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Geology glossary

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obsidian

Volcanology
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Obsidian from the Rocce Rosse lava flow on Lipari island (Italy)
Detail of obsidian with spherolites (small bubbles where the glass has crystallized around the bubble)
Detail of obsidian with spherolites (small bubbles where the glass has crystallized around the bubble)
Black volcanic glass
Obsidian is a naturally occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of cr... -> See whole entry

olivine

Minerals
Olivine from basalt lava on Hawai'i (image: By incidencematrix - Basalt with Olivine, CC BY 2.0, Link)
Olivine from basalt lava on Hawai'i (image: By incidencematrix - Basalt with Olivine, CC BY 2.0, Link)
Olivine is one of the earth's most common rock-forming silicate minerals, typically contained in mafic to ultrafafic magmatic rocks with low silica content (e.g. basalt, gabbro, peridotite), which make up much of the earth's upper mantle, the oceanic crust and lava flows etc from basaltic shield volcanoes on land. Its name refers to its often olive-green color.
Nameolivine
Category
Formula(Mg2+, Fe2+)2SiO4
Crystal systemorthorhombic
ColorYellowish-green to olive green
StreakNone
LusterVitreous
Crystal habitMassive to granular
Mohs hardness6.5–7
Specific weight3.2-4.5
Uses
Other
Chemically, it combines two interchangeable ions of magnesium (Mg2+) or iron (Fe2+) with one silica ion (Si4+) in a densely-packed crystal lattice. The two end-members from this compositional range are forsterite (magnesium-olivine, Mg2SiO4) and and fayalite (iron-olivine, Fe2SiO4). In rare cases, Mg/Fe can also be replaced by calcium (Ca), manganese (Mn), or nickel (Ni). -> See whole entry

P-wave

Earthquakes
Illustration of a P-wave (image: USGS)
Illustration of a P-wave (image: USGS)
The P wave (short for primary wave, also called pressure or compressional wave) is the seismic wave that shakes the rocks forth and back in the same and opposite direction of the propagation direction (longitudinal).
The primary (P) waves of earthquakes are the same type of waves as the more familiar sound waves. Both are waves propagating spherically away from its source through a compressible medium (rocks, fluids and gasses) where particles are swinging in the same and opposite direction of the wave propagation.

Typical speeds of P-waves from earthquakes are between 1450m/s in water and 5000m/s in granite.... -> See whole entry

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.
Phreatomagmatic lava fountain.
Phreatomagmatic lava fountain.
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.

-> See whole entry

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)
Pumice deposit on Santorini, Greece, from the large Plinian "Minoan" eruption on Santorini in 1613 BC, showing the holes in the pumice where remants of an olive tree could be found and recovered by Tom in 2003. This material allowed the most recent and most precise dating of this eruption to date.
Pumice deposit on Santorini, Greece, from the large Plinian "Minoan" eruption on Santorini in 1613 BC, showing the holes in the pumice where remants of an olive tree could be found and recovered by Tom in 2003. This material allowed the most recent and most precise dating of this eruption to date.
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... -> See whole entry

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 from the Greek island Santorini
Pumice from the Greek island Santorini
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... -> See whole entry

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.
The billowing ash could of the advancing pyroclastic flow.
The billowing ash could of the advancing pyroclastic flow.
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... -> See whole entry

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 ... -> See whole entry

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.

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