#isostasy

The British Sea-saw: How Scotland is slowly dunking England into the Sea This is a map of how the Earth's Surface is changing in the UK, illustrating a phenomenon that geologists have characterized worldwide. Right now, the UK is acting a bit like a giant seesaw as the rising of Scotland causes the South of England to sink beneath the waves.

The crust is the thinnest layer of the Earth.  It is less dense than the mantle, as its rocks contain minerals with lighter elements, such as aluminium, calcium & silicon.  The boundary between the crust & mantle is marked by the Mohorovicic seismic discontinuity.

The continental crust is 30km thick, and the oceanic crust is 10km thick (on average).  There is over twice as much oceanic crust as continental.

Continental Crust

The continental crust forms all the major landmasses, and their surrounding shallow seas.  Its thickness ranges from 25-70km: the thickest parts are under young mountain belts.

Its composition varies hugely, with sedimentary, metamorphic and igneous rocks.  This is because light continental crust is not “recycled” within the Earth, as the denser oceanic crust is.  Therefore, much of the rocks above sea level have been through many cycles of erosion, formation into sedimentary rocks, and metamorphosis.  Some continental rocks are 4 billion years old.

Tectonic forces have major effects on the continents - causing fragmentation, amalgamation and long-term movement, which results in the formation of new oceans & mountains, volcanism, and sometimes the creation of supercontinents.  Also, new rock layers are constantly being added to the surface & margins of the continents.

Oceanic Crust

The oldest parts of the ocean floor are only 200 million years old, and covered with a thin layer of sediment.  Its thickness ranges from 6-11km.

The oceanic crust is made up of dense basaltic lavas and related rocks.  Because it’s denser, it is less elevated than the lighter continental crust.

New oceanic floor is constantly formed from mantle material within spreading ridges (long rifts).  The mantle is continually rising, heating and expanding the rocks above it to form an elevated submarine mountain chain.  This causes the oceanic plates on each side of the rift to move apart.  Basaltic lava erupts from fissures & cones, and cools to form new oceanic floor, which has numerous volcanoes sprouting up through it.  These volcanoes can grow big enough to rise above sea level.  They can also form chains of volcanic islands, in places where the crust is moving over a semi-permanent mantle hotspot plume.

The flip-side of the spreading ridges are the subduction zones, where oceanic crust descends into the mantle, at the same rate that it is created at the ridges.

Isostasy

Crustal blocks are “floating” on the mantle - this concept is called isostasy.  Because the continental crust is lighter than the oceanic crust, it is buoyant (like an iceberg is buoyant in seawater).  Of course, the more rock is above sea level, the deeper the crust’s “roots” will go - hence why continental crust reaches 70km deep into the earth below young mountain belts - well into the mantle.

Antarctica Rising

Antarctica is divided into two sections by the Transantarctic mountain range, East and West Antarctica. Both sections are covered by ice sheets; the eastern ice sheet is much larger and more massive than the western ice sheet, while the western ice sheet is less stable and has begun responding to a warming climate.

Despite the mountain range dividing these two ice sheets, there is a connection between them in the Earth’s mantle. The mantle is solid, but it’s so hot that it is able to flow as a fluid. The mantle under east and west Antarctica is connected by this fluid, hundreds of kilometers beneath the surface.

When ice sheets form at the surface, the mantle responds. When large ice sheets formed on Canada and Scandinavia during the last ice age, they acted like giant weights. They pushed down on the mantle and the mantle flowed away in response. When the ice sheets melted, the mantle began flowing back in, and even now, 10,000 years after the ice sheets collapsed, GPS stations in Canada and Scandinavia can detect the surface gradually rebounding.

Today, West Antarctic Ice Sheet is losing mass; a consequence of a warming climate. As a consequence of this mass loss, material in the mantle has begun flowing in to Western Antarctica, creating signals that scientists from The Ohio State University detected using GPS instruments.

Mantle flow is very slow, but it can respond rapidly to changes. Western Antarctica has been losing mass for years, perhaps decades; long enough for the mantle deep beneath the continent to respond.

The bedrock is moving by about 10 millimeters per year. It seems like a small amount, but the mantle doesn’t flow rapidly even when the changes above are large; that’s why the land in Canada and Scandinavia is still moving today.

The details of the mantle flow may actually enable new science. At the 2013 meeting of the American Geophysical Union, the scientists presented results from around the continent, showing that the mantle was flowing most rapidly in to areas that also had the greatest degree of ice loss.

The mantle deep beneath Antarctica today is responding Sheet just as the mantle did when the ice caps of the last ice age melted. The mantle slowly is flowing back in, with mass flowing most towards the areas of greatest mass loss.

Beyond that pattern, the flow directions are revealing differences in the strength of the mantle beneath Antarctica. Some areas are, surprisingly, flowing horizontally, with mass moving around more rigid parts of the mantle. It is possible that, as we’re witnessing the beginnings of mantle flowing in response to melting ice (rather than thousands of years later), we are seeing heterogeneity in the mantle due to variations in either its temperature or composition that will be mixed and homogenized as the flow continues. Thus, seeing this process at the start may well give us new insights into the composition and properties of the mantle beneath Antarctica in a way that would not have happened had the ice sheet remained stable.

Image credit: NASA/Michael Studinger http://commons.wikimedia.org/wiki/File:Marie_Byrd_Land,_West_Antarctica_by_NASA.jpg

Source picture and information from Geología Online

🌍 What do the Himalayas, Greenland, and the Trans-Mexican Volcanic Belt have in common? ⛰️ Although they are separated by thousands of kilometers and have very different landscapes, they all reveal the same phenomenon: the Earth is never completely still. 🌎 Beneath our feet, the Earth's crust constantly sinks, rebounds, and readjusts to maintain equilibrium. ⚖️ This process is known as isostasy.

⛰️ The Himalayas continue to rise due to the collision between the Indian and Eurasian tectonic plates.

❄️ Greenland experiences isostatic rebound because the weight of the glacial ice gradually decreases.

🌋 And the Trans-Mexican Volcanic Belt reflects the internal dynamism of the crust associated with volcanism and tectonic movements.