
</p>
<p>Meet our micropaleontology team! From left to right: Freya for Diatoms, Wen-Huang for Foraminifers, Jan for Nannofossils and Sarah for Radiolarians.</p>
<p><strong>Why are we using microfossils?</strong></p>
<p>Microfossils especially valuable for determining the relative ages of marine rock layers for several reasons:</p>
<ul>
<li>They have been around for millions of years.</li>
<li>They show fairly continuous evolutionary development, so different species are found at different times.</li>
<li>They are abundant and widespread, found in nearly all marine environments.</li>
<li>Finally, they are small and easy to collect, even from deep wells.</li>
</ul>
<p>On board, the four micropaleontologists are each specialized in a different fossil group. Take a look at the kinds of microfossils which we found a site U1480, 4126 deep on the seafloor.</p>
<p align="center"><img loading="lazy" decoding="async" alt="Fossils-test" src="/sites/default/files/u376/Fossils-test.jpg" style="border: 0px solid; margin: 0px;" width="697" height="692" /></p>
<p align="center"><em>Pictures of the four microfossils groups found on the seafloor from Expedition 362 (Sarah Kachovich)</em></p>
<p align="justify">Each fossil type are single-celled plankton. Radiolarians and diatoms both have a glass skeleton (made of silica), but diatoms are phytoplankton whereas radiolarians are zooplankton. This means that Diatoms are capable of photosynthesis, so are restricted to the photic zone (water depths down to about 200 m if the water clarity is good).</p>
<p align="justify">Foraminifer and nannofossils have skeletons made of calcium carbonate. Most foraminifers (forams for short) have shells that are commonly divided into chambers and are added during growth. The simplest forms are open tubes or hollow spheres. Most of the species live on or in the sand, mud, rocks and plants at the bottom of the ocean and the remainder are planktonic (living in the water column).</p>
<p align="justify">Nannofossils are so small that they are barely discernible under a light microscope! One extant group that produces “nannofossils” is the Coccolithophorids, which are a planktonic alga that are very abundant in the world’s oceans. A coccolith is a single disc-like plate which is secreted by the algal organism and held in combination with several other to form the coccosphere. On death the individual coccoliths invariably become separated: the calcareous plates accumulate on the seafloor and are preserved as nannofossils. The picture illustrates the tremendous size difference between a radiolarian made of silica and a calcareous nannofossil made of calcium carbonate (“calcareous”).</p>
<p align="center"><img decoding="async" alt="nanno" src="/sites/default/files/u376/nanno.jpg" style="border:0px solid;margin:0px;" />
</p>
<p align="center"><em>Comparison between the size of a radiolarian and a nannofossil (pictures of Sarah Kachovich)</em></p>
<p align="justify">Since nannofossils are phytoplankton, like diatoms, they are distributed throughout the photic zone (especially the upper 50m of the water column). The planktonic mode of life, rapid evolution and the tremendous abundance of calcareous nannofossils makes them very useful tools for biostratigraphy.</p>
<p align="justify"><strong>Tracking the age of sediments using microfossils?</strong></p>
<p align="justify">Paleontologists can dertermin the age of sediments by using the microfossil assemblages contained within them. To work well, the fossils used must be widespread geographically and the sediments must contain a lot of different species.</p>
<p align="justify">When we are drilling into sediments, generally the deeper we drill the older the fossils we find. Different species have appeared, evolved and became extinct. The main goal of our micropaleontolgy team is to find and identify these fossils to create an age model for the sedimentation accumulation rate through time.</p>
<p align="justify">A biozone is a body of sediment or rock characterized by its fossil content (absence or combinations of fossils). There are 5 types of biostratigraphic interval zones (see the picture below):</p>
<ul>
<li>
<div align="justify">Taxon Range Zone (TRZ): the lower, upper and lateral limits of this zone are determined by the range of occurence of species A,
</div>
</li>
<li>
<div align="justify">Concurent Range Zone (CRZ): the body of sediments including the concurrent, coincident, or overlapping parts of the range zones of two specified species (A and B),
</div>
</li>
<li>
<div align="justify">Base Zone (BZ): defined by the first appearance of species A and at the top by the first appareance of species B,
</div>
</li>
<li>
<div align="justify">Top Zone (TZ): the opposite of the BZ: the base of the biozone is defined by the lowermost documented occurence of species A and the top by uppermost documented,
</div>
</li>
</ul>
<div align="justify">
<ul>
<li>
<p>Partial Range Zone (PRZ): Interval zones established to partition the range of species C on the basis of the occurence of two other species (A and B) whose ranges do not overlap.</p>
</li>
</ul>
</div>
<div align="center">
<img decoding="async" alt="BB" src="/sites/default/files/u376/BB.jpg" style="border:0px solid;margin:0px;" />
</div>
<p align="center"><em>The five logical possibilities for biostratigraphic characterization of biozones (Backman et al., 2012)</em></p>
<p align="justify">Not every fossil is useful. Some species evolved slowly and have existed for long time periods. Therefore, these fossils don’t give a good precision of the age of the sediment. Short lived species make our micropaleontology team happy. Some particular species give very specific constraints on the ages making the work of the micropaleontology team easier. Statiscally, having juste one or two species is not very useful for them, even if the number of the fossils is very important.</p>
<p align="justify">Using these techniques, our team of micropaleontologists are able to date the sediments drilled during Expedition 362. The sedimentologists and other geologists can use their work to analyse the sequeneces of sediments and rocks encountered. For example, they can calculate sedimentation rates, relate these to climatic changes or major tectonic changes such as the uplift of large mountain ranges and get information on the depositional environment at the site.
