The Earth Story

Birth of the Atlantic

This is a gorgeous computer rendering of the opening of the Atlantic Ocean and the breakup of the Supercontinent Pangaea.

Why does the ocean get deeper?

This is a map of the seafloor elevation or bathymetry in the Indian Ocean. The red and yellow areas represent shallow parts of the ocean; the blue and violet areas are deeper. Some obvious features, like Indonesia and the Ninety-East ridge, stand out obviously. If you look at the overall seafloor there is also another clear pattern. The Indian Ocean is shallow near the mid-ocean ridge, and the ocean gets deeper farther away from the ridge.

This property of the ocean floor is almost entirely due to heat. At mid-ocean ridges, the hot mantle comes up nearly to the depth of the seafloor. That mantle can be about 1200°C. Once that mantle melts to make ocean crust and the ocean crust solidifies and begins cooling, everything starts to contract. Over a period of millions of years, heat diffuses out of the ocean crust and even the upper mantle, cooling off the upper hundred kilometers of the planet. As the rocks cool, they contract and their density goes up, gradually dragging the ocean floor downwards under its own weight.

Some of the deepest places on Earth, including areas like the Mariana Trench, are found where there is exceptionally old oceanic crust. The older the oceanic crust, the colder it is and the denser it is, and the more it wants to sink into the planet.

This feature is observed on land as well. Where the land surface was recently rifted or active, it can be warmer than an ancient, cold continent. As this land surface cools, it will settle and subside, creating space for sediment or water to flow in and be trapped.

This simple story still leaves room for some complexities – for example, right next to the Sunda trench in this photo, the ocean floor actually rises because it’s being bent just before it goes down the subduction zone. Other areas can be warmed up by volcanic activity or by extra heat coming up from the mantle.

-JBB

Image credit: https://go.nasa.gov/2o8a1NX

Read more: http://topex.ucsd.edu/geodynamics/07cooling.pdf http://www-gpsg.mit.edu/12.201_12.501/BOOK/chapter5.pdf

Forming an ocean basin

This image shows the bathymetry of the South China Sea. Don’t ask me why the southern section is labeled Dangerous Ground, I honestly don’t know; this was just the best shot of the seafloor I could find with a usable license. Anyway, a recent scientific drilling expedition just produced some cool results about this area – it actually started turning into an ocean basin. The South China Sea basin opened over about 10 million years, from the late Oligocene 34 million years ago into the Miocene. Sometimes blocks of continental crust get pulled apart and shifted by larger plate tectonic forces – that’s what happened here. Some of the continent-like crust currently found in the Philippines was literally ripped away from the rest of the continent, leaving behind a basin and forming a series of normal faults that are now buried by sediment coming off of the continent.

In some places, continents are pulled apart without creating true ocean crust. For example, the Iberian Peninsula has substantially rotated compared to the rest of Europe, but it did so slowly enough to avoid creating ocean crust. For a contrast, the Red Sea is opening somewhat slowly, but there is legitimately basaltic ocean crust forming at the bottom.

As part of an expedition by the International Ocean Discovery Program, which runs the scientific drilling ship Joides Resolution, a team of researchers drilled through the sediment layers to sample the rocks in what they thought would be remnants of the continental break up in this area. To their surprise, they found layers of oceanic basalt that had erupted as the continent pulled apart.

This area did not fully pull apart into an ocean, but it started the process. Between 34 and 30 million years ago, this part of the world rifted rapidly, so rapidly in fact that it pulled the crust apart and the mantle began upwelling to fill the space. When hot mantle moves to low pressures, that triggers melting, which lead to the formation of igneous rocks and eventually ocean-like crust. That ocean crust has since been buried, the rifting slowed in the middle Miocene, but it’s still down there making up part of the South China Sea.

-JBB

Image source: http://bit.ly/2BxbUxt

Original paper: https://www.nature.com/articles/s41561-018-0198-1_ _

A new Gondwanan reconstruction

Over 150 million years ago, the continents of South America, Africa, Australia, and Antarctica, along with other slices of land such as the Indian subcontinent, were joined together in a single giant landmass known as Gondwana. Since that time, that landmass has progressively been torn apart by the opening of new rift zones and creation of new ocean, as is happening today at the Red Sea rift.

The fit between Africa and South America is so good that it was recognized over a century ago by Alfred Wegener when he first proposed the concept that plates could migrate across the surface of the Earth.

If you’ve ever tried to fit together puzzle pieces shaped like the African and South American coasts, you’ve probably found that they come very close to fitting together, but there are some indentations that just don’t match up well. If they were puzzle pieces the shapes wouldn’t be quite right, there’s just a little more complexity.

