Earth's Crust Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

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 3800 m or Planet Earth Oceanic Crust 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 Planet Earth Ridges And Trenches 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

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