The Lithosphere. The Layers. Isostasy

The Layers. The large continental landmasses are formed primarily, from granite-type rock that has a high content of aluminum, sodium, potassium, and silica; quartz and feldspar are the principal mineral components.

The rock of the continental crust is highly variable in composition and distribution. Continental crust has an average density of 2.7 g/cm¹ and is approximately 40 km (25 mi) thick on average. The crustal rock layer lying under the ocean is basalt-type rock, which is lower in silica and higher in iron, magnesium, and calcium.. This more homogeneous crustal rock layer is only about 7 km (4 mi) thick and has an average density of 3.0 g/cm³.

The boundary between the crust and the mantle is the Mohorovicic discontinuity, named for its discoverer, and usually called the Moho. The Moho is a chemical boundary at which there is a sudden change in the speed of seismic waves. At one time some thought that the Moho was the structure at which the Earth’s rigid crust moved relative to the mantle. Current research places this zone of movement within the mantle, from as much as 100 km (62 mi) below the ocean crust to as much as 150 km (93 mi) below the continental crust.

The mantle just below the crust is rigid, solidified, and fused to the crust but at the same time separated from it by the Moho. This rigid layer of crust and the upper mantle form the lithosphere. Because seismic waves pass through it at high speeds, we can infer that the lithosphere is both strong and rigid. The oceanic lithosphere thickens with the increasing age of the sea floor. It reaches a maximum thickness of about 100 km (62 mi) at an age of 80 million years.

The region of the mantle below the lithosphere, extending to a depth of about 350 km (217 mi), is the asthenosphere. Seismic waves travel more slowly through the asthenosphere, indicating that it may be as much as 1% melt. The asthenosphere behaves like a plastic, flowing slowly when stressed. The lithosphere is less dense than the asthenosphere; both continental and oceanic lithosphere float on the asthenosphere. As oceanic lithosphere ages, it cools and thickens, and its density increases.

It is this denser oceanic lithosphere that can sink as rigid slabs into the mantle (see above). The continental lithosphere is less dense than the asthenosphere and remains at the surface. The lithosphere and the asthenosphere are shown in figure 2.4 and are compared in table 2.2.

Fig. 2.4. The lithosphere is formed from the fusion of crust and upper mantle. It varies in thickness: thinner in ocean basins and thicker in continental regions. The lithosphere rides on the weak, partially molten asthenosphere. Notice that the Moho is relatively close to the Earth’s surface under the ocean’s basaltic crust but is depressed under the granitic continents

Isostasy. The distribution of elevated continents and depressed ocean' basins requires that a balance be kept between the internal pressures under the land blocks and those under the ocean basins. This is the principle of isostasy. The balance is possible because the greater thickness of low-density granitic crust in the continental regions is compensated for by the elevated higher-density mantle material under the thinner crust of the oceans (fig. 2.5).

Fig. 2.5. Isostasy. Columns of crustal material are unequal in height and density but generate the same pressure at the same depth within the mantle

The situation is often compared to the floating of an iceberg. The top of the iceberg is above the sea surface, supported by the buoyancy of the displaced water below the surface. The deeper the ice extends below the surface, the higher the iceberg reaches above the water. The less-dense continental land blocks float on the denser mantle in the same way, with most of the continental volume below sea level.

In other words, the asthenosphere offers buoyant support to a section of lithosphere sagging under the weight of a mountain range. Because the lithosphere is cooler, it is more rigid and stronger than the asthenosphere. If a thick section of lithosphere has a large area and is mechanically strong, it depresses the asthenosphere slightly and is able to support a mountain range such as the Himalayas or the Alps. In other places where the crust is fractured and mechanically weak, a mountainous region must penetrate deeper into the mantle in order to provide buoyancy. The Andes are a mountain range with roots deep in the mantle.

If material is removed from or added to the continents, isostatic adjustment will occur. For example, parts of North America and Scandinavia continue to rise as the continents readjust to the lost weight of the ice sheets that receded at the close of the last ice age ten thousand years ago. Newly formed volcanoes protruding above the sea surface as islands often subside or sink back under the sea as their weight depresses the oceanic crust.

The outer mantle gradually changes its shape in response to weight changes in the overlying, more rigid crust. In the middle 1800s the concept of isostasy was firmly rooted in geological studies. It was thought that landmasses separated and oceans changed boundaries in response to changes in isostasy as sections of the Earth’s crust moved up or down.