acceleration | accretionary lapilli | aftershock | agate | andesite | ash | ashfall | asthenosphere | basalt | base surge
- EarthquakesGenerally, 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.
- VolcanologySmall spherical balls of volcanic ashAccretionary 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. -> See whole entry
- EarthquakesAftershocks 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 ... -> See whole entry
- MineralsAgate 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... -> See whole entry
- VolcanologyAndesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica (SiO2). Andesites are typical for lava domes and stratovolcanoes.Andesite is an igneous, volcanic rock, of intermediate composition, containing between about 52 and 63 weight % silica (SiO2).
Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene (clinopyroxene and orthopyroxene) and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine. Andesite magma ... -> See whole entry
- Volcanology: volcanic ashVolcanic 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... -> See whole entry
- Volcanology: volcanic ash fallVolcanic ash fall is the deposition of ash from the air from eruption plumes emitted during explosive volcanic eruptions.
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- GeologyThe 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 ... -> See whole entry
- VolcanologyThe most common type of volcanic rock, with a relatively low silica content and typically erupted at shield volcanoes.Basalt is the usually hard and black volcanic rock formed from (liquid) balsalitc lava. Balsaltic lava contains less than about 52 percent silica (SiO2) by weight. Because of its low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava can quickly and easily flow more than 20 km from a vent. The low viscosity typically allows volcanic gases to escape without genera... -> See whole entry
- VolcanologyBase surges are ground hugging, fast outward moving and turbulent, dilute clouds of gas and ash. They result from water magma interactions (violent steam explosions). During the 1965 eruption of Taal volcano (Philippines), such base surges were first observed. Some of them traveled 4 km and killed 189 people. Base surges were first identified during ocean nuclear weapons explosions in the Pacific.... -> See whole entry