Atlas of the UnderworldGuest Post, · Categories: Guest Posts
Atlas of the Underworld –
Just let that roll off your tongue and imagine what it might look like. Probably not like what it is – The Atlas of the Underworld is, according to D.G. Van der Meer and D.J.J van Hinsbergen, “the first complete mapping of subducted plates in the Earth’s mantle and their geological interpretation.” Subducted plates refers to parts of the surface of the earth’s crust that have been pushed under other parts of the earth’s crust. This is a change in thinking, and an important new window on Earth’s crustal processes.
You might remember, from middle school Earth Sciences, some diagrams showing different types of crustal boundaries. There are three basic ones –
Transform Boundary (we don’t care about these today)
Creative boundaries happen where plates are pulling or being pulled apart. The mid-Atlantic ridge is a classic oceanic example. The African Rift Valley and the Red Sea are examples of creative boundaries on continents. At a creative boundary, melted crust and mantle material reach the surface of the earth, and harden. The mechanism for this is not entirely clear – it could be a push mechanism, or a filling action. We are not worrying about these boundaries today either.
Destructive boundaries are where two plates come together. Generally a subduction zone is formed, where one plate is pulled or pushed beneath another. If you look at the illustration, you can see that what happens to the plate as it descends into the mantle is unclear – the illustration ends in a fuzzy mess of uncertainty. Until recently, it was assumed that the descending plate melted between the mantle and the bottom of the crust. While some of the melted material rose through the crust to form volcanoes, the remainder was unaccounted for.
It turns out that the crust subducted into the mantle retains unexpected integrity. The old plates, called slabs, can be mapped beneath the mantle, and this mapping project forms the Atlas of the Underworld. The Atlas covers the last 300 million years, a small part of the Earth’s 4.6 billion year age, but significantly more than we had before. Because the sinking slabs are colder and thus denser than the surrounding mantle rock, sound and shock waves from earthquakes travel through them at different speeds, allowing the shape and location of the slabs to be seen. The subducted slabs do not melt but in fact are dangling like curtains in the Earth’s molten mantle. That dangling means they influence the way heat moves through the mantle, and likely also constrain where convection happens in the mantle and thus where plate boundaries can or will form.
The slab locations before subduction can also be modeled. This allows geologists to see where they used to be on the surface of the planet, and see what the configuration of plates and continents used to look like.
The images in the Atlas of the Underworld can be hard to read, but the colors are pretty. Take this example, and we’ll try to parse it together.
The Hatteras slab (because it is under Cape Hatteras) anomaly is shown map view in the top left image (A).
The anomaly is the way they found it was there to begin with, from the reflections of sound and shock waves from earthquakes. The top of the slab is 850 km beneath the surface of the earth, and it extends 1400 km into the mantle. You can see this in the cross-section on the top right (E) – the blue is the cooler slab descending (it is headed east and down). The cross section is made along the red line on the map in fig. D. They feel the top of the slab was at the surface 59-50 million years ago and the base of the slab was at the surface 200-155 million years ago. So it started subducting in the middle of the Triassic (early dinosaurs) and stopped subducting close to the boundary between the Cretaceous and the Paleocene (end of the dinosaurs, early mammals). Fig C shows where the slab was being subducted during that time, under the California coast.