Tag Archives: geophysics facts

Continental Crust Facts

  • The Earth’s crust is its hard outer shell.
  • The crust is a thin layer of rock that floats on the mantle. It is made mainly of silicate minerals (minerals made of silicon and oxygen) such as quartz.
  • There are two kinds of crust: oceanic and continental.
  • Oceanic crust is the crust beneath the oceans. It is much thinner – just 7 km thick on average. It is also young, with none being over 200 million years old.
  • Continental crust is the crust beneath the continents. It is up to 80 km thick and mostly old.
  • Continental crust is mostly crystalline ‘basement’ rock up to 3800 million years old. Some geologists think at least half of this rock is over 2500 million years old.
  • It is estimated that approximately one cubic kilometer of new continental crust is probably being created each year.
  • The ‘basement’ rock has two main layers: an upper half of silica-rich rocks such as granite, schist and gneiss, and a lower half of volcanic rocks such as basalt which have less silica. Ocean crust is mostly basalt.
  • Continental crust is created in the volcanic arcs above subduction zones (see converging plates). Molten rock from the subducted plate oozes to the surface over a period of a few hundred thousand years.
  • The boundary between the crust and the mantle beneath it is called the Mohorovicic discontinuity.

Converging Plate Facts

  • In many places around the world, the tectonic plates that make up the Earth’s crust, or outer layer, are slowly crunching together with enormous force.
  • The Atlantic is getting wider, pushing the Americas further west. Yet the Earth is not getting any bigger because as the American plates crash into the Pacific plates, the thinner, denser ocean plates are driven down into the Earth’s hot mantle and are destroyed.
  • The process of driving an ocean plate down into the Earth’s interior is called subduction.
  • Subduction creates deep ocean trenches typically 6-7 km deep at the point of collision. One of these, the Mariana Trench, could drown Mt Everest with 2 km to spare on top.
  • As an ocean plate bends down into the Earth’s mantle, it cracks. The movement of these cracks sets off earthquakes originating up to 700 km down. These earthquake zones are called Benioff–Wadati zones after Hugo Benioff, who discovered them in the 1950s.
  • As an ocean plate slides down, it melts and makes blobs of magma. This magma floats up towards the surface, punching its way through to create a line of volcanoes along the edge of the continental plate.
  • Subduction creates a ring of volcanoes around the Pacific Ocean called the ‘Ring of Fire’:
  • If volcanoes in subduction zones emerge in the sea, they form a curving line of volcanic islands called an island arc. Beyond this arc is the back-arc basin, an area of shallow sea that slowly fills up with sediments.
  • As a subducting plate sinks, the continental plate scrapes sediments off the ocean plate and piles them in a great wedge. Between this wedge and the island arc there may be a fore-arc basin, which is a shallow sea that slowly fills with sediment.
  • Where two continental plates collide, the plate splits into two layers: a lower layer of dense mantle rock and an upper layer of lighter crustal rock, which is too buoyant to be subducted. As the mantle rock goes down, the crustal rock peels off and crumples against the other to form fold mountains (see mountain ranges).
  • This is a cross-section through the top 1000 km or so of the Earth’s surface. It shows a subduction zone, where an ocean plate is bent down beneath a continental plate.

Fold Facts

  • Rocks usually form in flat layers called strata. Tectonic plates can collide (see converging plates) with such force that they crumple up these strata.
  • Sometimes the folds are just tiny wrinkles a few centimeters long. Sometimes they are gigantic, with hundreds of kilometers between crests (the highest points on a fold).
  • The shape of a fold depends on the force that is squeezing it and on the resistance of the rock.
  • The slope of a fold is called the dip. The direction of the dip is the direction in which it is sloping.
  • The strike of the fold is at right angles to the dip. It is the horizontal alignment of the fold.
  • Some folds turn right over on themselves to form upturned folds called nappes.
  • As nappes fold on top of other nappes, the crumpled strata may pile up into mountains.
  • A downfold is called a syncline; an upfolded arch of strata is called an anticline.
  • The axial plane of a fold divides the fold into halves.