Essential Geology Terms: Earth’s Layers, Quakes, and Volcanoes

Posted on Aug 13, 2025 in Geology

Understanding Earth’s Structure and Geological Phenomena

Earth’s Internal Layers

Crust

The outermost layer, primarily composed of aluminum-rich silicates. The continental crust has an average thickness of 35 km and can reach 80 km. The oceanic crust has a maximum thickness of 10 km and consists of basaltic rocks.

Mantle

Extends from the base of the crust to approximately 2900 km deep. It is separated from the crust by the Mohorovičić discontinuity and is rich in iron and magnesium silicates. The mantle is divided into two parts: the upper mantle and the lower mantle, which vary in chemical composition. The Repetti discontinuity lies between the upper and lower mantle.

Core

Extends from 2900 km to the Earth’s center at 6370 km. It is separated from the deep mantle by the Gutenberg discontinuity. The core is distinguished into two parts: the outer core, composed of liquid iron and nickel, and the inner core, which is heavier and solid, formed primarily of iron.

Lithosphere

The outer, rigid layer of the Earth, encompassing the entire crust and a portion of the upper mantle. The continental lithosphere is between 100 and 200 km thick, while the oceanic lithosphere varies between 50 and 100 km.

Asthenosphere

A layer beneath the lithosphere, reaching a depth of 670 km. Its rocks are in a solid state, but close to their melting point, making it a plastic and deformable layer.

Mesosphere

The mantle layer beneath the Asthenosphere. It behaves more rigidly because the materials are less plastic, as high pressures outweigh the elevated temperatures.

Outer Core

Located beneath the lower mantle, it extends to the Lehmann discontinuity at 5100 km. This area is fluid due to high temperatures, around 4000 degrees Celsius, producing intense convection currents caused by temperature differences.

Inner Core

Comprises the rest of the core. Although temperatures reach 6,000 degrees Celsius, high pressures cause the materials to remain solid.

Earthquakes and Volcanic Activity

Magnitude

Refers to the amount of energy released by an earthquake. The Richter scale measures the magnitude of an earthquake.

Intensity

A measure of an earthquake’s destructive effects, which can be measured with the MSK scale.

Hawaiian Eruptions

Characterized by very fluid lava that slides down quickly and travels long distances.

Strombolian Eruptions

Involve lava flows accompanied by more or less violent explosions and the emission of large quantities of gases.

Vulcanian Eruptions

Feature very viscous lava that obstructs the volcanic chimney until enormous gas pressure produces a violent explosion.

Geological Processes and Structures

Orogeny

The process of mountain formation resulting from internal Earth processes.

Glyptogenesis

The wear that occurs due to the effects of erosion, where eroded materials are transported to lower areas.

Lithogenesis

The formation of new rocks caused by internal and external geological processes.

Folds

Ripples or bends caused in rocks of a plastic nature when subjected to compressional forces.

Axial Plane

The plane that unites all hinges and divides the fold into two parts or flanks.

Fold Axis

The line of intersection of the axial plane with the hinge of the fold.

Fold Nucleus

The inner part of the fold.

Hinge

The area of maximum curvature in a fold.

Flank

Each of the parts into which the fold is divided by the axial plane.

Anticline

A fold that is curved upward, where the core contains the most ancient materials.

Syncline

A fold where the core contains the most modern materials.

Faults

Fractures in rock masses that form two blocks, which move along the plane of rupture.

Joints (Diaclases)

Fractures in rock masses where the blocks do not move relative to one another, or if they do, it is to widen the gap.

Earthquakes

Vibrations and shocks of the Earth’s surface that originate at a specific point within the lithosphere.

Focus or Hypocenter

The point inside the Earth where an earthquake originates.

Epicenter

The point on the Earth’s surface closest to the hypocenter.

Seismic Waves

Vibrations that originate from the hypocenter and are transmitted radially in all directions.

P-waves (Primary Waves)

Originate in the hypocenter and are transmitted through both solid and liquid media; they are the fastest seismic waves.

S-waves (Secondary Waves)

Originate in the hypocenter but are only transmitted through solid media; they are slower than P-waves.

L-waves (Surface Waves)

Originate at the epicenter and are transmitted only along the Earth’s surface; they are the slowest seismic waves but often cause the most damage.