Earth's Interior Structure and Seismic Wave Behavior

Seismic Wave Behavior

The behavior of seismic waves includes the following:

• The speed of seismic waves increases with the stiffness and density of the rocks they cross.
• The speed of waves in a layer formed by the same materials increases with depth because the pressure causes the materials to become more dense.
• The P-waves propagate faster than S-waves and can cross liquid materials, unlike S-waves.
• All types of seismic waves pass through solid materials.
• When waves pass from one material to another, they are reflected or refracted. These alternations through the inner Earth are called seismic discontinuities, which reveal the internal structure of the planet into layers of different composition and density. The major discontinuities are the Mohorovičić, Gutenberg, and Lehman boundaries.

Models of the Earth's Interior

Dynamic Model

This model uses the mechanical behavior, density, and physico-chemical properties of materials inside the Earth as criteria. It suggests the existence of the lithosphere, mesosphere, and endosphere (formed by the outer core and the inner core).

Geochemical Model

This model uses the chemical composition of Earth's interior materials as a criterion and proposes three main layers: the crust, mantle, and core.

Current Model: The Crust

The Crust

The crust is the Earth's solid, rigid surface layer, separated from the mantle by the Mohorovičić discontinuity. There are two types of crust beneath the surface, differing in thickness and composition: the oceanic and continental.

Oceanic Crust

Horizontal Structure

The oceanic crust forms the ocean floors, except on the continental shelves. Its surface features three main areas:

• The abyssal plains, interspersed with seamounts, volcanic islands, and ocean trenches.
• Ridges, which are low mountains thousands of kilometers long.
• Continental margins, where sediments accumulate from continents.

Vertical Structure

It is between 3 and 15 km thick and consists of two zones:

• The superficial zone, which may be covered by a thin layer of sediment formed by basaltic lavas.
• The profound zone, bordered by the mantle, consisting of gabbros.

Age

This crust is considered young, as it is never older than 180 million years because the lithosphere's dynamics continuously create and destroy it.