Geol. Ch. 5 study guide

Which of the three lava types are typically associated with lava domes?

Basalt to rhyolite

What type of lava is typically associated with fissure eruptions?

Basaltic

What lava type typically constructs shield volcanoes?

Basaltic lava

Pillow Lava

Basaltic lava that solidifies in an underwater environment and develops a structure that resembles a pile of pillows.

Describe the characteristics of a basaltic magma in terms of temperature, composition and type of eruption.

Basaltic magma - 1000 to 1200oC - high in Fe, Mg, Ca, low in K, Na - violent eruption

Cinder cones are primarily constructed of what material?

Basaltic pyroclastic

Crater

The depression at the summit of a volcano, or that which is produced by a meteorite impact.

Describe composite/stratovolcanoes.

Erupt both lavas and pyroclastic material Slopes composed of lava flows alternating with layers of pyroclastic deposits Steeper slopes than shield volcanoes, Common along convergent plate boundaries

How do volcanic eruptions affect climate?

Explosive eruptions emit huge quantities of gases and fine-grained debris

Nuee Ardante

Incandescent volcanic debris buoyed up by hot gases that moves downslope in an avalanche fashion

How are dikes and sills similar?

The difference between a dike and a sill is that dykes are formed across or vertical to the rock while sill are on horizontal cracks. However, they are both sedimentary in nature. Both are intrusive, where magma flows through the outer crust. They are relatively thin.

Scoria

Vesicular ejecta that is the product of basaltic magma

Stratovolcano

also known as a composite volcano is a conical volcano built up by many layers (strata) of hardened lava, tephra, pumice, and volcanic ash.

Pegmatites

coarse-grained veins that cut across finer-grained rock, typically felsic in composition.

Interaplate Volcanism

Igneous activity that occurs within a tectonic plate away from plate boundaries.

What type of volcanic eruption produces pyroclastic material?

A fissure eruption

Sheild Volcano

A broad, gently sloping volcano built from fluid basaltic lavas.

Lava Dome

A bulbous mass associated with an old-age volcano, produced when thick lava is slowly squeezed from the vent; may act as plugs to deflect subsequent gaseous eruptions.

Island Arc

A chain of volcanic islands generally located a few hundred kilometers from a trench where there is active subduction of one oceanic plate beneath another.

Hot Spot

A concentration of heat in the mantle, capable of producing magma that, in turn, extrudes onto Earth's surface. The intraplate volcanism that produced the Hawaiian Islands in one example.

Pyroclastic Flow

A highly heated mixture, largely of ash and pumice fragments, traveling down the flanks of a volcano or along the surface of the ground

How does a laccolith differ from a sill?

A laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base. A sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock.

Caldera

A large depression typically caused by collapse or ejection of the summit area of a volcano.

Batholith

A large mass of igneous rock that formed when magma was emplaced at depth, crystallized, and subsequently exposed by erosion.

Pahoehoe Flow

A lava flow with a smooth-to-ropy surface.

Pumice

A light-colored, glassy vesicular rock commonly having a granitic composition.

How does composition (silica content) affect the viscosity of magma?

A magma with low silica (<45%) will be runny and so the eruption will not be explosive. A magma with high silica (>60%) will be quite viscous anso the eruption will be violent.

Mantle Plume

A mass of hotter-than-normal mantle material that ascends toward the surface, where it may lead to igneous activity. These plumes of solid yet mobile material may originate as deep as the core-mantle boundary.

Laccolith

A massive igneous body intruded between preexisting strata

Viscosity

A measure of a fluid's resistance to flow

How is viscosity related to the explosiveness of an eruption?

A more viscous (thick) magma will produce a more violent eruption

What are the factors that determine the explosiveness of a volcanic eruption (i.e. composition, temperature and dissolved gases)? How does each of these factors affect the explosiveness of an eruption?

