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.... Read all
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.
Read all
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
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 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
Batu Tara photos A remote island in the Flores Sea of Indonesia formed by a single volcano that has been in strong strombolian eruption for years. We regularly lead expeditions to there where we camp several days to observe it.
Latest quakes in Greece Greece has a lot of earthquake activity, but most of them occur around the margin of the Aegean microplate. The Cycladic islands are located in its center and therefore seismically comparably quiet.
Glossary of geologic terms A growing list of often illustrated explanations and definitions of geological terms and keywords, with focus on volcanology and seismology.
