Seismic Waves, Mantle Discontinuities and Meteorites

Seismic Waves: P Waves and S Waves

P waves (primary): P waves are the fastest seismic waves and therefore arrive first at seismic stations. They are longitudinal waves: ground particles vibrate in the same direction as wave propagation. As P waves pass through rock, the material is alternately compressed and dilated.

S waves (secondary): S waves travel more slowly than P waves and are transverse waves: particles vibrate perpendicular to the direction of propagation. S waves cannot travel through liquids, so they are unable to propagate through Earth’s liquid outer core.

Earth's Internal Discontinuities

Discontinuities are sudden changes in seismic wave propagation speed that mark boundaries between layers inside Earth.

Mohorovičić (Moho) Discontinuity

The Mohorovičić discontinuity, commonly called the Moho, is the boundary between the crust and the mantle. It lies at depths of approximately 5–10 km beneath the oceans and about 25–70 km beneath continental regions. The Moho is marked by a sharp increase in seismic wave speeds and is used to distinguish the thin surface layer (the crust) from the layer beneath it (the mantle).

Gutenberg Discontinuity

The Gutenberg discontinuity is the boundary between the mantle and the core, located at roughly 2,900 km depth. It is characterized by a significant decrease in P‑wave speed and by the disappearance of S waves (because the outer core is liquid). This discontinuity separates the solid mantle from Earth's core.

Lehmann Discontinuity

The Lehmann discontinuity differentiates the outer core from the inner core: the outer core is liquid, while the inner core is solid. Seismic observations of P waves that travel through the inner core, and their behavior at this boundary, reveal the Lehmann discontinuity.

Meteorites: Types and Origins

Definition and origin: Meteorites are small planetary bodies that cross Earth's orbit and fall onto its surface. The asteroid belt, located between Mars and Jupiter, is a major source of these objects. Meteorites are about 4,500 million years old (4.5 billion years), roughly the same age as Earth, and are composed of primordial material from the early solar system. They provide diverse information about the nature and formation of planetary bodies. Some meteorites are compositionally similar to terrestrial rocks.

Major meteorite types by composition:

• Chondrites: Represent about 86% of known meteorites. They consist of a mixture of minerals and chondrules, and their composition is similar in some respects to peridotitic mantle minerals.
• Achondrites: Represent about 9% of known meteorites and have compositions similar to terrestrial basalts.
• Siderites (iron meteorites): Mainly composed of iron and nickel and represent roughly 4% of known meteorites.
• Other types: The remaining meteorites include various rare types and transition forms; all provide key clues to solar system history.

Fossils and Fossilization Processes

Fossils: Fossils are the preserved remains, impressions, or traces of past life. Fossilization can occur in several ways. Sometimes small organisms are preserved with mineralogical changes that replace original tissues. More often, the shell of an organism (for example, a mollusk) dissolves and is subsequently filled by minerals carried by circulating groundwater, creating a cast or filling the void as a mineral replacement.

Fossil preservation can also result in external molds (the impression of the outside of the organism) or internal molds (where sediment covers the shell and then fills the interior). These processes record the shape and sometimes fine details of the original organism, providing important paleontological information.