A more viscous (thick) magma will produce a more violent eruption This is controlled in part by the concentration of silica in the magma. A magma with low silica (<45%) will be runny and so the eruption will not be explosive. A magma with high silica (>60%) will be quite viscous and so the eruption will be violent. The temperature of the magma is also a factor. A hotter magma will be less viscous One other factor is the amount of dissolved volatiles in the magma (chlorine gas, water and other gases). A magma with a lot of dissolved gases will form a lot of large bubbles. It is the bursting of these bubbles that causes an eruption to be explosive, so the more volatiles in the magma, the more violent the eruption

Conduit

A pipelike opening through which magma moves toward Earth's surface. It terminates at a surface opening called a vent.

What are plutons?

A pluton is a body of intrusive igneous rock (called a plutonic rock) that is crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, stocks, dikes, sills, laccoliths, lopoliths, and other igneous bodies.

Stock

A pluton similar to but smaller than a batholith.

How does volcanic material ejected into the atmosphere affect incoming solar radiation?

A portion of the incoming solar radiation is reflected and filtered out

Cinder Cone

A rather small volcano built primarily of ejected lava fragments that consist mostly of pea- to walnut-size lapilli.

Volcanic Breccia

A sedimentary rock composed of angular fragments that were lithified.

How does a batholith differ from a stock?

A stock is an intrusive igneous rock that has an exposed surface area of less than 100 km2 while a batholith has an exposed surface area greater than 100 km2

Pluton

A structure that results from the emplacement and crystallization of magma beneath the surface of Earth.

Sill

A tabular igneous body that was intruded parallel to the layering of preexisting rock.

Dike

A tabular-shaped intrusive igneous feature that cuts through the surrounding rock.

Concordant

A term used to describe intrusive igneous masses that form parallel to the bedding of the surrounding rock.

Discordant

A term used to describe plutons that cut across existing rock structures, such as bedding planes.

Aa Flow

A type of lava flow that has a jagged, blocky surface

Fumarole

A vent in a volcanic area from which fumes or gases escape.

Pipe

A vertical conduit through which magmatic materials have passed.

What are fissure eruptions?

A volcanic eruption originating along an elongate fissure rather than a central vent

How does a volcanic neck form?

A volcanic neck forms inside a volcano when the heat and pressure from below are reduced so that the magma inside the volcano's vent solidifies. Over a period of a few hundred thousand years, the surrounding softer earth erodes away, leaving the volcanic neck standing alone.

Composite Volcano

A volcano composed of both lava flows and pyroclastic material.

How does a caldera form?

After violent eruption, the empty magma chamber below the vent no longer able to support its roof. Overlying rock collapses to form a large depression. Resulting caldera many times the size of original crater. Can range from a few kilometers up to 50 km in diameter.

Fissure Eruption

An eruption in which lava is extruded from narrow fractures or cracks in the crust.

What is Shiprock, New Mexico?

An exposed volcanic neck, a giant volcano plug

Xenolith

An inclusion of unmelted country rock in an igneous pluton

Volcanic Neck

An isolated, steep-sided, erosional remnant consisting of lava that once occupied the vent of a volcano

What are cinder cones?

Built from ejected lava (mainly cinder sized) fragments. Pyroclastic eruptions sometimes associated with lava flows. Steep slope angle, small size and frequently occur in groups.

How does a caldera differ from a volcanic crater?

Caldera and volcanoes that form craters in the ground. An enormous magma chamber bulges up beneath the ground from the extremely high pressures of the trapped gases within. Ring-shaped cracks form outward from the magma chamber toward the surface and these act as relief valves for the magma to escape. Once the accumulated pressure has been sufficiently released through a serious of extremely powerful pyroclastic and plinian eruptions, the ground above the magma chamber subsides or caves in, leaving a large depression.

Where in the U.S. would you find flood basalt today?

Columbia river, Northwest

What are pyroclastic deposits?

Consist of an accumulation of tiny rock fragments, glasses and mineral grains • Form from magmas that contain relatively abundant water and gases • Wet, gaseous magma erupts explosively • Lava and any surrounding rock shattered into fragments

Describe the characteristics of a shield volcano.

Constructed from successive flows of basaltic lava - Lavas are fluid and flow easily over great distances - Broad, gentle slopes, - classic type of volcano in Hawaii.

Which of the three types of plate boundaries is characterized by composite/stratovolcanoes?

Convergent plate boundaries.

Lahar

Debris flows on the slopes of volcanoes that result when unstable layers of ash and debris become saturated and flow downslope, usually following stream channels.