</p>
<p align="justify">My blog in french: here
</p>
<p>
</p>
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<title>Daily Science Report Explained</title>
<link>https://joidesresolution.org/daily-science-report-explained/?utm_source=rss&utm_medium=rss&utm_campaign=daily-science-report-explained</link>
<comments>https://joidesresolution.org/daily-science-report-explained/#respond</comments>
<dc:creator><![CDATA[Naomi Barshi]]></dc:creator>
<pubDate>Tue, 13 Sep 2016 17:44:18 +0000</pubDate>
<category><![CDATA[carbonate sediments]]></category>
<category><![CDATA[carbonate sediments core description]]></category>
<category><![CDATA[core description]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[Expedition 362 Sumatra Seismogenic Zone]]></category>
<category><![CDATA[sediment cores]]></category>
<category><![CDATA[sedimentary rocks]]></category>
<category><![CDATA[Structural Geology]]></category>
<category><![CDATA[Sumatra Seismogenic Zone sediment cores sedimentary rocks Structural Geology veins]]></category>
<category><![CDATA[veins]]></category>
<guid isPermaLink="false">https://joidesresolution.org//daily-science-report-explained</guid>
<description><![CDATA[Each day, our Staff Scientist/Expedition Project Manager sends out an update to the ship and to our colleagues on shore.... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/daily-science-report-explained/" title="Continue reading Daily Science Report Explained">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p>Each day, our Staff Scientist/Expedition Project Manager sends out an update to the ship and to our colleagues on shore. The daily report summarizes the scientific findings from the day before. Here’s an example of a daily report, explained with photos!<br />
<!--break--><br />
Click on the photo at left to make it bigger. </p>
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<title>How we drill and core in hard sediments and rocks</title>
<link>https://joidesresolution.org/how-we-drill-and-core-in-hard-sediments-and-rocks/?utm_source=rss&utm_medium=rss&utm_campaign=how-we-drill-and-core-in-hard-sediments-and-rocks</link>
<comments>https://joidesresolution.org/how-we-drill-and-core-in-hard-sediments-and-rocks/#respond</comments>
<dc:creator><![CDATA[Agnes Pointu]]></dc:creator>
<pubDate>Mon, 12 Sep 2016 10:26:05 +0000</pubDate>
<category><![CDATA[Coring]]></category>
<category><![CDATA[Coring drilling how science works RCB]]></category>
<category><![CDATA[drilling]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[how science works_1421]]></category>
<category><![CDATA[RCB_1430]]></category>
<guid isPermaLink="false">https://joidesresolution.org//how-we-drill-and-core-in-hard-sediments-and-rocks</guid>
<description><![CDATA[When we want to drill into hard sediments and rocks, we have to change the drill bit. In the deeper... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/how-we-drill-and-core-in-hard-sediments-and-rocks/" title="Continue reading How we drill and core in hard sediments and rocks">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p align="justify">When we want to drill into hard sediments and rocks, we have to change the drill bit. In the deeper part of Site U1480, we used an RCB (Rotary Core Barel) drill bit rather than the APC/XCB we used before for coring the younger, softer sediments.*</p>
<p align="justify">One main difference between the two drills bits is that with the RCB the bit and the outer core barrel turn with the drill string.</p>
<p align="justify">The RCB barrel is made of 2 parts: an outer core barrel and an inner core barrel.</p>
<p align="justify">The RCB inner core barrel is dropped from the surface without a line attached and free falls trough the drill string until it lands in the bottom hole assembly (BHA). The RCB bit and the outer core barrel rotate with the drill string while the inner core barrel remains stationary. The main bit turns the core which does not rotate inside the inner core barrel as it is cut.</p>
<p align="justify">The inner core barrel is retrived by a wireline when the core has been cut. This is a much faster method of coring than the conventionnal system often used in oil industry. With a conventional coring system, the core barrel is attached to the end of the drill string. Cores are recovered by moving the entire dill string up to the rig floor and back down to the bottom of the hole (called tripping). But here, we are in more than 4 km of water and we have to drill into ~1 km of sediments. Using this system of retrieving each core would be very slow and take many over 1/2 day for each core.</p>
