Volcanoes midterm

Lahar

A "volcanic mud flow" that contains lots of water mixed with old rock debris from a volcano, or newly formed volcanic products. _____ may form by mixing rock material with rain or water released from melting glaciers.

Magma Chamber

A body of liquid rock beneath a volcano.

Composite Volcano

A volcano that forms a high, central main vent, whose flanks are constructed of alternating lavas and pyroclastic rocks. This structure allows relatively steep slopes to be constructed. Also known as a stratovolcano.

Lapilli

Blobs of magma thrown out of a volcano (through the air) that are between 2 and 64 mm in size.

Bomb

Blobs of magma thrown out of a volcano (through the air) that are larger than 64 mm in size.

Gas Eruption

Eruptions that simply vent volcanic gasses that were dissolved in the magma at depth. The most common gasses produced by volcanoes are water vapor, sulfur dioxide, and carbon dioxide (the last two can be deadly).

Lava Dome

Formed by slow extrusion of highly viscous magma (often rhyolite). Small pyroclastic flows are often associated with growth of ___ due to collapse of hot blocks from over-steepened slopes on the dome.

Littoral Core

Formed during hydrovolcanic eruptions, these cones are built through piling up lava flows where they meet large bodies of water (lakes or ocean) and form mounds of exploded lava. Lava flows do not emanate from them.

Pyroclastic

From the root "fire-broken", describes the products of very violent volcanic eruptions where crystals, glass (quenched, rapidly-chilled magma), and rock fragments are highly broken.

Magma

Liquid (melted) rock that still lies below the surface (not yet erupted).

Nuees Ardentes

Name given pyroclastic flows by French volcanologist Alfred Lacroix, when he observed "glowing avalanches" or "glowing clouds" that did not go up into the sky but followed the ground in the 1902 eruption of Mount Pelee.

Shield Volcano

Named for the broad, curving slope of a warrior's shield, ___ have very large bases and gentle slopes. They are generally built by low viscosity basaltic magma (in contrast to the more viscous andesite to rhyolite magmas that build composite volcanoes). ___ are the largest volume volcanoes on the Earth. The most famous ___ are those of the Island of Hawaii

Volcanic Arc

Where oceanic crust is being subducted into the mantle. The arc-like distribution of volcanoes in this setting reflect the arc-like shape of magma generation locations in the mantle, where the subducted plate is warm enough deep in the mantle to either melt or induce melting in the mantle above the subducted plate. The volcanoes do not form a linear (straight) line because the subducted plate is not flat, but is instead curved because the Earth is a sphere.

Rift Zone

This is an extensional plate boundary, but unlike the mid-ocean ridges, a ___ forms on the continents where two continents are separating. The most famous example of this is the Red Sea, where Africa is separating from Saudi Arabia.

Hot Spot (Mantle Plume)

This plume of upwelling mantle often produces volcanoes that are NOT at plate boundaries, the most famous of which is Hawaii, which is in the middle of the Pacific tectonic plate.

Sulfur Dioxide

A gas that is minor in abundance in the atmosphere, but common in magmas at depth. As a magma body ascends in the crust, or a magma body gets larger as it accumulates batches of magma over time, the amount of _ emissions from the volcanic vent may increase. However, in a high-viscosity rhyolite magma, degassing of _ may be difficult and so the trends of _ levels with time may be hard to interpret. In fact, sometimes _ emissions actually decrease toward an impending eruption because a sticky plug has formed at depth that prevents degassing - this is quite a dangerous situation and may be indicative of a very explosive eruption; such a situation was observed in the 1991 eruption of Mt. Pinatubo.

Vesicle

A hole in a volcanic rocks (cinder or pumice) that represents bubbles that formed during exsolution (vesiculation or "boiling") of dissolved gasses in the magma during an eruption.

Caldera

A large depression in the center of a volcano.

Pumice

A low-density volcanic rock that has many bubble holes (vesicles) that form due to rapid exsolution ("boiling off") of dissolved gas during eruption.

