Magma and Igneous Rocks

magma

found beneath Earth’s surface

Lava

found on the Earth’s surface

decompression melting

occurs when a hot rock at high pressure moves toa shallower crustal level with lower pressure, causing the rock to melt. Characterized by a decrease in pressure while temperature remain constant

Geologic environments where decompression melting occurs

mantle plumes, continental rifts, and divergent plate boundaries

flux melting

occurs in subduction zones. Volatiles are driven from the oceanic crust into the asthenosphere above the subducting plate. Volatiles help break chemical bonds in the rock, creating a melt. Involves the addition of volatiles (flux) to the rock, which lowers its melting point.

Heating Transfer

how rocks surrounding magma chambers can be melted

Mafic melts

relatively high proportion of magmnesium oxide and iron oxide relative to silica

Ultramafic melts

even higher proportion of magnesium oxide and iron oxide relative to silica

Felsic melts

relatively high proportion of silica relative to magnesium oxide and iron oxide

Intermediate melts

a composition that lies b/t mafic and felsic melts

what controls the speed of flow?

viscosity of a liquid affects the speed at which it moves

Viscosity

depends on temp, volatile content, and silica content

Hotter melt

tend to be less viscous than cooler melt

Melt containing more volatiles

have lower viscosity than a dry melt

Mafic melt

less viscous than felsic melt

What the silica content of a melt determines

a melt’s viscosity and the name given to a particular melt

The source rock

dictates the initial melt composition

Partial melting of rocks

makes the melt silica-enriched because felsic minerals melt first

Assimilation

occurs as a melt rises and assimilates the wall rock, changing the composition of the melt

Low viscosity

melts are hotter, have less SiO2 (mafic), and more volatiles

High viscosity

melts are cooler, have more SiO2 (felsic), and less volaties.

Circulating groundwater

removes heat

Fractional crystallization

occurs as minerals crystallize in cooling magmas

Mafic minerals

crystallize first

Felsic minerals

crystallize last

Bowen’s reaction series

indicates the succession of crystallization in cooling magmas

Pyroclastic Flows

deadly, fast-moving avalanches of superheated volcanic ash and debris

Explosive volcanoes

erupt various sizes of debris, ranging from microscopic ash to large pumice

Dikes

run vertically, cut across rock layers and spread rock sideways

Sills

run horizontally, injected parallel to rock layers, typically close to the surface. Result in a change in the elevation of the land surface.

Laccoliths

magma that cannot spread laterally accumulates in a blister-shaped intrusion

Plutons

a blob-shped intrustion that solidify from magma chambers

Batholith

a group of plutons that covers a large area

Rising Magma

moving upward by percolating b/t grains, wedging open cracks, melting, and breaking off blocks of the wall rock

Xenoliths

chunks of wall rock incorporated into the magma

Subduction Zone

many volcanic arcs are part of subduction zone volcanism

Mid-Ocean Ridges

most igneous activity occurs along the mid-ocean ridge

Continental Rift

rift-related lithospheric thinning causes decompressional melting of the asthenosphere

Hot Spots

mantle plume hot spots generate igneous activity independent of tectonic plate boundaries. Hot spot volcanoes track plate movement

Large Igneous Provinces

large areas of mafic melt that appear periodically over geologic time

at subduction zones…

addition of volatiles from the subducting plates initiates partial melting

at mid-ocean ridges and continental rifts…

magma forms because of decompression melting