<p align="justify">On the JR, once we have cut the core by drilling ahead the desired amount (usually 9.7 m), the drillers drop sinker bars down the drill string to the top of the core barrel. These lock onto the upper end of the core barrel called the “pulling neck”. Then, the drillers pull the core barrel back up with the wireline.</p>
<p align="justify">Once the core barrel is on the rig floor, the core liner, a plastic tube containing the core of sediments and rock, is removed and carried onto the catwalk where it is inspected, gas measurements are taken, and it is cut into sections before analysis and sampling.</p>
<p align="justify">
<p align="center"><img loading="lazy" decoding="async" style="border: 0px solid; margin: 0px;" src="/sites/default/files/u376/RCB-drill.jpg" alt="RCB-drill" width="744" height="1150" /></p>
<p align="justify">
<p align="center"><img loading="lazy" decoding="async" style="border: 0px solid; margin: 0px;" src="/sites/default/files/u376/rem2.jpg" alt="rem2" width="675" height="615" /></p>
<p align="justify">
<p align="justify">
<p>* technical sepcs on IODP drilling systems can be found <a href="http://iodp.tamu.edu/tools/index.html" target="_blank" rel="noopener">here<br />
</a></p>
<p><a href="http://iodp.tamu.edu/tools/index.html" target="_blank" rel="noopener">To follow my blog in french, it’s </a><a href="https://expedition362joides.wordpress.com/" target="_blank" rel="noopener">here</a><!-- [if gte mso 9]>
<![endif]--></p>
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<title>From Mud to Rocks</title>
<link>https://joidesresolution.org/from-mud-to-rocks/?utm_source=rss&utm_medium=rss&utm_campaign=from-mud-to-rocks</link>
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<dc:creator><![CDATA[Naomi Barshi]]></dc:creator>
<pubDate>Sun, 11 Sep 2016 19:36:50 +0000</pubDate>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[Geology_482]]></category>
<category><![CDATA[sedimentary rocks]]></category>
<category><![CDATA[Sumatra Seismogenic Zone]]></category>
<guid isPermaLink="false">https://joidesresolution.org//from-mud-to-rocks</guid>
<description><![CDATA[Agnes just gave us a nice primer on mud. But what happens next to make mud into a rock? First, we... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/from-mud-to-rocks/" title="Continue reading From Mud to Rocks">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p class="p1">Agnes just gave us a nice primer on mud. But what happens next to make mud into a rock?</p>
<p class="p1"><strong>First, we need to answer the question, What’s a rock? </strong></p>
<p class="p1">This is a simple question, but it’s tricky to tie down a simple AND satisfying answer. Most of the geologists I asked on board said, “A rock is a solid aggregate of minerals.” That’s what a textbook says, but others disagree with the boundaries of this definition. What about coal? That’s not made of minerals. What about our bones? They’re not rocks.</p>
<p class="p1">“Making boxes is difficult in a messy world,” sedimentologist Kitty Miliken told me when I asked her about the definition of a rock. “If you’re going to be a scientist”, she said, you’ll want to make a lot of boxes. “But you need to be prepared for Nature not to respect your boxes.” So, according to Kitty, the boundaries of the box around “rock” are:<br />
– naturally occurring<br />
– hard<br />
– solid<br />
– that’s not easily disaggregated or broken apart<br />
– not just made by a single organism, though it may contain parts of organisms (fossils)<br />
– and is any combination of the following:<br />
– an aggregate of minerals and/or organic material<br />
– monocrystalline or polycrystalline<br />
– all of one composition or a mixture of different compositions.</p>
<p class="p1">This seems to work for our purposes of what’s rock and what’s not yet a rock. Note that some rocks do break apart easily. Shale by definition breaks into sheet-like layers. But it’s still a coherent mass of mineral grains that doesn’t just fall apart in your hands.</p>
<p class="p2">The sediments we’re looking at are well on their way to fitting this definition: they’re naturally occurring, the individual grains or minerals or fossils are hard (though the combination of them is not yet hard), they’re individually solid, and they’re not made just by one organism. There are lots of different minerals, so in this case the rocks that our sediments will turn into are polycrystalline and a mixture of different compositions. Some rocks are made of just one mineral, like quartzite (quartz) or limestone (calcite).</p>