Sill

A magma conduit or flat "mini" magma chamber that parallels the preexisting rock layers of the volcano. Formation of a ___ often reflects inflation of the volcano as it is filled with magma.

Dike

A magma conduit that may have fed a vent on the surface (not all reach the surface). A ___ cuts across the preexisting rock layers of the volcano. In some volcanoes, the ___ radiate out from the central vent, reflecting homogeneous inflation of the volcano as it was filled with magma. Often the magma that slowly cools in the ___ forms more resistant rock (as compared to the broken up lavas or pyroclastic rocks of the flanks of the volcano), and ___ in old eroded volcanoes often form prominent ridges.

Extensional Boundary

A plate tectonic boundary where two of the Earth's plates are separating away from each other.

Single Eruption

A series of eruptive phases that are clearly separated from longer periods of low or no activity. May last days to months.

Eruptive Phase

A series of eruptive pulses of similar type, lasting hours to days.

Eruptive Pulse

A single explosion during an eruption, lasting seconds to minutes.

Maar

A small volcano that has a wide, flat crater and was formed by steam explosions when magma and groundwater come into contact (hydrovolcanic eruption). Also called a "tuff cone".

Harmonic Tremor

A special type of earthquake pattern that is only associated with volcanic systems. Reflects a continuous resonance or vibration of the volcanoes plumbing system as magma moves through its conduits at depth. Often indicates a volcanic eruption is imminent.

Earthquake

A sudden release of energy in the Earth, caused by the build up of strain (deformation), followed by breakage of the rock.

Pyroclastic Flow

A superheated flow of gas, rocks, minerals, and ash (glass shards) that tends to travel down slopes and valleys of a volcano. Speeds can reach 200 km/hr and the flows can travel up to 100 km from the vent. Small ___ may occur by collapse of lava domes or direct eruption from the vent. Large ___ occur during plinian eruptions, through collapse of the high-altitude eruption cloud or direct eruption from central or perimeter ("ring") vents.

Pahoehoe

Basaltic lava that has a smooth "ropy" shape, reflecting emplacement while still hot and relatively fluid (low viscosity). Relatively fast-moving basaltic lava flow. Most common near the vent.

Hawaiian Eruption

Because basaltic magmas have such low viscosities, the volcanic forms produced include long, smooth lava flows that are distinct from the volcanic forms produced by eruption of higher viscosity andesite and rhyolite magmas. Near-vent eruption styles include lava fountains and some ejection of magma blobs as bombs and lapilli. Lava flows are common, both on the surface, or beneath crusted-over chilled tops, which are called lava tubes. Eruptions at the summits may include formation of lava lakes, followed by draining and eruption on the lower flanks of the volcano. Flank eruptions are often along rift zones producing long "curtains of fire" of erupting magmas.

Viscosity

Defined as the "resistance to flow". The increase in ___ from basalt to andesite to rhyolite magmas is mostly due to the increase in silica in this series, which greatly increases the complexity of the structure of the magma as it evolves toward rhyolite. This increase in ___ is the primary reason rhyolite magmas erupt far more explosively (and less predictably) than basaltic magmas.

Hydrovolcanic Eruption

General description of eruptions that are formed through interaction of water (either groundwater of ocean water) and magma. ___ include "littoral" eruptions, which reflect interaction of an incadescent lava flow with water. Phreatomagmatic eruptions are ___ that involve direct contact between magma beneth a volcano with ocean or ground water.

Sediment Gravity Flow

General term applied to a cohesive mixture of sediment (volcanic fragments) and gasses that allows flowage by gravity. Without the cohesion of the sediment load, volcanic gasses would escape upward, and the mass would not flow downhill.

Flamme

Glassy collapsed pumice lumps in an ignimbrite.

Hyaloclastite

Highly fractured glassy rims of lavas erupted under the ocean due to rapid quenching and explosive magma-water interaction. May also form where volcanoes erupt beneath glaciers (such as Iceland).

