AS Igneous Processes Flashcards
Adiabatic process
a thermodynamic process in which no heat enters or leaves the system during expansion or compression.
Adiabatic cooling
occurs when crust or mantle material rises, undergoes expansion and the temperature falls; with no loss or gain of thermal energy.
Adiabatic heating
occurs when crust or mantle material descends and temperature rises as it contracts; with no loss or gain of thermal energy.
Divergent plate margins
occur where two plates are moving apart and magma is riding up between them.
Partial melting
occurs where some of the minerals in a rock melt to form a magma.
Hot spots
are formed by a fixed mantle plume bringing magma to the surface.
Convergent plate margins
are where two plates are colliding and magma is formed above a subduction zone or deep in the crust.
Batholith
a very large igneous intrusion, with an undefined depth, in the Earth’s crust.
Decompression melting
accounts for most volcanism, including mid-ocean ridges and hot spots. Decompression of the ultramafic peridotite causes partial melting and produces mafic magmas by decompression melting.
Concordant intrusions
are parallel to the existing beds.
Discordant intrusions
cut across the existing beds.
Country rock
any rock into which an igneous rock intrudes.
Dyke
a discordant, sheet-like intrusion.
Minor intrusions
cool at hypabyssal depth below the surface and include sills and dykes.
Sill
a concordant, sheet-like intrusion.
Diapir
a body of relatively low density material that pierces and rises up through overlying material of a higher density.
Contact
where the igneous rock meets with country rock.
Baked margin
in the country rock where it was heated by the intrusion and altered.
Chilled margin
where the igneous rock has cooled rapidly so it has fine crystals.
Major intrusions
are plutonic and cool deep below the surface and include batholiths.
Plutons
large igneous intrusive bodies. If they are greater than 100 km² then they are called batholiths, usually made of several plutons.
Batholith
a large igneous intrusion which may be an aggregate of plutons.
Metamorphic aureole
a large area around a batholith where the rocks have been metamorphosed.
Contact
where the igneous rock meets the country rock.
Partial melting
occurs when only a portion of a rock is melted. When a rock is heated, those minerals with lower melting temperatures will melt. Those with higher melting temperatures will remain solid.
Stoping
the process that accommodates the magma, as it moves upwards into the country rock, by the mechanical fracturing of the surrounding country rock.
Xenoliths
clasts or blocks of pre-existing rock contained within an igneous rock.
Assimilation
the melting process that incorporates blocks of country rock, freed by stoping, into the magma.
Tiltmeters
designed to measure very small changes in vertical level.
GPS
stands for Global Positioning System and is the radio navigation system that allows the determination of an exact position.
Fumaroles
openings in or near a volcano, through which hot gases emerge.
Cumulate
an igneous rock produced by gravity settling of crystals in a magma body.
Chilled margin
where the igneous rock has cooled rapidly so it has fine crystals.
Baked margin
in the country rock where it was heated by the intrusion and altered.
Palaeosol
a soil horizon that was formed in a past geological age.
Melt
the name given to magma or lava, in the liquid phase.
Aa
lava flows that have a rough, blocky, jagged surface.
Pahoehoe
lava flows that have a smooth or ropy surface.
Scoria
a volcanic rock that is usually mafic but can be intermediate in composition.
Pyroclast
an individual fragment ejected during an eruption. Pyroclastic describes all the fragmental materials formed by explosive eruptions, including bombs, blocks, lapilli and ash.
Nuée ardente
(French, ‘burning cloud’) is an incandescent, gaseous, pyroclastic cloud of magma droplets and ash. When cooled and solidified the deposit is called an ignimbrite.
Pyroclastic flow
is a hot mixture of pyroclastic material and gas. A nuée ardente is a type of pyroclastic flow.
Volcanoes
vents at the surface of the Earth through which magma and other volcanic materials are ejected.
Low viscosity
where magma or lava are fluid and flow freely.
Shield volcanoes
have gentle slopes of less than 10 degrees and a roughly circular shape around a central vent.
Fissure eruptions
where magma reaches the surface along long, linear cracks or fissures.
Submarine eruptions
where magma comes from a vent or fissure on the sea floor.
Composite volcanoes
tall, conical shaped and are composed of alternate layers of lava and ash.
Caldera
a large volcanic crater that has undergone collapse, following an eruption.
Effusive
the term used to describe the fluid, non-explosive, basalt lava.
Volcanic Explosivity Index (VEI)
a measure of the explosiveness of volcanic eruptions, allowing them to be compared. It is a 0 to 8 index of increasing explosivity.
Hawaiian eruptions
have large amounts of very fluid basaltic magma from which gases escape, but few pyroclasts.
Strombolian eruptions
are more explosive with less fluid basalt and andesite lava. They have regular explosions of gas and pyroclastic material.
Vulcanian eruptions
violent with viscous andesitic lava and large quantities of pyroclastic material from large explosions.
Plinian eruptions
extremely explosive with viscous gas-filled andesitic and rhyolitic lava and tremendous volumes of pyroclastic material blasted out.
Isopachyte
a line joining points of equal thickness of a deposit such as ash. The maps may be called isopach maps.
