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• 1 2016-05-18T09:12:42+00:00 Crust 25 image_header 2021-01-26T16:32:02+00:00 The crust-mantle boundary of the Earth is called the Moho (short for Mohorovicic Discontinuity), which is the depth where the low-density mineral plagioclase feldspar (also most abundant mineral in the Earth’s crust) first appears.  Relative to the mantle, the crust is richer in silicon and aluminum. 
  
  The crusts of rocky bodies range in thickness.  For the Earth, the crust varies from 5 km to up to 70 km thick.  The thinnest parts of the Earth’s crust underlie the oceans and are called oceanic crust.  The thicker parts (averages ~35 km) make up the Earth’s continents, known as continental crust.
  Rocks that make up the crust include a crystalline basement of igneous and metamorphic rock and volcanic rock.  The average composition of Earth’s oceanic crust is basalt, which is a dark-toned, iron and magnesium-rich volcanic rock. If you ever visit Kilauea, the most active volcano on the oceanic island of Hawaii, you can see basalt actually erupting at the surface. Basalt is also the most common rock type making up the crusts of other planetary bodies. 
  
  The Earth’s continental crust is richer in aluminium and silicon and has an average composition closer to that of the igneous rock granite. You may know granite best as a quality stone for countertops and tombstones. Continental crust is also thicker and less dense than the basaltic oceanic crust, which explains why it stands higher above sea level.
  
  On the Earth and other geologically active planets like Mars, the outermost crust may also contain abundant layered, sedimentary rocks like sandstone and shale. On Mars, the Curiosity rover is currently exploring layered sedimentary rocks making up Mount Sharp shown in the image below.
  
  Bodies that have not been so geologically active, like the Moon have been subjected to bombardment by meteorites since their formation.  The outermost crust of these bodies (a few to 10s meters) is commonly composed of regolith, which is a layer of loose heterogeneous material covering solid rock.  Regolith includes dust, soil, and broken rock. The powdery texture of the lunar regolith is well conveyed by this famous image of a footprint taken by Buzz Aldrin in 1969 on the Moon's surface.
  
  In some cases, the broken up regolith is more durable and indurated as a solid rock, such as the meteorite below.  This meteorite is called a eucrite and has been sourced to the surface of the asteroid Vesta.