<p class="p1">When sediments settle to the seafloor, they form a layer of particles, whether they’re grains of sand, silt, and clay from land or tiny micro-organisms that lived in the ocean. Sediments are already made of the right stuff, but they might not yet be rocks. The young sediments in the upper kilometer or so of seafloor that we drilled through were not yet rocks. For the first few hundred meters of the holes at Site U1480, the sediments were soft and easily disaggregated. That is, we could easily take a tiny toothpick scoop of sand or mud and smear it around on a smear slide, as Agnes explained in a recent blog post.</p>
<p class="p1">What exactly the sediments are made of determines how quickly they’ll become a true rock. Carbonate minerals eventually make up rocks like limestone. They’re reactive and can quickly turn into rock. If you go to the beach in the Caribbean, where there are high concentrations of carbonate minerals in the seawater, you can see rocks forming over a time period of years. It can take millions or even tens of millions of years for sand and mud to turn to rock. This process of becoming a rock can be sped up if there’s carbonate-rich water flowing between the grains of sand and mud or carbonate grains (shell fragments, calcareous algae flakes, etc.). The calcium, carbon, oxygen, and other atoms that make up carbonate minerals dissolved in the water can then grow together as carbonate minerals between grains of sediment and glue them together. This is called <strong>cementation</strong>. Carbonate minerals can also glue non-carbonate chunks together. We found a layer of proper sandstone in one of our cores, with carbonate minerals cementing the sand grains together.</p>
<p> </p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23606" src="https://joidesresolution.org//wp-content/uploads/2016/09/Figure-U1480-C-F36_sm.jpg" alt="" width="640" height="986" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/Figure-U1480-C-F36_sm.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/Figure-U1480-C-F36_sm-195x300.jpg 195w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p> </p>
<p class="p1">The image shows a core section scan with close-up photos taken with a microscope, looking through a very thin slice of the rock glued to a glass slide.</p>
<p class="p1">There were layers of unconsolidated sand above and below that were not yet sandstone because the grains weren’t cemented together. We interpret that this rock layer must have been a place where underground water could flow because sand is very porous and permeable, meaning water can easily flow through it. The water was rich in carbonate ingredients, so carbonate minerals were able to glue the grains of sand together. Other kinds of cement are silica and iron oxide. The iconic red sandstones of the American Southwest owe their color to iron oxide cement. The grains of sand that comprise them are actually mostly colorless quartz!</p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23610" src="https://joidesresolution.org//wp-content/uploads/2016/09/UtahRocks.jpg" alt="" width="500" height="312" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/UtahRocks.jpg 500w, https://joidesresolution.org/wp-content/uploads/2016/09/UtahRocks-300x187.jpg 300w" sizes="auto, (max-width: 500px) 100vw, 500px" /></p>
<p> </p>
<p class="p1">Sandstones with iron oxide cement in Arches National Park, Utah. (Photo © Naomi Barshi)</p>
<p class="p1">In addition to cementation, <strong>compaction</strong> also helps turn loose sediments into rocks. As sediments get buried by more sediments and water above, they “feel” more and more pressure. Even just 4200 m of water, where we are now, provides 400 times as much pressure as the atmosphere exerts on us. At 1000 <dfn title="Look up the definition of Mbsf.">mbsf</dfn>, there’s an additional 200 times atmospheric pressure just from the overlying sediments. We could very clearly see the difference between young, uncompacted sediments at the top of the hole and much older (more than 60 million years older!) compacted sediments and true sedimentary rocks near the base of the hole. Here are some photos for comparison. The first is some silty, clayey land-derived sediment very characteristic of the mid-shallow subsurface of the ocean.</p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23609" src="https://joidesresolution.org//wp-content/uploads/2016/09/siltyClay.jpg" alt="" width="640" height="496" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/siltyClay.jpg 640w, https://joidesresolution.org/wp-content/uploads/2016/09/siltyClay-300x233.jpg 300w" sizes="auto, (max-width: 640px) 100vw, 640px" /></p>