Subduction Zone

If new oceanic crust is created at extensional plate boundaries, and if the Earth is not expanding, then oceanic crust must ultimately be destroyed by return into the mantle from which it came. Return of oceanic crust into the mantle occurs at ___. When this crust (a tectonic plate) descends into the mantle, it melts and/or adds water (obtained when the rocks were lying on the ocean floor) to the mantle, which also induces melting.

Lava

Moving magma (molten rock) on the surface of the Earth. May also refer to the hardened rock after cooling.

Aa

Rough, blocky basaltic lava that reflects emplacement when relatively cool. Commonly far from the vent. Relatively slow moving and steep volcanic front.

Cinder Cone (Scoria Cone)

Small volcanic cones of commonly basaltic rock, often representing a single eruption or period of limited eruptive duration.

Silica Tetrahedron

Structurally, the most stable way to combine these elements is in the form of a four-sided pyramid, where each corner is an oxygen atom. The silicon atom is in the center of the pyramid. ___ is the fundamental building block of the vast majority of minerals in the Earth's mantle and crust. Because this structure (written as [SiO4] 4-) has a -4 charge, the ___ are linked together by positively charged atoms like sodium (Na), potassium (K), magnesium (Mg), calcium (Ca), and iron (Fe) to form minerals.

Mantle

The "middle layer" of the Earth, lying between the crust and the core. The upper part is the source for the vast majority of magmas in the Earth, produced by melting during upwelling.

Asthensophere

The "plastic", near-outer layer of the Earth's mantle that can move by convection as part of the plate tectonic system.

Debris Avalanche

The catastrophic collapse of Mt. St. Helens during the climactic 1980 eruption vividly demonstrated how volcanoes can catastrophically collapse, producing large ___ deposits. It has since been recognized that catastrophic collapse is a common occurrence during the lifetimes of many composite or stratovolcanoes, and some exceptionally large collapses have been recognized from ancient ____ deposits.

Plate Tectonics

The concept that the surface of the Earth is composed of large individual plates (about a dozen major ones) that slide around on top of the "plastic" (asthenosphere) mantle.

Pyroclastic Surge

The lighter, gas-rich portion of a pyroclastic flow, which can ride over ridges, in contrast to the heavier component of pyroclastic flows (due to high abundances of volcanic fragments), which tend to hug the valleys. The paths of ___ are very unpredictable.

Epicenter

The location of an earthquake when plotted on the surface of the Earth (i.e.: a map); this is a point directly above the earthquake, which would generally be between 1 and 15 km below the surface in most earthquakes that are directly related to volcanic eruptions.

Silica

The molecule SiO2, which in its pure form is the mineral quartz (the mineral quartz is only found in rhyolite). ___ is enriched during evolution of magmas from basalt through andesite to rhyolite compositions, and this enrichment greatly increases the magma's viscosity and decreases its temperature - properties that produce very explosive eruptions.

Basalt

The most "primitive" volcanic rocks (less than 55% silica), reflecting near-direct derivation from melting the mantle (asthenosphere). Most common rock type on Earth, and forms the crust under virtually all oceanic basins. The most common volcanic rock erupted at mid-ocean ridges and hot-spot volcanoes.

Rhyolite

The most evolved volcanic rock type, crystallizing from magma that has greater than 70% silica. __ are produced by extreme crystallization of basaltic magmas and/or melting of the crust. They are the lowest temperature and highest viscosity magmas. Because they have high viscosities, they often do not smoothly exsolve ("boil off") dissolved gas in the magma as the magma ascends through the crust. Therefore, dissolved gasses often boil out of ___ magma only when it reaches very shallow levels in the crust, at which point the still over-pressured gas is released catastrophically. The most violent eruptions, pyroclastic eruptions, are therefore generally associated with ___ magmas. Most ___ are restricted to the continental crust

Crust

The outermost layer of the solid Earth.

Welding

The process of compaction and "sticking together" of glass shards in an ash-rich pyroclastic flow after the flow has come to a stop. Compaction of the interior of the flow, which may be up to 500 oC after it comes to a rest, molds the glass shards together, making a very hard, relatively dense, and strong rock.

