Understanding the Geosphere: Structure, Seismic Waves, and Volcanic Hazards

Posted on May 16, 2024 in Geology

ITEM 6: GEOSPHERE and Geohazards

Geosphere

The geosphere refers to the solid part of the Earth, encompassing everything beneath the Earth’s crust. It is the focus of study for phenomena related to the Earth’s internal dynamics, including its solid materials, surface features, and the planet as a whole.

Structure of the Geosphere

The geosphere’s structure can be categorized based on both physical and chemical composition:

Chemical Composition

  • Core: Composed primarily of iron (Fe) and nickel (Ni).
  • Mantle: Rich in iron (Fe), calcium (Ca), and magnesium (Mg). It is further divided into the lower and upper mantle based on the structure of its constituent minerals.
  • Crust: The continental crust is rich in aluminum (Al), sodium (Na), and potassium (K), while the oceanic crust is rich in calcium (Ca), iron (Fe), and magnesium (Mg).

Physical Composition

  • Endosphere: Corresponds to the core, consisting of a solid inner core and a liquid outer core.
  • Mesosphere: Encompasses the entire lower mantle and part of the upper mantle. It behaves as a rigid and stable region.
  • Asthenosphere: Located within the upper mantle, it exhibits semi-plastic behavior.
  • Lithosphere: Includes part of the upper mantle and the entire crust. It is divided into plates that slide, collide, are destroyed, and are created.

(Drawing of the composition)

Types of Seismic Waves

Deep Waves

Originating from the hypocenter, deep waves propagate through the Earth’s interior, making them valuable for studying its internal structure.

  • Primary (P) Waves: These are the fastest seismic waves and are detected first by seismographs. They compress and dilate the Earth in the direction of propagation, similar to a spring.
  • Secondary (S) Waves: Slower than P waves, S waves cause particles to move perpendicular to the direction of propagation. They can only travel through solids.

Surface Waves

Surface waves result from the interaction of deep waves with the Earth’s surface. They spread out from the epicenter and are responsible for most of the damage caused by earthquakes.

  • Love (L) Waves: Love waves produce horizontal motion perpendicular to the direction of propagation. They vibrate on a single plane, corresponding to the Earth’s surface.
  • Rayleigh (R) Waves: The slowest type of seismic wave, Rayleigh waves cause the most noticeable ground motion. They exhibit an elliptical motion in the direction of propagation and on a vertical plane, similar to ocean waves before they break.

Seismograms

Seismograms are used to determine an earthquake’s epicenter, magnitude, and focal depth.

Epicenter

The epicenter is the point on the Earth’s surface directly above the hypocenter. It experiences the greatest magnitude of the earthquake.

Hypocenter

The hypocenter, also known as the focus, is the point within the Earth where the earthquake originates. It is not a single point but rather a zone of slippage along a fault.

Location of the Epicenter

Seismic stations distributed globally record seismic waves from earthquakes worldwide. These stations are interconnected, allowing seismologists to access data and calculate the epicenter’s location, the hypocenter’s depth, the type of tectonic stress that caused the earthquake, the path of the waves, and other relevant information.

Determining the Focus of an Earthquake

To pinpoint an earthquake’s epicenter, two key pieces of data are needed: the distance from the seismic station and the direction.

  1. Distance from the Epicenter: Seismograms reveal that S waves always arrive at the seismograph after P waves. The greater the delay between the P and S waves, the farther the epicenter is from the seismic station. Each delay interval corresponds to a specific distance to the epicenter.
  2. Direction: The distance from the station to the epicenter indicates that the epicenter could be located anywhere on a hypothetical circle with that radius, centered on the station. To determine the exact location, data from at least three seismic stations in different locations are compared. The intersection of the three circles indicates the epicenter.

Geological Processes: Volcanoes

Volcanoes are direct expressions of geothermal energy. They are fractures in the Earth’s crust through which magma erupts. While volcanoes contribute to fertile land, mineral resources, and geothermal energy, they also pose a significant geological risk due to their potential for causing death and destruction.

Geographical Distribution of Volcanoes

Earth has approximately 40,000 volcanoes, with only a quarter of them above sea level. Their distribution is not random but concentrated along plate boundaries, particularly subduction zones that form the”Pacific Ring of Fire” Volcanoes can also occur within plates or between plates. This distribution is attributed to two main factors:

  • Presence of a Hot Spot: Hot spots are areas of the lithosphere situated above a thermal plume in the mantle. These plumes remain fixed relative to the moving plates.
  • Presence of Fractures or Weak Points in the Lithosphere: While the Canary Islands were initially thought to have formed due to a hot spot, this theory has been challenged. Many scientists now believe that the archipelago arose from the accumulation of volcanic material erupting through fractures in the African plate, possibly caused by tensions from the opening of the Atlantic Ocean.

Parts of a Volcano

  • Crater: The opening through which lava erupts. If the diameter exceeds 1 km, it is called a caldera.
  • Volcanic Cone: A mound formed by the accumulation of erupted material.
  • Magma Chamber: An underground reservoir where magma is stored before eruption.
  • Chimney: The conduit through which magma travels from the magma chamber to the crater.
  • Eruptive Column: The height reached by materials ejected into the air during an eruption.
  • Lava Flow: The stream of lava that flows from the crater.
  • Parasitic Cone: A secondary cone on the volcano, typically emitting gases.

Volcanic Risk Factors

  • Exposure: Volcanic areas are often densely populated due to the benefits they offer, such as fertile land, mineral resources, and geothermal energy. This concentration of population can exacerbate the impact of volcanic eruptions.
  • Vulnerability: The extent to which a population is susceptible to damage from a volcanic eruption depends on the availability of resources to cope with the event.
  • Dangerousness: This factor relates to the magnitude of the volcanic event itself, including the type of eruption, its geographic distribution, the total area affected, and its recurrence interval. Different volcanic manifestations pose varying levels of danger:

Volcanic Manifestations and Their Impacts

  • Gases: Gases within magma drive eruptions by expanding and escaping rapidly after a fracture occurs, allowing other materials to rise. These gases can cause respiratory problems or even death by asphyxiation in humans and animals.
  • Lava Flows: The danger posed by lava flows depends on their viscosity. Acidic lavas, derived from magmas with high silica content, are highly viscous and therefore more dangerous because they trap gases that can be released abruptly. Lava flows can damage crops, ignite fires, disrupt roads, destroy villages, and block valleys, leading to flooding.
  • Pyroclastic Flows: Pyroclastic flows are dense, fast-moving currents of hot gas and volcanic matter that cascade down volcanic slopes. They are extremely destructive, causing combustion, severe burns, asphyxiation from inhaling hot dust, and the destruction of all material goods in their path.
  • Explosions: Volcanic explosions are influenced by the viscosity of the lava. Viscous lavas tend to produce more explosive and dangerous eruptions than fluid lavas. Explosions can release massive amounts of pyroclastic material into the atmosphere, tear apart volcanic slopes, clog valleys, cause flooding, damage human constructions, and generate burning clouds or calderas.
  • Volcanic Dome Formation: When lava is extremely viscous, it can accumulate in the crater instead of flowing, forming a dome-shaped plug that obstructs the exit of lava. The sudden explosion of a dome can enlarge the crater, intensify the eruption, and trigger a pyroclastic flow.
  • Caldera Formation: After a major eruption that ejects vast amounts of pyroclastic material, the magma chamber may become partially emptied and unstable. This can cause the roof of the chamber to collapse, enlarging the crater into a caldera. Caldera formation can lead to volcanic collapse, earthquakes, and tsunamis.