Geology glossary

Updated: Dec 1, 2021 03:25 GMT -

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phreatomagmatic

Volcanology: phreatomagmatic activity
Lava fountain at Etna during the 2001 eruption. The activity is phreatomagmatic in origin, which explains the large amount of ash involved in the fountain: the rising water on its way meets wet layers where the contact between water and magma produces violent fragmentation.
Lava fountain at Etna during the 2001 eruption. The activity is phreatomagmatic in origin, which explains the large amount of ash involved in the fountain: the rising water on its way meets wet layers where the contact between water and magma produces violent fragmentation.
Volcanic activity where fresh magma AND external water are involved.
Phreatomagmatic activity means that erupting magma reacts with external water, e.g. ground water, lake water, sea water etc. In contrast, if only magma is erupted and driven only by gasses originally contained in the magma, it is called magmatic activity. If no magma itself erupts, but heated ground water drives explosions and eruptions of older material, the activity is called phreatic.
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Plinian eruption

Volcanology
Plinian eruption of Mt. St. Helens on May 18, 1980 (USGS Photograph taken on May 18, 1980, by Donald A. Swanson)
Plinian eruption of Mt. St. Helens on May 18, 1980 (USGS Photograph taken on May 18, 1980, by Donald A. Swanson)
The most explosive and largest type of volcanic eruptions. Plinian eruptions erupt more than 1 cubic kilometer of magma often within less than a few days and produce ash columns that can reach 20-50 km height.
Plinian eruptions are large explosive events that form enormous dark columns of tephra and gas high into the stratosphere (>11 km). Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows and surges, and extensive ash fall. Many thousands of people evacuate... Read all

primary waveSynonym of: P-wave

Earthquakes
Illustration of a P-wave (image: USGS)
Illustration of a P-wave (image: USGS)

Pumice

Volcanology: pumice stone
Our tour guide Marta posing in Lipari's pumice...
Our tour guide Marta posing in Lipari's pumice...
Pumice is a very light, porous volcanic rock that forms during explosive eruptions. During the eruption, volcanic gases dissolved in the liquid portion of verz viscous magma expand very rapidly to create a foam or froth; the liquid part of the froth then quickly solidifies to glass around the gas bubbles.
The volume of gas bubbles is usually so large that pumice is lighter than water and floats.
Pumice is an important industrial mineral used to produce high-quality cement and lightweight, isolating building materials.
Pumice is a textural term for a volcanic rock that is a solidified frothy lava composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. It is commonly, but not exclusively of silicic or felsic to intermediate in composition (e.g. rhyolitic, dacitic, andesite, pantellerite, phonolite, trachyte), but occurrences of basaltic and other com... Read all

pyroclastic flow

Volcanology
Pyroclastic flow travelling down the Krasak ravine at Merapi volcano on 27 May 2006.
Pyroclastic flow travelling down the Krasak ravine at Merapi volcano on 27 May 2006.
Fluid avalanche of turbulently mixed ash, lava and or rock fragments, and air, that flows down the flanks of a volcano, driven by gravity. Pyroclastic flows are usually very hot and highly destructive.
A pyroclastic flow is a ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour or more. The temperature within a pyroclastic flow may be greater than 500°C, sufficient to burn wood. Once deposited, the ash, pumice, and rock fragments may deform (flatten) and weld together because of the intense heat and the weight... Read all

quartz

Minerals
Quartz from Milos island (Greece)
Quartz from Milos island (Greece)
Quartz (crystalline SIO2) is the most abundant mineral in the Earth's continental crust.
Quartz is made up of a lattice of silica (SiO2) tetrahedra. Quartz has a hardness of 7 on the Mohs scale and a density of 2.65 g/cm³.

Pure quartz is colorless or white, colored varieties include rose quartz, amethyst, smoky quartz, milky quartz, and others. Quartz goes by an array of different names. The most important distinction between types of quartz is that of macrocrystalline (individual ... Read all

rhyolite

Volcanology
Rhyolite from Milos island (Greece)
Rhyolite from Milos island (Greece)
A type of highly viscous magma with high silica content; it is found as pumice (in airfall deposits or ignimbrites), lava or obsidian. Rhyolite is also the name given to the volcanic rock formed from rhyolitic magma.

sediment

Geology
Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid.
Sediments are also transported by wind (aeolian processes) and glaciers. Desert sand dunes and loess are examples of aeolian transport and deposition. Glacial moraine deposits and till are ice transported sediments. Simple gravitational collapse also creates sediments such as talus and mountainslide deposits as well as karst collapse features. Each sediment type has different settling velocities, ... Read all

seismic waves

Earthquakes
Diagram showing the elements of an earthquake. (source: Tarbuck, Lutgens, and Tasa. 2012. Essentialsof Geology. 11th Ed. Page 337)
Diagram showing the elements of an earthquake. (source: Tarbuck, Lutgens, and Tasa. 2012. Essentialsof Geology. 11th Ed. Page 337)
During an earthquake, large amounts of energy are released from the hypocenter or focus (i.e. point where the quake occurs) in the form of seismic waves. These cause the vibrations felt during a proper quake.
As a thought experiment, try imagining throwing a pebble in the middle of pond. Ripples will form from the point of contact between the pebble and the water surface, and propagate outwards ... Read all

seismograph

Earthquakes
A seismograph at a volcano observatory in Indonesia
A seismograph at a volcano observatory in Indonesia
An instrument that displays vibrations and movements in the Earth = the recording of a seismometer.
Traditional seismographs transfer the recorded shaking to a precise pencil that writes a continuous shake line onto a sheet of paper wrapped around a turning drum.
The drum slowly turns at steady speed and at the same time, the pencil is slowly moved horizontally, so that the line forms a continuous spiral line on the paper until it is filled. Each turn of the drum (or each line on the unwrapped... Read all

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