We like to think of our planet as a solid ball of rock. Perhaps it’s comforting to think that no matter what happens to our left, right, or above our heads, at least what’s beneath our feet is solid. Alas, it is once again not so simple. Whilst slow, the ground beneath our feet is moving and morphing, building and breaking up. This week we will be exploring tectonic plates, how they change, and some of the rather dramatic impacts they have, including what may happen if you stick a volcano under a glacier.
The structure of the earth
The earth is made up of several layers. In the centre is the aptly named core which is thought to be around 5000oC. Well, we think. It is almost 3000km below the surface, so a little difficult to stick a thermometer into. It is made up of the inner core, then the outer core. Outside of this is the magma filled mantle, and finally we have the earth’s surface, called the crust. The mantle, being lava-like, is liquid, whilst the crust is solid, so it sort of slides around on top of the mantle. The crust however isn’t just one big solid circle, but it is thought to be made up of a number of ‘plates’, sections of crust that meet one another at their edges.
These plates vary in size. The entire continent of Africa and the seas many miles to the west sit on one plate- the African Plate. The largest one is the Pacific plate, which, you guessed it, is in the Pacific and is 103 million km2 in size. The smallest is the Juan de Fuca plate off the west coast of the US, at a mere 250,000 km2.
Movement and disasters
These plates are solid, and to us it usually feels like they are still, but as mentioned earlier they are actually floating on top of the molten mantle. The boiling, bubbling and swilling of this liquid rock does actually cause these massive plates to move. The Pacific plate for example is moving in a roughly north westerly direction at around 7cm per year. This might not sound like much, but if the Pacific plate is trying to force itself 7cm further into another solid plate, issues can arise. After all, not all plates move in the same direction so they quickly start to rub up against one another.
A few things can happen in this situation. If the plates are sliding past each other horizontally, but not pushing too much into one another, you get what is called transform margins. An example of this is the San Andreas Fault in California. If one plate is ploughing straight into another you get a subduction zone or convergent margin, where one of the plates is forced to slide underneath the other and into the earth’s mantle. Finally, in areas where two plates are being pulled away from another, called a divergent margin, a gap forms. An example is the East African rift.
When land masses larger than entire continents are sliding around, you would think we would notice. Well, we might not feel the 7cm journey northwest every year, but we definitely can feel and see the effects of tectonic movement. When plates are sliding past one another, or being subducted, occasionally bits of the plates snag on one another. This can start to build up more and more pressure, and when this tension is finally released, you get an earthquake. This is one reason why areas near or on plate boundaries, like Japan, which is near the meeting place of four plates, tend to have a lot of earthquakes. As these earthquakes may involve a sudden jerk and change in the sea floor, where these plate boundaries often are, they can cause the water above them to swill around. At these scales, the movement of water can be huge, sometimes triggering tsunamis. The effect of the devastation varies depending on the type of tectonic snag and the amount pressure built up. It can destroy entire cities or simply feel like a mild tremor.
Plate movements are not always a disaster. At convergent margins, where two plates come together, sometimes the rocks on each side bunch up from the impact of crashing in to one another. This forces them to be pushed up into the air, creating mountains. If you notice from the picture above, India is on its own plate, which is moving northwards into the Eurasian plate. It is at the boundary of these two plates that you find the Himalayas. This bunching up is still happening, though in slow motion to our eyes, so the mountain range is actually getting slightly taller each year.
There is another way we can see the effects of tectonic plates moving, and that’s volcanoes. When one plate is subducted under another, it is pushed down into the mantle where the rock melts. If there is a lot of water, e.g. if the subduction is happening underwater, this water that has been pulled down evaporates in the mantle and starts to cause the rock above it in the crust to melt. With the heat and pressure there, this continues to make the rock above it melt until it reaches the plate surface and explodes out. It becomes a volcano, and what we call magma when it is underground is now called lava.
Cracks in the crust, either from plates being pull apart or shoved together, can also allow some of the magma from the mantle to seep through, creating volcanoes. This again is why volcanoes tend to be more common along plate boundaries, though they tend to be most common in the divergent margins where plates are being pulled apart.
Now when you think of volcanoes, tsunamis and earthquakes, it is easy to think of places like Hawai’i, Japan and South East Asia. However, the icy regions of the planet have their volcanoes too. West Antarctica is thought to have over 100 volcanoes, but as they are hidden beneath thick ice it has been difficult for scientists to study them. There isn’t thought to have been a proper eruption in the area for the last 2000 years, but researchers have noticed that one of the fastest melting glaciers there, the Pine Island Glacier, is sitting right on top of one of these volcanoes. This is the glacier which in 2017 lost a chunk of ice the size of four Manhattans. They think the volcano might still be releasing heat, even though it’s not fully erupting, and so it and many others in the region may be speeding up the melting of some of the glaciers from beneath.
The same story is going on in the Arctic too. The Gakkel Ridge, a divergent crack in the crust, is a volcanically active ridge stretching from Greenland to Siberia under the Arctic Ocean. Recent evidence shows that these volcanoes have been erupting, pumping heat into the Arctic Ocean above them. Their effect, like those of the volcanoes in Antarctica, are a little controversial- they probably do cause some melting to the sea ice above, but some say they cause lots and others say they cause quite little. Historically it is known that erupting Arctic volcanoes have caused a lot of ice to melt, but this relationship today still needs a little more research.
Whilst a glance out the window might tell you the ground is solid, the earth is made up of giant tectonic plate islands, floating on a sea of magma thousands of kilometres below the surface. Where these plates meet there can be tremors, tsunamis and volcanic eruptions, both on land and under water, sometimes even under ice. What precise effect this all is having on our ice sheets and glaciers is a question that scientists are working on this very minute. This field, studying glaciovolcanism, is very new and has only really started receiving attention in the 2000s. Even with heat and tremors aside, the gases spewing out of volcanoes has an impact on the environment creating more greenhouse gases, and on human life directly too, like when entire countries closed their airspace in 2010 after the eruption of the Icelandic Eyjafjallajökull. What goes on below the surface of our planet most certainly affects what’s going on here on top.
For more info:
Sub-glacial volcanoes in the Antarctic: http://www.antarcticglaciers.org/glacial-geology/antarctic-ice-sheet/subglacial-volcanoes/
A great video on plate tectonics and how they have influenced our continents:
How to pronounce Eyjafjallajökull: