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.
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.
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.
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.
The oceanic crust is thinner & denser, and has less buoyancy. Its equilibrium point is well below sea level, so it is hardly ever dry land. An exception is Iceland, which is raised above the sea by exceptional heat flow.
