VIDEO
Crust
TheEarth’s crust is the hard, outermost covering of the Earth. This is the layer exposed to weathering like wind, rain, freezing snow, hurricanes, tornadoes, earthquakes, meteor impacts, volcano eruptions, and everything in between.
It has all the wrinkles, scars, colorations, and shapes that make it interesting. Just as people are different, with their own ideas and histories depending on their experiences, so the Earth has different personalities. Lush and green in the tropics to dry and inhospitable in the deep Sahara to fields of frozen ice pack in the Arctic, the Earth’s crust has many faces.
Continental Crust
The landmass of the crust is thin compared to the rest of the Earth’s layers. It makes up only about 1% of the Earth’s total mass. The continental crust can be as much as 70km thick. The land crust with mountain ranges and high peaks is thicker in places than the crust found under the oceans and seas, but the ocean’s crust, about 7km thick, is denser. The continents are the chunks of land that are above the level of ocean basins, the deepest levels of land within the crust. Continents are broken up into six major landmasses: Africa, Antarctica, Australia, Eurasia, North America, and South America.
This hard continental crust forms about 29% of the Earth’s surface and 3% of the Earth’s total volume. Besides dry land, continents include submerged continental shelves that extend into the ocean, like the crust framing the edge of a pie. The continental shelf provides a base for the deposit of sand, mud, clay, shells, and minerals washed down from the landmass. A continental shelf is the thinner, extended edges of a continental landmass that are found below sea level. The continental shelf can extend beyond the shoreline from 10 to 220 miles (16–320 km) depending on location.
The water above a continental shelf is fairly shallow, between 200 and 600 feet deep (60–180 m), compared to the greater depths at the slope and below. There is a drop off, called the continental slope, that slips away suddenly to the ocean floor. Here, the water reaches depths of up to 3 miles (5 km) to reach the average level of the seafloor to the different depths of the ocean floor. A ‘‘land’’ or ‘‘dry’’ continent has more variety than its undersea brother, the oceanic crust, because of weathering and environmental conditions.
The continental crust is thicker, especially under mountains, but less dense than the ‘‘wet crust’’ found under the oceans. Commonly, the continental crust is around 30km thick, but can be up to 50–80km thick from the top of a mountain. The continental crust is made up of three main types of rock. These are: sedimentary, igneous, and metamorphic rock.
Oceanic Crust
The land below the levels of the seas is known as the oceanic crust. This ‘‘wet’’ crust is much thicker than the continental crust. The average elevation of the continents above sea level is 840 m. The average depth of the oceans is about 3800m or 4 ½ times greater. The oceanic crust is roughly 7–10km thick.
Though not changed by wind and rain as is the continental crust, the oceanic crust is far from dull. It experiences the effect of the intense heat and pressures of the mantle more than the continental crust, because the oceanic crust covers more area. Even slow processes like sediment collection can trigger important geological events.
This happens when the build up of heavy sediments onto a continental shelf by ocean currents causes pieces to crack off and slide toward the ocean floor like an avalanche. When this takes place, the speed of the shift can be between 50 and 80 km/hr. The sudden movement through the water causes intense turbidity currents that can slice deep canyons along the ocean floor.
