20. Planet Earth in Profile: The Layered Interior

Photo 34: Sediment layers at Drumheller. 2011-05-12

The Earth’s Interior

Drumheller is a unique location, a valley revealing many layers of soil and rock (photo 34). Yet these layers represent a small fraction of what the Earth is made up of. The radius of our planet is 6370 km, the continental crust is on average 45 km thick, the oceanic crust about 6-7 km thick. The deepest human drilling in Russia went to a mere 12.262 km. We know about the Earth's interior through the Seismic waves of earthquakes, which reveal what the inside of the Earth is like because of the way waves hit certain rock layers.

Earth has a Solid Inner Core with a radius of 1220 km, consisting of iron and nickel in a solid state due to the enormous pressure. Going outward, there is the 2250 km thick liquid Outer Core, consisting of iron and nickel as well, but in liquid state due to less pressure. Beyond that is the Solid Lower Mantle of about 2230 km thick, consisting of oxides of iron, magnesium, and silicon. The Upper Mantle is a thin mantle of only 670 km and borders the Earth’s crust. This Upper Mantle is mostly liquid syrup like rock, which feeds the magma chambers of volcanoes and on which the continental plates move. The distinct boundary between the upper mantle and the continental and oceanic crust is called the Mohorovičić discontinuity.

Photo 35: Atlantic Ocean, France. 2005-07-03

The Structure of Oceans

Humans know quite a bit about the continental Earth, but sometimes the Oceanic realms can still hold great surprises. The Oceanic floor has many landforms similar to that of the continents. Some of the deepest oceanic parts are very close to the continents and not in the middle of the oceans: not at all what one would expect when looking out over the flat surface of the ocean (photo 35). Expanding a bit on the post in Unit 2, there are 3 main Oceanic topographical features:

·         The Mid-Oceanic Ridge are systems of mountain ranges, varying from 1000 km to 4000 km wide, in the Atlantic and Pacific Oceans. These ranges are about 50,000 to 60,000 km long, mostly a couple of kilometers under water, but sometimes form islands like Iceland and St. Helena. These mountains have rift valleys, volcanoes, and some seismic activity. The Mid-Oceanic Ridges are not mountains due to plate collision like those on land, they are the result of seafloor spreading. New magma comes up from the Upper Mantle and pushes the old aside.

·         Deep Ocean Basins are the flat bottoms of oceans, and lay between 4.5 and 6 km below sea level. Active basins expand due to plate movement; inactive basins collect sediment, like the Gulf of Mexico.

·         Deep Marginal Trenches are long and narrow trenches up to about 11 km below sea level. They are located at the edges of deep oceanic basins and near continental shelfs.