Continental Drift Theory and Earth's Internal Structure

The Theory of Continental Drift

Continental Drift: In the past, all dry land was united into a large continent called Pangaea. Eventually, Pangaea split, and the resulting fragments were displaced, leading to the current configuration of continents. It was initially thought that advances in the continents would form serious wrinkle ridges.

Hypothesis Limitations

Two possible causes were proposed for this movement, though both were later deemed impossible:

• The centrifugal force due to the Earth's rotation, which would move the continent towards the Equator.
• The tidal attraction produced by the sun and moon.

Neither explanation was sufficient, and the proponent died before his theory was accepted. His ideas were regarded as an impossible hypothesis. Currently, it is still studied today, noting two classic mistakes: considering that the continents moved over the oceanic floor and the incorrect explanation given for the cause of these movements. This theory eventually evolved into modern geology: plate tectonics.

Characteristics of the Ocean Floor

Oceans cover 71% of the planet's surface. Key features include:

• The existence of the oceanic ridge.
• A shortage of sediment and its unusual distribution.
• The relative youth of the ocean floor.

Properties of the Earth's Interior

The interior of the Earth is denser: The Earth's average density is 5.5 g/cm³, while the average density of continental rocks is only 2.7 g/cm³.

The interior of the Earth is hot: As seen in mines, the temperature increases with depth.

The Earth acts like a magnet: There is a magnetic field to which a compass is oriented, suggesting a metallic core.

The Earth is structured in layers: The analysis of seismic wave propagation has allowed scientists to design a model of the planet's layered structure.

Chemical Composition of the Earth

The propagation speed of seismic waves shows three abrupt changes or discontinuities: Mohorovicic, Gutenberg, and Lehmann.

The Earth's Layers

• Crust: A thin surface layer. The continental crust is roughly 35 km thick and dominated by granite and gneiss. In the oceans, it is about 8 km thick and composed of basalt.
• Mantle: The thickest layer, extending up to 2,900 km in depth. It is separated from the crust by the Mohorovicic discontinuity and is composed of peridotite.
• Core: The central area, separated from the mantle by the Gutenberg discontinuity. It is composed primarily of iron and nickel.

Geodynamic Units and Layers

• Lithosphere: The outermost rigid layer. It includes the crust and the uppermost part of the mantle. The continental lithosphere is between 100 and 200 km thick, while the oceanic lithosphere is between 50 and 100 km thick.
• Sublithospheric Mantle: A plastic layer beneath the lithosphere bordering the core. The rocks in this state are ductile. It is divided into the asthenosphere (up to 670 km depth) and the mesosphere (reaching up to 2,900 km).
• Outer Core: Located below the Lehmann discontinuity at 5,150 km, this layer is in a liquid state.
• Inner Core: Includes the remainder of the core.