Section 10: Geohazards & Weather Systems

GES 108

Geohazard

Any naturally occurring geological or atmospheric event that poses a threat to human life, property, or the environment. Examples include volcanoes, earthquakes, tsunamis, and landslides.

Lava

Molten rock that reaches Earth’s surface during a volcanic eruption. It cools and solidifies into igneous rock.

Magma

Molten rock located beneath Earth’s surface, composed of liquid rock, gases, and crystals.

Mafic Magma

The hottest and most fluid type of magma, rich in magnesium and iron, producing dark-colored rocks.

Intermediate Magma

Magma of moderate viscosity and temperature, with intermediate silica content.

Felsic Magma

The coolest and most viscous type of magma, rich in silica and light-colored minerals such as quartz and feldspar.

Volcano

A vent or opening in Earth’s crust through which magma, gases, and ash are expelled, forming landforms such as cones, domes, and calderas.

Stratovolcano

A large, steep, cone-shaped volcano built from alternating layers of lava flows and pyroclastic material. Stratovolcanoes produce explosive eruptions. Example

Pyroclast

Fragmented solid material ejected from a volcano during an eruption, including ash, pumice, and volcanic bombs.

Lava Dome

A small, dome-shaped volcano formed when thick, viscous lava piles up near a volcanic vent instead of flowing away. Lava domes often form over short periods (weeks to years).

Cinder Cone

A small, steep, cone-shaped volcano built almost entirely of pyroclastic material (ash, lapilli, and volcanic rock fragments). These volcanoes erupt briefly before becoming inactive.

Angle of Repose

The steepest angle at which loose material can accumulate without sliding, often describing the slope of cinder cones.

Flank Eruption

An eruption that occurs along the side (flank) of a volcano rather than its summit.

Effusive Eruption

A gentle volcanic eruption in which low-viscosity lava (usually mafic) steadily flows from a vent.

Explosive Eruption

A violent eruption that ejects pyroclastic material high into the atmosphere. Typically associated with stratovolcanoes and felsic magma.

Volcanic Gas

Gases released during volcanic eruptions, making up about 8% of magma. The main gases are water vapor, sulfur dioxide, and hydrogen sulfide.

Large Igneous Province

A vast accumulation of basaltic lava that covers large areas of Earth’s crust, formed by extensive volcanic activity over millions of years.

Flood Basalt

A thick, widespread lava flow that erupts over long periods, creating large igneous provinces.

Caldera

A large, basin-shaped depression formed when the summit of a volcano collapses into the emptied magma chamber after an eruption. Example

Lahar

A fast-moving, cement-like mudflow composed of volcanic ash, rock, and water. Lahars often occur when snow or ice melts rapidly during volcanic eruptions, burying entire communities.

Pyroclastic Flow

A hot, fast-moving avalanche of gas, ash, and volcanic debris that rushes down a volcano’s slope at speeds up to 450 mph and temperatures near 900°F. Extremely deadly, as seen in Pompeii.

Volcanic Explosivity Index (VEI)

A scale used to measure the explosiveness of volcanic eruptions based on the height of the ash cloud and volume of material ejected. Ranges from 0 (gentle) to 8 (catastrophic).

Earthquake

A sudden shaking of the ground caused by the release of energy from stress built up along faults or plate boundaries.

Fault

A fracture in Earth’s crust along which movement has occurred, releasing energy as seismic waves.

Foreshock

A smaller earthquake that occurs before a major earthquake event.

Aftershock

A smaller earthquake that occurs after a major earthquake, as the crust adjusts to new stress conditions.

Seismic Wave

A wave of energy that travels through Earth’s interior and surface, produced by an earthquake or explosion.

Body Wave

A type of seismic wave that travels through Earth’s interior, including primary (P) and secondary (S) waves.

Surface Wave

A seismic wave that travels along Earth’s surface, producing the shaking felt during earthquakes (L-waves and R-waves).

Seismograph

An instrument that records the motion of the ground during an earthquake.

P-Wave (Primary Wave)

The fastest type of seismic wave, compressing and expanding rock as it travels. It is the first wave detected by a seismograph.

S-Wave (Secondary Wave)

A slower seismic wave that moves the ground side to side. It cannot travel through liquids.

Intensity (of an Earthquake)

A measure of the damage or shaking caused by an earthquake at a specific location.

Mercalli Intensity Scale

A scale that measures earthquake intensity based on observed damage, ranging from I (barely felt) to XII (total destruction).

Tsunami

A large ocean wave generated by undersea earthquakes, landslides, or volcanic eruptions.

Lightning

A powerful electrical discharge produced during thunderstorms. Lightning can be up to five times hotter than the surface of the Sun.

Fulgurite

A glassy, hollow tube formed when lightning strikes sand, melting the silica into the shape of the bolt.

Tropospheric Lightning

The most common type of lightning, produced within thunderstorms.

Blue Jet

Lightning that extends upward into the stratosphere.

Red Sprite

Lightning that occurs in the mesosphere.

Elf

Lightning that occurs in the thermosphere.

Heat Lightning

Distant lightning that is visible but inaudible due to the speed of light exceeding that of sound.

Thunder

The sound produced by lightning as air rapidly expands and contracts due to intense heating.

Thunderstorm

A storm that produces thunder and lightning, often accompanied by rain, wind, or hail. Thunderstorms are most common near the equator and occur about 40,000 times daily worldwide.

Supercell Thunderstorm

The least common but most intense type of thunderstorm, characterized by a rotating updraft. Only about 25% of supercells produce tornadoes.

Multicell Thunderstorm

A cluster or line of multiple storm cells forming along boundaries where different air masses meet. These can produce hail, strong winds, or tornadoes.

Single-Cell Thunderstorm

A small, short-lived thunderstorm (lasting about an hour) that typically forms in the late afternoon due to surface heating.

Developing (Cumulus) Stage

The early stage of a thunderstorm when rising warm air condenses into clouds.

Mature Stage

The active stage of a thunderstorm when heavy rain, lightning, and thunder occur.

Dissipating Stage

The final stage when the storm weakens as precipitation and downdrafts dominate.