A Journey to the Unrest Beneath… The Earth’s Mantle

So much for slowing down as you age. Earth’s plates are moving faster now than at any point in the last 2 billion years, according to the latest plate movement study.

The processes that occur right beneath our feet can not only destroy and create dramatic features on our planet’s surface but can also have major impacts on humans, animals, and their habitats, by driving the movement of whole continents, and causing earthquakes and volcanic eruptions.

The interior of the Earth is divided into layers. Between the crust that we live on and the planet’s core (the layer responsible for generating the Earth’s magnetic field), lies the mantle. It is the processes occurring in this rocky shell, 2,886 kilometres thick, which are responsible for the incredible effects that can be seen at the Earth’s surface. We all know that the Earth’s continents are moving, but new, controversial data shows that they might be speeding up!

Planet Earth: the biggest, moving jigsaw known today!

The lithosphere is Earth’s strong, rigid outer shell of rock, and is broken into about a dozen plates that fit together like jigsaw pieces. These plates can slide past, collide with, or separate from each other over time. Their motion is driven by the flow of rock in the Earth’s mantle, just like biscuit pieces floating on the surface of golden syrup. Heat from the Earth’s core warms the bottom of the mantle, so that hotter regions rise whilst cooler parts of the mantle sink down, in a process known as convection.

Heat from the core forces mantle rock to rise whilst tectonic plates and cool mantle fall forming a convecting system which can destroy, create and move huge masses of land.

Heat from the core forces mantle rock to rise whilst tectonic plates and cool mantle fall forming a convecting system which can destroy, create and move huge masses of land.
Image used under a Creative Commons license from Wikimedia Commons

Over time, tectonic plates have moved great distances, taking our continents with them. The concept of Pangea, a supercontinent which broke up and moved large distances into the continents as we know them today, was first proposed by Alfred Wegener in 1915. However the basic ideas of plate tectonics were only put together into a single theory about 50 years ago, tying together a whole range of unknown geological processes, from mountain building to destructive natural hazards).

Pangea, the supercontinent proposed by Alfed Wegner, 1915.

Pangea, the supercontinent proposed by Alfed Wegner, 1915. A single theory was created to tied together all geological processes.
Image used under a Creative Commons license from Wikimedia Commons

Finally, it became clear that processes in the Earth’s deep interior could affect the actual surface of our planet, as seen in the mountains of the Andes, the powerful earthquakes along the San Andreas Fault and the violent eruptions of Mount Vesuvius.

So how fast are our continents moving? Faster than previously thought, according to Kent Condie, a geochemist at the New Mexico Institute of Mining and Technology. Condie and his colleagues looked at how often new mountains form when tectonic plates collide with one another1. They then combined these measurements with magnetic data from volcanic rocks, to work out where the rocks formed and how quickly the continents had moved. Both techniques showed that plate motion has accelerated; Earth’s tectonic plates have doubled their speed over the last 2 billion years.

This result is controversial, since previous work seemed to show the opposite effect, also predicted by theory. As the planet ages and its heat energy depletes, plate motion was expected to slow, not speed it up. “It was a surprise,” says Condie.

But Condie’s findings could be explained by another shocking discovery, that more water is present in the Earth’s mantle than in all of the oceans combined!2 When a plate crashes into another and is forced down into the mantle, oceanic water gets sucked down too. Although a lot of this water is ejected in the form of volcanic gas, over billions of years, the store of water in the mantle has grown vast. This water makes the mantle runnier and speeds up the flow of rock, and therefore the speed of the land we stand on.

Not only does the mantle control the large movements of massive continents, it is also responsible for swallowing (subducting) tectonic plates, and ejecting material to form new islands like Hawaii.

The Peruvian flat slab

The Peruvian flat slab: a region generating large earthquakes and embedded between apocalyptic volcanism either side.
Image from Eakin and Long (2013)3

The circle of life… upwellings and downwellings in the mantle!

Tectonic plates are consumed in head-on collisions at plate boundaries, normally creating spectacular volcanoes, and frequent earthquakes. Examples of such explosive volcanic activity can be seen in Chile, also the site of the largest earthquake in the world, which measured 9.5 on the Richter scale in 1960.

This, and other earthquakes in the region are caused by the subduction of the Nazca plate under the South America plate. Further North, the Nazca plate sinks to a depth of 100km, and then runs horizontally instead of subducting further into the mantle. These so called “flat slabs” are a mystery even today, but the most popular hypothesis is that they form from the combined effect of many factors, including the buoyancy of the subducting plate.

The upwelling part of mantle convection are plumes.

The upwelling part of mantle convection are plumes. This rising molten rock can form island such as Iceland and Hawaii.
Image used under a Creative Commons license from Wikimedia Commons

Physics tells us that matter cannot be created or destroyed; if material is subducting down into the mantle, in the form of a tectonic plate, there must be an upwelling of material elsewhere.

This upwelling material is the reason we can visit enchanting islands such as Iceland and Hawaii. These islands are formed from “mantle plumes”, which were first proposed in 1971, spawning the field of plume research across the world.

Peaking beneath the crust

When we want to know more about the rocks in a mountain range, we can go there and look. When we want to know about processes going on in the deep Earth, we can drill holes, up to about 12km deep, but we can’t drill as deep as the mantle!

Bring in seismic tomography – this is like taking a CT scan of the Earth – it uses seismic waves from earthquakes, which travel at different speeds depending on the material they are travelling in, to image the inside of the Earth.

Seismic tomography can reveal the rough locations of plumes. For instance, recent studies have been able to detect multiple plumes below East Africa, when previously it thought there was just a single superplume causing the continent to start splitting!

Ultimately, the fate of plate motion, volcanic activity and earthquake frequency lies in the hands of our Earth’s mantle, which lies beneath.

References

1 Condie, Pisarevsky, Korenaga and Gardoll (2015), Is the rate of supercontinent assembly changing with time?
2 Lovett (2014), Tiny diamond impurity reveals water riches of deep Earth. Nature.
3 Eakin, Long (2013) Complex anisotropy beneath the Peruvian flat slab from frequency-dependent, multiple-phase shear wave splitting analysis. Journal of Geophysical Research.

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