The Earth's Crust.

One of the most conspicuous features of the Earth's surface is it's division into continents and ocean basins. Both the oceanic and the continental crust lie on top of the mantle, but the two types of crust are significantly different.

The lighter, thicker continental crust floats higher in the mantle than the denser , thinner oceanic crust. About two-thirds of the Earth's surface is oceanic crust, but the continental crust has a far wider variety of rocks.

Oceanic Crust.

The oceanic crust is much younger than the continental crust - even the oldest parts of the ocean floor are only 200 million years old. This is because oceanic crust is continually being recycled. It forms from mantle material rising within rifts and oceanic ridges, and spreading to either side of the ridge. This material returns to the mantle at the same rate in areas called subduction zones.

The oceanic crust is composed of several layers, the topmost of which is a layer of sediments, primarily very fine muds overlying a layer of basalt. The lower layers are primarily gabbro, with increasing olivine content as the depth increases.

The Continental Crust.

The continental crust covers about a third of the Earth's surface, forming the main landmasses and the beds of the shallow seas that surround them. It is where most of the rocks and minerals we will be discussing in later blogs, are found.

The continental crust varies from 25 to 70 km (16 to 45 miles) in thickness, with the greatest thickness under the mountain ranges. At the heart of each continent is a stable mass of crystalline rocks from the Precambrian period (4,560 million to 543 million years ago) covering thousands of square kilometres. This is known as the continental shield. Australia's shield is located in the central area of Western Australia. Other rocks of the continental crust have more recently been subjected to the processes of the rock cycle such as erosion, metamorphism and sedimentation.

Floating Rock.

The Earth's crust behaves as if it were floating on the underlying denser, flexible rock of the mantle. This concept is known as "isostasy". The crust can be visualized as resembling an iceberg floating on water, and like an iceberg it extends downwards into the medium on which it floats. Everest is part of the Himalayas (see previous blogs), a relatively young mountain range created by a collision of two continental plates. Mountains such as Everest have a ""root" extending many kilometres into the mantle.

Because the Earth's crust is not all of the same density or thickness, different parts of it protrude into the mantle to different depths. Thus the crust extends deeper down into the mantle under tall mountain ranges than it does in low-lying areas, because it requires a deeper "root" to buoy up the mountain ranges' additional weight. About two-thirds of the thickness of the continental crust forms the root that supports the rest.

(Source: DK Publishing, Rocks and Minerals. Photo courtesy of Bing Images).

Here at The Emporium, we are finding it increasingly fascinating to learn about the history of the physical Earth, and where our beloved Marble Eggs come from.

Next week, we will be looking at plate tectonics, or the building and movements of the Earth's crust. Stay tuned!