Flood Basalt

Flows of basaltic lava that issue from numerous cracks or fissures and commonly cover extensive areas to thicknesses of hundreds of meters

What are volcanic domes?

Formed by highly viscous felsic lavas, Steep-sided and small: ~100's m wide, Associated with violent eruptions

Where are shield volcanoes found today?

Hawaiian Islands

How does temperature affect the viscosity of magma?

High temperature=low viscosity Low temperature=high viscosity

Describe a pyroclastic flow (nuée ardente). Why are they so dangerous and destructive?

Hugh cloud of hot ash, dust, and gases - Cascade downhill at speeds up to 200km/hr - Devastating, causing many deaths - Can bury entire towns

Give two examples of batholiths found today in the U.S.

Idaho and Sierra Nevada

Continental Volcanic Arc

Mountains formed in part by igenous activity associated with the subduction of oceanic lithosphere beneath a continent. Examples include the Andes and the Cascades.

Volcanic Tuff

One of the most common pyroclastic rocks composed of tiny, ash-size fragments that were later cemented together.

Where in the U.S. would you find stratovolcanoes today?

Oregon, Cali, Washington

How does a pahoehoe lava flow differ from an Aa lava flow?

Pahoehoe - very thin, fast fluid flow that takes on a ropey texture as it cools. Aa - Very thick, rocky and rough slow moving

What are volcanic pipes and necks?

Pipes are conduits that connect magma chamber to surface : Volcanic necks are relict volcanoes that stand higher than surrounding landscape: Volcanic rocks occupying the pipes are more resistant to erosion than the surrounding volcanic cone-Resistant pipes are left standing after erosion has removed the cone

What materials comprise these types of volcanoes?

Pyroclastic deposits.

What criteria are used to classify and name pyroclastic material and pyroclastic rocks?

Pyroclasts are the loose fragments associated with explosive eruptions: Bombs (> 64mm): twisted shapes due to solidification from fluid while airborne. Blocks (> 64mm): large chunks of rock broken from within and around the volcanic conduit. Lapillus (2-64mm): individual fragments of minerals, glass or rock 2 - 64 mm in diameter. Ash grains (< 2 mm): mineral, glass and rock fragments less than 2 mm in diameter

Which two of the three lava types are most likely to erupt explosively? Why?

Rhyolitic and basaltic because of low viscosity

How does Rhyolitic lava differ from andesitic lava and basaltic lava in terms of temperature, composition, viscosity, and type(s) of eruptions?

Rhyolitic: erupt at lower temp (800 - 1000 C), contains more silica (>65 wt% SiO2), more sluggish, flows slowly and builds thicker lava flows than does basalt. Crystallizes mostly felsic minerals (K-feldspar, Na-feldspar, quart, etc) and solidifies into rhyolite. Andesitic: Silica content intermediate between basalt and rhyolite (55 - 65 wt% SiO2). Flow characteristics intermediate between basalt and rhyolite. Solidifies into andesite and common along convergent plate boundries

What is the name of a famous cinder cone in Arizona?

Sunset Crater

Vent

The surface opening of a conduit or pipe.

What are flood basalts?

The term flood basalts describe the fluid basaltic lava flows that cover an extensive region in the northwestern United States known as the Columbia Plateau. When silica-rich magma is extruded, pyroclastic flows, consisting largely of ash and pumice fragments, usually result. Extensive flows are called flood basalts: Fluid nature of the lava allows it to flow great distances (~150 km) from the source - Successive flows can build up to great thicknesses (tens to hundreds of meters thick) and cover large areas (thousands of square km)

How does a dike differ from a sill in terms of concordance versus discordance?

The term sill is synonymous with concordant intrusive sheet. This means that the sill does not cut across preexisting rocks, in contrast to dikes, discordant intrusive sheets which do cut across older rocks.

Pyroclastic Material

The volcanic rock ejected during an eruption. Pyroclastics include ash, bombs, and blocks.

Veins

intrusions injected into rock fractures and range from several millimeters to several meters wide.

Volatiles

the group of chemical elements and chemical compounds with low boiling points that are associated with a planet's crust and/or atmosphere