When continents rift apart, they don’t just break cleanly along lines. Large rift zones form, the crust thins, faults propagate in multiple directions, rock units are stretched out and broken, and new rocks can be created by volcanism or sedimentary deposition. Consequently, the boundaries of continents never match up perfectly. If you’re trying to piece South America and Africa back together, this isn’t a big deal; the continents fit together with an accuracy of a few kilometers anyway and the match is hard to miss.

But, for the other landmasses that made up Gondwana, these small mismatches are much more important. At some point, Australia, India, and Antarctica were all hooked together, but as you see in this image, the coastlines don’t have the type of puzzle-piece matching shape that South America and Africa do. These coastlines are pretty smooth, and given the distortion that happens when continents rift apart, there can be tens to hundreds of kilometers of variation in fitting these continents together.

That’s exactly the case right now; in scientific publications there are a variety of reconstructions for how Gondwana fit together. This lack of agreement on where these continents joined leads to other issues in understanding the geology. If the map can’t reconstructed accurately, the dates the continents rifted apart can’t be determined accurately, leading to errors in understanding how species evolved once continents became isolated and in deciphering why the supercontinent broke up in the first place.

This image is a reconstruction from a just-published model putting India, Australia, and Antarctica together again based on newer data. There are a variety of techniques to do these types of reconstructions, and so in this piece I’m not going to endorse theirs as the correct answer; I’ll let peer review figure that out.

To reconstruct the plate positions, these authors tried to use geologic features common to the different continents. This can be tricky as well, because units can be folded, they can be continuous over hundreds of kilometers, and glaciers make everything more complicated.

These authors used a series of faults and basins that developed as rifting began as their tie-points. It looks like a good setup and it’s been done well, now we’ll wait to see how it holds up as new data is collected.

In fact, one of the dirty little secrets of geology is that this type of work will probably become much more accurate in the future…because information with bearing on how Australia and Antarctica fit together is buried under the East Antarctic Ice Sheet. It probably won’t happen in my lifetime, but with the rate CO2 is going up in the atmosphere, it won’t be long before someone gets to map these rocks and really put together the history of this part of the world.

-JBB

Image credit/Press report: http://phys.org/news/2013-07-reveals-ancient-jigsaw-puzzle-supercontinent.html

Study (subscription required): http://www.sciencedirect.com/science/article/pii/S1342937X1300213X

Always happy to find videos like this, this is an animation of the rifting between Australia and Antarctica over the last 100 million+ years. This is the kind of video content scientists seem like they produce for conferences and talks but which I never know where to find online.

Marie Tharp and the Mid-Atlantic Ridge

In honor of International Women’s Day and the beginning of Women’s History Month, we thought some of our favorite female geologists deserved a shout out. One of my personal favorites is this girl, Marie Tharp, who did something very important for our modern understanding of geology—she discovered the Mid-Atlantic Ridge.<!-- more --.

The ocean floor has always been a mysterious place and in the 1950’s it was even more so; most believed it was simply a flat, boring plain. After World War II, many feared the next warfront would be underwater, so the quest to gather more information about the ocean floor was in full force, which proved to be good for science.

Marie was working at the Lamont Geological Observatory when the funding for ocean research started pouring in. Many of her colleagues would go out to sea and come back with mounds of sonar data that could be used to determine the depth of the ocean floor. Back at the lab, Marie began piecing together the numbers and using them to make a map of the ocean floor.

Marie, and her colleague Bruce Heezen, began to notice something interesting about the map (which she made by hand, by the way)—the ocean floor was not flat. In fact, there were mountains! Underwater! Most notably, these mountains formed a very long chain, one that went right down the middle of the Atlantic Ocean. Marie thought it was a volcanic rift center, an idea that was initially dismissed as “girl talk”.

Marie and Bruce (who came around to the idea that it was a volcanic rift center) published their first map in 1957. At this time, most people still did not like the idea of a mid-ocean ridge because it was too closely linked with continental drift, which was considered geological nonsense at the time. A few years later, Harry Hess would use the Mid-Atlantic Ridge to support his theory of seafloor spreading, which ultimately led to the unifying theory of plate tectonics.

Marie devoted much of her life to mapping; she and Bruce released a map of the entire ocean floor in 1977. Our modern understanding of Earth processes and plate tectonics would not be possible without these maps. The discovery of mid-ocean ridges was a crucial piece to the puzzle of how continents move and ultimately why Earth works. So thank you Marie!

-CM

For more information: http://bit.ly/1BfCjWE http://huff.to/18oityd A brief autobiography: http://bit.ly/1mEqFR7 This book:http://amzn.to/1wMccZ0

Photo (Marie with Bruce Heezen) credit: Marie Tharp Maps http://bit.ly/1NqP5YX