<p><img loading="lazy" decoding="async" class="alignnone size-full wp-image-23608" src="https://joidesresolution.org//wp-content/uploads/2016/09/SedRocks.jpg" alt="" width="800" height="577" srcset="https://joidesresolution.org/wp-content/uploads/2016/09/SedRocks.jpg 800w, https://joidesresolution.org/wp-content/uploads/2016/09/SedRocks-300x216.jpg 300w, https://joidesresolution.org/wp-content/uploads/2016/09/SedRocks-768x554.jpg 768w" sizes="auto, (max-width: 800px) 100vw, 800px" /></p>
<p class="p1">Compaction by increased pressure, increasing temperature with depth, and the flow of fluids in the spaces between the rocks can also change the chemical composition of the sediments. Minerals can have many different elements inside their crystal lattice structures, including hydrogen and oxygen together as hydroxide. These can be squeezed or heated off. The oxygen and hydrogen they combine into to pure water plus a new, chemically dry mineral left behind. These kinds of mineral reactions are called “dehydration reactions” and are very important for the behavior of rocks and sediments, especially when they get hot and are put under pressure. Water weakens rocks and sediments, lets rocks melt at lower temperatures, and makes sediments more slippery. We can tell if these reactions are happening in the sediments by measuring the composition of interstitial water in the cores. We squeeze the water out of the sediments in the <dfn title="Look up the definition of Chemistry.">chemistry </dfn>lab (physically only—we don’t squeeze so hard that we cause chemical de-watering). We expect that the interstitial water should be seawater, so if we see freshwater, we know it’s from dehydration reactions. Water moving between the grains of sediments and rocks can also react with the minerals to form new, chemically wet minerals. This can cause changes in volume and has all sorts of implications for rock composition and strength. Lest I write a <dfn title="Look up the definition of Geology.">geology</dfn> textbook instead of a blog post, I’ll cut this discussion short here. (Email me or send us a <a href="https://www.facebook.com/joidesresolution/">Facebook<</a> message if you want to know more about the effect of water on rock behavior at elevated temperatures and pressures. You’ll find my email in my blogger profile.)</p>
<p class="p1">Once the sediments are cemented together, possibly including compaction, they are no longer easy to break apart. They’re now hard and solid on a larger scale, not just individual hard and solid particles. This process of sediments turning into rocks is called <strong>lithification</strong>. “Lithos” means “rock” in Greek. Keep an eye out for that word root! Lithology, lithography, monolith, lithic fragments…</p>
<p class="p1">There are of course other ways of making rocks, but we’ll leave <dfn title="Look up the definition of Igneous.">igneous</dfn> and metamorphic rocks for another time.</p>
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<title>A Successful Story at Site U1480</title>
<link>https://joidesresolution.org/a-successful-story-at-site-u1480/?utm_source=rss&utm_medium=rss&utm_campaign=a-successful-story-at-site-u1480</link>
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<dc:creator><![CDATA[Naomi Barshi]]></dc:creator>
<pubDate>Fri, 09 Sep 2016 22:05:00 +0000</pubDate>
<category><![CDATA[drilling]]></category>
<category><![CDATA[EXP362]]></category>
<category><![CDATA[Expedition 362 Sumatra Seismogenic Zone]]></category>
<category><![CDATA[operations]]></category>
<category><![CDATA[Sumatra Seismogenic Zone operations]]></category>
<guid isPermaLink="false">https://joidesresolution.org//a-successful-story-at-site-u1480</guid>
<description><![CDATA[We finished our first site with success! We’re now at our second site, so here’s a little summary of what... <div class="read-more"><a class="excerpt-read-more" href="https://joidesresolution.org/a-successful-story-at-site-u1480/" title="Continue reading A Successful Story at Site U1480">Read more<i class="fa fa-angle-right"></i></a></div>]]></description>
<content:encoded><![CDATA[<p>We finished our first site with success! We’re now at our second site, so here’s a little summary of what we did at Site U1480. We met our primary science goal at the site: core the entire sedimentary sequence from seafloor down to the oceanic crust that forms the basement that the sediments rest on. That’s almost 1.4 km (0.8 mi)!<br />
<!--break--></p>
<p>While the scientists are writing up their reports, we’ll summarize a few of the things that we’ve been thinking about. I’ll have a few more posts for you in the next few days. Stay tuned!</p>
<p>(Click on the image at left to see it bigger.)
</p>
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