Geologic Hazards

Common Geologic Hazards

Geologic Hazard

• It is any geologic condition which have the ability to cause damage to property and life.
• There are three common types of geologic hazards: earthquakes, volcanoes, and landslides.

Tectonic Earthquakes

Earthquake

• It is a trembling or shaking movement of the ground on the Earth’s surface.
• They can be classified as volcanic or tectonic.
• Tectonic quakes happens along faults or cracks in the Earth’s crust and in places where the ground experiences differential stresses.
  • These stresses build up along faults due to tectonic plate movements.
  • When these stresses become too great for the rock to handle, the fault slips and the ground moves.

Fault

• It is a crack along the Earth’s crust.
• It is mainly responsible for generating earthquakes along them.
• It has two main types: dip-slip and strike-slip.
  • Dip-slip Fault: when blocks of rocks fall along the angle of the crack (also known as the dip)
    • It is composed of two blocks of rocks: the footwall block and the hanging wall block.
      • The footwall block is the block below the crack.
      • The hanging wall block is the block above the crack.
    • It also has two types:
      • Normal fault: when the hanging wall drops downward relative to the footwall.
      • Reverse fault: when the hanging wall moves upward relative to the footwall.
  • Strike-slip: when blocks of rocks neither drops or move upward, but slide past one another.

Elastic Rebound Theory

• It is the mechanism at which why earthquakes form.
• When a force or stress is applied in a system, it gains potential energy.
• But when the stress is removed, the energy is released and it propagates like a wave through a medium
  • This process is known as rebound.
• In a fault line, when the rock cannot handle the stress, the fault slips and the stress is removed.
  • The energy will then propagate like a wave in all directions and in through the rocks as a seismic wave.
  • When this energy reaches the surface, it makes the ground shake, which forms the earthquake.

Aftershocks

• These are earthquakes formed after a previous earthquake.

Volcanoes

Volcano

• It is a geographical feature that is a rupture in the Earth’s crust that allows hot lava, volcanic ash and gasses to escape to its surface.
• Its name came from the god of fire in Roman Mythology, Vulcan.

Volcanic Eruption

• It is a geological phenomena when enough magma accumulates inside the volcano that it causes itself to release gas, rock fragments, lava flows, mudslides in its opening.
• It has two types: explosive and non-explosive/effusive.
  • Explosive eruptions release and eject volcanic materials into the air.
  • Effusive (non-explosive) eruptions let volcanic materials steadily flow on the surface of the volcano.

Parts of a Volcano

• Crater: an oval depression surrounding the main volcanic vent.
• Throat: the main passageway that connects the magma chamber to the surface of a volcano.
• Vent: an opening in the volcano from which volcanic materials escape.
• Conduit: an underground network of pipes where magma passes through
• Sills: an underground rock formation of solidified magma that forms when magma tries to fill in horizontal gaps in layers between rocks as it rises on the surface.
• Dikes: forms in the same way as sills but instead of between rock layers, it forms when there are cracks in the Earth’s crust, giving a more vertical and angled appearance.
• Parasitic Vent: also known as a secondary vent; alternate pathways other than the main vent that connects the magma chamber or the throat to the surface of the volcano.
• Parasitic Cone: a small, cone-shaped protrusion on the side of a volcano that formed due to accumulation of debris and ash coming off a parasitic vent that is surrounding it.
• Magma Chamber: it is a section underneath a volcano where magma is stored.

Types of Volcanic Material

• There are three types of volcanic material
  • Lava: magma that comes from the mantle when it reaches the Earth’s surface.
  • Tephra/Pyroclasts: any solid or semi-solid material that is ejected by a volcano
    • This includes ash, lapilli, or volcanic bombs.
  • Gas: may be composed of water vapor, carbon dioxide, sulfur dioxide and some impurities.

Type of Volcanoes

• Volcanoes can be classified according to volcanic activity or structure and magma composition.
  • According to volcanic activity, volcanoes can be classified as:
    • Active: has erupted lately; is most likely to erupt soon.
    • Dormant: has not erupted for a long time but it has a chance to possibly erupt again.
    • Extinct: had erupted before but it does not have a chance to erupt again.
  • According to structure, they can be classified as:
    • Composite: has a more steeper slope; ejects high viscosity lava; composed of hardened layers of lava and tephra.
    • Cinder Cone: the simplest type of volcano; composed of loose pyroclastic fragments; less steeper than composite volcanoes
    • Shield: ejects less viscous lava; it has a shield-like appearance with a gently steep slope.
    • Caldera/Crater: forms when a volcano expels vast amounts of magma that it collapses into itself, forming a depression on the surface, known as a caldera.

Landslides

Landslide

• It is any form of rapid mass movement or earth down a slope.
• There are 4 main causes of landslides:
  • a less cohesive, more loose layer of soil atop a steep slope becomes fluid
  • when soil atop a steep slope is saturated with water, either during heavy rainfall or snowmelt, or in areas near rivers and lakes
  • when a mass of soil is disturbed during an earthquake, causing it to fall down
  • due to human activities, such as deforestation, blasting, construction, or logging.
    • trees hold soil via their roots and absorb water well, one factor in landslides.