Earth's Crust Help (page 2)
Introduction to the Earth's Crust
The Earth’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.
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 70 km 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 7 km 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. Figure 1-7 shows the steady thinning of the continental landmass to the different depths of the ocean floor.
Fig. 1-7. A continental shelf extends the landmass before sloping to 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 30 km thick, but can be up to 50–80 km 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. We will learn more about these rock types in later chapters.
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 3800 m or times greater. The oceanic crust is roughly 7–10 km 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. We will learn more about ocean currents in Chapter 13.
Ridges And Trenches
In the middle of the Atlantic Ocean is a north to south mountain range called the Mid-Atlantic Ridge. This ridge is made of many layers of cooled, pushed-up rock from inner crustal depths that have been broken and lifted to form a 16,000km seam that stretches from Greenland to Antarctica.
Similarly, the East Pacific Ridge contains peaks or seamounts of flattened, dead volcanoes called guyouts . These ancient volcanoes were 3660 m above the water level originally, but were eroded down over time by waves crashing against them. Now they are found 1500 m below the waves of the Pacific.
The oceans also contain deep, narrow cuts known as trenches that stretch for thousands of miles. Trenches are formed when layers of the crust slam into each other and instead of pushing up like the ridges, they fold at a seam and slide further downward into the layer below. The largest of these trenches, the Mariana, is found in the eastern Pacific.
The Mariana Trench is the deepest trench of this kind on Earth. Located in a north/south line east of the Philippines, it descends over 11,000 m downward and slowly gets deeper. Compared to the height of Mount Everest, the tallest peak on the Earth at 8850 m, the Mariana Trench is gigantic. All of Mount Everest could fit into the Trench with nearly 2200 m of ocean above it to the waves on the surface.
It is times deeper than the Grand Canyon which is an average of 5000 m deep. We will learn more of this folding action in Chapter 4, when we study plate movement.
It is no wonder the Mariana Trench has been the subject of several science fiction films. It excites the imagination to think about what amazing mysteries of nature might still be discovered at such tremendous depths.
For more information, review our Planet Earth Practice Test