THE NATURAL ENVIRONMENT
Geography 101 Online
In the previous chapter, we discussed our planet's layered structure and how heat flowing out from Earth's interior drives lithospheric plate motion and mantle plumes at hot spots. We continue that theme by more carefully examining endogenic processes and the landforms they create.
A very fundamental feature of Earth's surface is the distinction between the ocean bottoms and the continents. Simply put, ocean bottoms are lower in elevation than continental surfaces by an average of 5 kilometers (3 miles). This distinction is illustrated in a hypsometric curve, which shows the accumulated area of the entire Earth's solid surface above each elevation and in relation to sea level. This sounds a little confusing, but if you look at the graph below, it should make more sense.
Oceans bottoms are lower because oceanic crust is thinner and denser than continental crust. Notice the sharp drop in the curve near the average elevation of Earth's surface (which is below sea level). This represents a clear separation between the two crustal types. Ocean crust, which covers most of the planet's surface, lies mostly between about 3 and 5 kilometers (2 and 3 miles) depth, while continental crust, including the offshore continental shelves, averages less than 1 kilometer (0.6 miles) above sea level.
Unlike oceanic crust, most continental crust has been around for billions of years. It is so light that it "floats" on the denser rock beneath and resists being pushed down into the mantle for remelting. It has been crushed, folded, broken, moved, tumbled, and otherwise abused, but it is still around. Billions of years of weathering and erosion have bared areas of ancient original rock, exposing continental shields. Continental shields make up the majority of the continent of Africa, and substantial portions of South America, Australia, Asia, and North America. Earth's oldest rocks have been found in these venerable formations. The oldest true rocks, intact aggregates of minerals, date to about 4 billion years old. The oldest individual mineral, a fragment of zircon found in Western Australia, dated to 4.374 billion years old.
While most continental crust has been around a long, long time, new continental crust does occasionally form. The westward movement of the North American plate, for example, has scraped up a band of ancient islands, coral reefs, chunks of ocean floor, and other debris along the western edge of the plate. These additions to the old original continent are called terranes. The different colors in the diagram show different accretions of material over time.
While terranes stand out on geologic maps, a casual observer on the ground could not tell these recently accreted mountains from any other mountains. The Wrangell mountains, shown in the photograph, for example, are composed of the plastered together remnants of an ancient island arc that used to reside near the equator.
The life of oceanic crust is less exciting. It forms at areas of sea floor spreading, moves with the plates, and subducts back into the mantle to be remelted. As a result, it has very little geologic history compared to continental crust. The oldest oceanic crust known, found in the Western Pacific, dates back only 280 million years.
Kapiolani Community College Geography