Plate Tectonics Works

We have just arrived at the last coring Site for this Integrated Ocean Drilling Program Expedition (“PEAT-6C”, from today also know as Site U1335), with three more planned as part of this science programme for our colleagues on the next Expedition.

It is amazing how much we have already done, and even though it feels as if we have been on the ship forever, time seems to have passed incredibly quickly; a conflicting feeling shared by many here on the ship.

Having been involved in the planning of this project, I thought I’d share some of the amazement I feel when we try to find the best place in the ocean for coring, so we can answer specific questions about Earth history. When we then finally get to that specific spot in the huge Pacific Ocean my excitement of true exploration is huge: we get the first cores on deck, which tell us whether all our planning, predictions, and sometimes “educated guesses” are confirmed by reality.

We are trying to get sediments from special time periods of Earth’s history, for example from the switch in the Earth’s climate state from a “Greenhouse” world to the current “Icehouse” world, which is a more imaginative way of identifying when huge ice-sheets first grew near the poles of the planet.

The sediments we are coring work almost like a history book, where each layer of sediment corresponds to another page, written by Earth’s oceans, climate, and living creatures. If one was to take each centimeter of sediment (Tom, Kirsty and Paul already described what one can find in these) as a page of this book, it would correspond to about 500 to 2000 years of time. Each page of sediment, sometimes with riddles and puzzles, can tell us about what happened in the ocean above, as well as the Earth as a whole.

We are trying to target these specific time periods of Earth’s history for the last 53 million years, and for us to be successful and achieve our aims, we need to plan where exactly on the planet we need to take our temporary home, the JOIDES Resolution, to give us sediments of the right age.

This is where the title of this Blog comes into play. The oceanic plates, on top of which the sediments are deposited, are constantly on the move. For example, London and New York move apart by about the same length that fingernails grow: about a couple of centimeters per year. While this sounds small, it amounts to a fair distance over millions of years of Earth’s history.

Core samples

All of our drill sites were estimated to have been located at the Equator during the time when our main sediment targets were deposited. The Pacific plate, however, is moving through time, and you can see this directly from the trend of the Hawaiian island chain, spread out from southeast to northwest. The Pacific oceanic plate has been moving towards the north-west for the last 40 million years or so. The Hawaiian volcano has not moved very much, and is actually fed from underneath the oceanic crust, leaving a signature volcanic island on top of the plate, almost like a bunsen burner (you might know from chemistry lessons) leaving its mark.

Each site moves towards the north and away from the Equator by about 2.5 degrees latitude every 10 million years: that’s almost 300 km, or a little bit less than the distance between Southampton and Leeds, each 10 million years. That’s why our first drill site, PEAT-1C, is now actually at 12 degrees latitude, as it was first “born” on ocean crust of about 50 million years age. PEAT-6C, the site we have just arrived at, is a meagre 5 degrees north of the Equator, and is only about 26 million years old. All of our sites share the fact that they were on the Equator when they first formed!

The amazing thing is, then, that after all this planning, and detailed calculation, we arrive at a new coring site, and Paul and Kirsty and Tom analyse the microfossil shells in the sediments from each core, and can tell us their age … and once we hit “basement”, or the top of the hard basaltic rocks underneath a few hundred meters of sediment, we can see whether our predictions for the age and position of each site were correct or not.

For me, that is one of the most exciting, and also the most fascinating moments on the ship. We are using theories of plate tectonics that were only first formulated in 1912 by Alfred Wegener, and much refined even later by the British scientists Vince and Matthews in 1963, to decide where we need to take the ship to get the right sediments for our work. So far, it has worked each time for the previous four drill sites – we’ll find out in a couple of day’s time whether plate tectonics worked again!

Photo:  Sediments lying on ocean crust basalt (basement). The right hand core has basalt and limestone, and those to the left are younger carbonate sediments. The microfossils in these sediments let us date the age of the ocean crust formation at this site.

Posted to NERC’s Planet Earth Online, where you can see descriptive blogs by all the Brits on board, on 15 April 2009.