Volcanic Processes

Craters differ from calderas both in size and origin. Craters are much smaller features than calderas and are typically defined as being less than 1 km in diameter. Although both craters and calderas are most often associated with explosive eruptions, craters are typically formed by the explosive ejection of material in and surrounding the upper part of the conduit, rather than by collapse. Steep-walled pit craters, in contrast, often found on shield volcanoes, are more passive features formed when magma drains from a fissure, leaving overlying lava flows unsupported. Multiple explosive eruptions can form overlapping or nested craters, and adjacent craters may reflect localized areas of eruption along fissures, as seen in craters rows in Iceland that extend for tens of kilometers. Maars and tuff rings are broad, low-rimmed craters formed during powerful explosive eruptions involving magma-water interaction. The accumulation of material ejected from craters contributes to the formation of their surrounding rims.

Pyroclastic Fall
Explosive eruptions are one of Earth's most dramatic geological processes and have been the subject of the brush and pen for thousands of years. Eruptions can involve new magma (referred to as juvenile material) or the ejection of pre-existing rock and debris from around the vent. They can range from minor, relatively passive gas-and-ash emissions reaching only a hundred meters or so above the vent to powerful magmatic explosions that can eject large volumes of material and produce eruption columns that reach tens of kilometers above the volcano.[Read more]

Pyroclastic Flow
Pyroclastic flow is a term that deserves a place in the popular lexicon. One of the most destructive volcanic processes, pyroclastic flows are mixtures of hot gas and rock that travel gravitationally down the flanks of a volcano at highway velocities. Their elevated temperatures and great mobility make them lethal to anything in their path. These flows include a basal component containing blocks of pumice or dense rocks and a dilute cloud of hot gas and ash that rises convectively above the moving flow.
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Magma Meets Water
The interaction of magma with water near Earth's surface can intensify explosive eruptions. Steam-driven explosive eruptions, known as phreatic eruptions, can occur when an ascending magma body encounters groundwater. The ensuing eruptions often do not involve any ejection of new magma, but rather the fragmentation and explosive expulsion of pre-existing rock along the path of the volcanic conduit.
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Submarine Eruption
Eruptions beneath the sea surface are the most common type on Earth, but are significantly underrepresented in the historical record. Submarine eruptions display characteristics that differ from their on-land equivalents. Submarine lava flows are emplaced by incremental extrusion of lobes that are rapidly chilled in contact with the water, forming structures known as pillow lavas. Explosive fragmentation of lavas forms thick fragmental deposits known as hyaloclastites.
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Lava Flows
Lava flows are one of the most visually dramatic manifestations of volcanoes. Their incandescent trace across the landscape has inspired countless paintings and photographs. Two common lava flow types are often produced during the same eruption. Aa and Pahoehoe are Hawaiian terms referring respectively to clinkery, rough-textured, higher-velocity flows, and less viscous, smooth-textured, lower-velocity flows.
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Lahar(Mudflows)
Mudflows are somewhat of a misnomer, because these volcanic flows include not only mud, but debris ranging up to boulder size. The term, however, has been commonly applied to water-saturated volcanic flows and is well entrenched in the literature. The Indonesian word lahar refers to mudflows in volcanic terrain. Lahars can occur both during an eruption and as secondary flows long after an eruption is over, as rainfall remobilizes volcanic ash deposits.
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Geothermal Activity
Ascending hot magmatic fluids mix with near-surface groundwater to form active geothermal systems with dramatic surface features such as hot springs, solfataras, fumaroles, mudpots, geysers, and hot acid lakes. These hydrothermal features are common at stratovolcanoes and young calderas and can persist for hundreds of thousands of years. Active hydrothermal systems often precipitate colorful deposits of native sulfur and other minerals.
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