Scales Are Not Just For Fish: Time and Space in Geology

Blog Series Installment #2: Space as Time

When you make a layer cake, you lay the first slab of cake on the platter, then icing, then the next layer, then more icing, and so on.  You’re a scientist, so you did an experiment with the layers of the cake.  You baked some thicker than others, and discovered that they took more time to bake.  The time you’re done with the cake, with your variety of layers in a stack, the layer at the bottom is oldest, and thicker layers took longer to form.  (Sure, you could flip the cake over or cut it up and turn some chunks over, but we’ll leave that for a post about deformation of rock units and structural geology.)

This depth-age and thickness-time relationship is also generally true in what we affectionately call “layer cake geology”.  This is approximately what we’re looking at in the cores of Expedition 362, except that thicker layers aren’t always the ones that took longer to form.  We’ve recovered layer upon layer of different combinations of mud and sand, varying in color from grey to green to beige depending on the minerals that comprise them.  (In geology, mud is a technical term!  It means a combination of silt-sized and clay-sized particles—anything smaller than about 1/10 of a mm, which is smaller than you can usually see without magnification.)  This is what we expect to find at the bottom of deep ocean water, which is mostly very quiet.  Even the tiniest particles can settle out of it.  As they do so, they make layers upon layers, oldest at the bottom, youngest at the top.

In this way, space becomes time: the deeper we go, the older the sediments get. Our micro-paleontologists have confirmed this for us, which is very comforting!  The sediments coming up during my shift from around 500 meters (~1650 ft) below the seafloor are already several million years old (late Miocene, for the geologists), more than twice as old as the first humans!

Space and time have another important relationship, if we know how fast things happen.  The cake you baked above had thicker layers that took longer to bake than the thinner layers.  The process of baking the same mixture of flour, eggs, sugar, etc., that you used in all your cake layers goes at a certain rate, perhaps 1 unit of baked-ness per minute.  Baking different kinds of things takes a different amount of time.  Likewise, how long it takes for different types of sediments to accumulate on the seafloor varies.  Sometimes thickness and how long something took to form aren’t exactly the same thing.

The photo-stitch of a segment of core below illustrates a few things that our scientists have been looking for and interpreting on this expedition.  This segment of core, like many we’ve seen on board, is a reminder that a thicker layer does not always mean it took longer to form.  Sometimes, thick layers of sandy sediment like this only took a few minutes to form!  These could be the result of an underwater landslide or a fast-moving current of water that slowed down all at once and dumped a lot of coarser sediment because it couldn’t carry it any longer.  In contrast, the few centimeters of clay-rich, compact mud at the top of the photo probably took thousands of years to accumulate by the slow settling of tiny particles out of the ocean water above.

This segment of core also reminds us that we don’t always know the answer right away.  Two geologists might not always have the same interpretation.  We only get to see a little bit of core at a time, a few centimeters in diameter, which isn’t the whole picture.  Still, we can use the combination of all our different expertise to put together a bigger picture than what’s on the table in front of us.  That’s why these expeditions bring on board a group of scientists with a wide range of knowledge, all working towards related questions that need different tools and eyes and minds to be answered.