Geology glossary

Updated: Dec 4, 2021 14:09 GMT -

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acceleration

Earthquakes
Generally, acceleration is increase of speed or velocity. During earthquakes, the shaking of ground is a swinging motion involving both acceleration and deceleration.
Its maximum value is the largest increase in velocity recorded at a particular point. It largely influences how damaging an earthquake is in this area.

accretionary lapilli

Volcanology
Accretionary lapilli in an ash deposit on Santorini.
Accretionary lapilli in an ash deposit on Santorini.
Small spherical balls of volcanic ash
Accretionary lapilli are small spherical balls of volcanic ash that form from a wet nucleus falling through a volcanic ash cloud. They can flatten on hitting the ground or may roll on loose ash and grow like a snowball. Read all

acidic

Volcanology
An adjective describing the chemical composition of magmas (or lavas) having relatively high content of silica (SiO2). It has little to do with the use of the term in chemistry where it is used to describe low pH values found in acids.
In petrology and volcanology, magmatic rocks (or the magmas they derived from) are often classified by their bulk silica (SiO2) content, which can typically range from around 40-75 weight %, an results in the 4 main types of magma by composition. These are, in order of increasing silica content: basaltic (45-53%), andesitic (53-62%), dacitic (62-69%), and rhyolitic (69-77%).
Rocks with high sili... Read all

aftershock

Earthquakes
Aftershocks following the M6.5 quake in Idaho on 31 Mar 2020
Aftershocks following the M6.5 quake in Idaho on 31 Mar 2020
Aftershocks are smaller quakes that follow a usually larger earthquake in the same rupture zone. The can occur up to weeks, months or even years after, depending on the size of the main shock.
Aftershocks are smaller than the mainshock and occur within 1-2 rupture lengths distance from the mainshock.
In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue. Large earthquakes can be followed by thousands of aftershocks.
The larger aftershocks following an earthquake can be particularly dangerous if many structures have been ... Read all

agate

Minerals
Agate from Milos island, Greece
Agate from Milos island, Greece
Agate is a microcrystalline variety of quartz (silica), chiefly chalcedony, characterised by its fineness of grain and brightness of color.
Agate is a microcrystalline variety of quartz (silica), chiefly chalcedony, characterised by its fineness of grain and brightness of color. Although agates may be found in various kinds of rock, they are classically associated with volcanic rocks but can be common in certain metamorphic rocks.
Colorful agates and other chalcedonies were obtained over 3,000 years ago from the Achates River, now ca... Read all

amplification

Earthquakes
Amplification of earthquake waves in the LA area (image: USGS)
Amplification of earthquake waves in the LA area (image: USGS)
Amplication is the effect of increasing seismic shaking as a result of surface or subsurface topography and geology at a given spot. Various factors can focus seismic energy, in particular the geometry and composition of the rock or sediment strata.
Amplification occurs when seismic waves are reflected or bended and overlap with themselves or other seismic waves from the same quake.
Two important local geologic factors that affect the level of shaking experienced in earthquakes are (1) the softness of the surface rocks and (2) the thickness of surface sediments.
Areas near steep cliffs are particularly prone to seismic wave amplification. At such exposed surfaces, seismic waves are reflected and overlap to create larger waves in a similar way as water waves increase when hi... Read all

andesite

Volcanology
Typical andesite from the Methana peninsula (Greece)
Typical andesite from the Methana peninsula (Greece)
Andesite is a gray to black volcanic rock with intermediate silica content, typical for lava domes and stratovolcanoes in subduction zones.
Andesite is an igneous volcanic rock containing between about 52 and 63 weight % silica (SiO2). Andesite is the extrusive equivalent to plutonic diorite - if a dioritic magma cools beneath the surface as an intrusion, it will be diorite, if the same magma erupts during a volcanic eruption, the rock formed is andesite.

Where does andesite usually occur?

Andesite and diorite are among the m... Read all

ash

Volcanology: volcanic ash
Ash plume from an explosive eruption at Etna volcano (Italy)
Ash plume from an explosive eruption at Etna volcano (Italy)
Volcanic ash is the term for all fine-grained volcanic products (smaller than 2 mm), normally magma or older rock fragmented during explosive eruptions.
Volcanic ash has nothing to do with fire, but is a mere definition of grain-size. Ash can range in size from sandy to extremely fine; any fragment ejected by a volcano less than 2 mm in diameter is called ash. It may consists of freshly ejected lava (usually turned into a glass shard because of rapid cooling), older fragmented rock, or small crystals.
Ash is produced by explosive activity when ex... Read all

ashfall

Volcanology: volcanic ash fall
Ash plume and ash fall from Etna during the eruption in 2002
Ash plume and ash fall from Etna during the eruption in 2002
Volcanic ash fall is the deposition of ash from the air from eruption plumes emitted during explosive volcanic eruptions.

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ash

asthenosphere

Geology
The uppermost viscous layer of the mantle where pressure and temperature conditions can allow partial melting of the mantle rocks, which forms magmas.
From the Greek "asthenos"=weak and "sphera"=sphere, the asthenosphere is the uppermost layer of the plastic, highly viscous mantle beneath the brittle crust. It extends to depths of 100-200 km.
Because the pressure in this zone is still comparably low while temperature is high, parts of the mantle rocks can be in a molten state and form magmas. This happens especially in areas where pressure is ... Read all

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