Convection

The process of rising material due to heating from below (which decreases density), followed by cooling near the outer surface of the system, which then induces sinking (due to an increase in density from cooling), and then starting the cycle again with heating at depth.

Density

The ratio of weight (grams) per volume (cm3 - "cubic centimenter") of a substance.

Lithosphere

The rigid, outermost solid part of the Earth. Because the ___ is defined as the rigid part of the Earth virtually all earthquakes are generated in the ___. All of the oceanic and continental crust is included in the ___, as well as the outermost part of the mantle that is sufficiently cold so that it behaves rigidly. Synonymous with a tectonic plate.

Geophysics

The scientific discipline that uses the physical properties of the Earth (magnetic field, gravity, behavior of earthquake waves, etc.) to study its inaccessible interior.

Ignimbrite

The solid rock deposited by a pyroclastic flow.

Glass Shard

The solid, quenched magma that existed as bubble walls in the magma immediately prior to eruption. Most commonly forms during violent pyroclastic eruptions, and literally reflects the explosive disintegration of the magma during eruption. A pile of ___ would be collectively called "ash".

Focus

The three-dimensional location of an earthquake, described in terms of latitude and longitude on a map, as well as depth below the surface.

Plinian Eruption

The top of the eruptive scale in terms of very violent and large-volume eruptions. Exclusively associated with andesite to rhyolite magmas. Produces huge, ash-rich eruption clouds that may ascend as high as 50 km into the atmosphere, as well as super-hot pyroclastic flows that flow down the side of a volcano for up to 100 km distances, traveling at speeds up to 100's of km/hour. Collapse of large ___ columns can produce large pyroclastic flows that descend the flanks of the volcano, producing ignimbrite deposits. Large ___ (greater than about 10 km3 of magma erupted) often form pyroclastic calderas during collapse of the volcano.

Mid-Ocean Ridge

The world's largest volume, longest, and highest mountain ranges circle the globe beneath the oceans as ridges that are 1,000's of kilometers long along individual segments. The center of these ridges are where two plates a separating (an extensional boundary), and the greatest volumes of magma (nearly all basalt) are produced on the Earth: about 20 km3/year.

Tectonic Plate

There are about a dozen large (and many more small) individual plates moving on the Earth's surface. ___ are defined for the rigid outer part of the Earth, and so are identical to the lithosphere.

Black Smoker

Underwater hydrothermal ("hot water") vents at mid-ocean ridges that reflect expulsion of superheated ocean water through the oceanic crust. As ocean water moves down through the highly fractured oceanic crust that surrounds mid-ocean ridge magma chambers, it is heated and dissolves metals from the surrounding crust. As this heated water ascends back through the crust, it cools and deposits these dissolved metals from black, "sooty" jets, forming chimney stacks on the surface of the crust.

Pillow Lava

Unusual, circular lava form that results from undersea eruptions. Because of the efficient quenching effect of large water bodies, even fluid basaltic magma erupted underwater does not form the long, fluid-like lavas that are characteristic of basaltic eruptions on land. Instead, lavas erupted under water have bulbous, "pillow-like" shapes.

Strombolian Eruption

Up the eruptive scale from Hawaiian eruptions in terms of percentage of rock fragments and magma blobs thrown out (but the volume of volcanic material erupted is much than that of Hawaiian eruptions). Forms very cinder-, lapilli-, and bomb-rich volcanic deposits (generally basaltic in composition) of relatively small volume.

Base Surge

Very fast, horizontally-moving explosion "front" or "wave" that was discovered during above-ground nuclear tests. Forms finely-bedded deposits that are quite distinct from coarse-grained and thicker pyroclastic flow deposits that are usually thought of when considering violent volcanic eruptions. ___ are very dangerous because the fast-moving explosion wave moves more quickly than pyroclastic flows.

Andesite

Volcanic rock of intermediate silica content (55 to 70% silica), reflecting greater magma evolution than basalt, but not as great as rhyolite. ___ is common in continental volcanoes that are related to subduction of oceanic crust beneath continental crust.