Earthquakes and Volcanoes: Manifestations and Distribution

Posted on Mar 5, 2025 in Geology

Manifestations of Earthquakes

Earthquakes occur when energy accumulated in the interior of the Earth is suddenly released.

Focus or hypocenter: The point within the Earth where the earthquake originates.
Epicenter: The point on the Earth’s surface directly above the hypocenter.
Faults: Fractures in the Earth’s crust where two blocks of rock move relative to each other.
Seismic waves: Generated by earthquakes and captured by seismographs.
Seismographs: Devices that record seismic activity in a specific area, creating seismograms.
Seismograms: Chart records of earthquakes.

Magnitude:
- Measures the energy released by an earthquake.
- Measured using the Richter scale (1-8 degrees).
- Numerical and logarithmic scale.
Intensity:
- Measures the effects of an earthquake at a specific location.
- Measured using the Mercalli intensity scale (1-12).

Depend on their causes:

Tectonic:
- Mass movement of rock along faults (hypocenter < 50km).
- Friction between lithospheric plates (subduction zones).
- Shakes or secondary tremors (hypocenter: 700km).
- These cause the most intense disasters.
- Can produce movement of two blocks (less than 50m) or in subduction zones (depth: 700m).
Volcanic:
- Movement of magma underground.
- Very explosive eruptions.
- Less frequent and intense.

Seismic risk factors: Earthquake magnitude, population density, and infrastructure (buildings, roads, highways, bridges).

First belt of high seismicity: Around the Pacific (Pacific Plate, Nazca, and Cocos).
Second belt of intense seismic activity: Begins in the Azores and Gibraltar, goes through the Mediterranean, reaching Indonesia.
Third belt: High seismicity through the center of the Atlantic, dividing through the Indian and South Pacific Oceans. These are areas of mid-ocean ridges.

Volcanoes are in active areas: young mountain ranges, mid-ocean ridges, transform faults, and subduction zones.

Hotspot: A volcano that rises inside a plate from magma in the deep mantle.

Based on the composition and temperature of lava:

Constructive boundaries: Mid-ocean ridges (volcanoes of Iceland).
Destructive boundaries: Subduction zones (Chimborazo, Cotopaxi in the Andes, and Krakatoa in Indonesia).
Neutral boundaries: Strike-slip faults (Vesuvius, Stromboli, Vulcano, Etna).
Hotspots: Mauna Loa, Kilauea, and Hualalai (Hawaii).

A volcano is a geological structure that originates from the output of magma to the exterior through cracks or fissures.

Magma: Molten rock material that comes from the mantle or the base of the Earth’s crust.

According to magma viscosity and the speed at which gases leave the magma (Lacroix, 1908):

Hawaiian: Very fluid magma, tranquil eruptions.
Strombolian: Small explosions. Lava and pyroclastic materials are deposited in alternating layers.
Vulcanian: Large explosions. Release of solid materials. The volcanic cone has a very steep slope.
Pelean: Very viscous magma. Often solidifies before leaving the crater. Very violent explosions.

Fissural volcano: Magma emerges through fractures (25km long and 15km wide). Very fluid. Originates from ejected basaltic lava, forming basaltic plateaus (3000m thick).
Central volcano: Magma accumulates in the magma chamber, rises through the chimney, and exits through the crater.

According to the characteristics of the magma and how gases are released:

Land eruptions:
- Fluid magma (gases are easily released, resulting in less explosive activity).
- Viscous magma (gas output is difficult, gases accumulate, and pressure becomes enormous, leading to large explosions).
Submarine eruptions:
- Superficial: Very violent underwater explosions and lava occur.
- At depths greater than 300m: Lava solidifies quickly upon contact with water (forming pillow lavas).