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Post on 13-Mar-2019
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what are igneous rocks?
Magma - molten rock below earth’s surface
lava - magma that flows out onto earth’s surface
igneous rocks - rocks that form when lava or magma cools and crystallizes
composition of magmatype of igneous rock depends on composition of magma
Magma is a mix of molten rock, dissolved gases, and minerals
silica is most abundant element in magma - has greatest effect on characteristics
affects melting temperature and viscosity (resistance to flow)
composition of magmamagma is classified based on silica content
Types: basaltic, andesitic, or rhyolitic
magma formationfour factors involved in formation:
pressure
water content
Temperature
mineral content of crust or mantle
pressure
Pressure increases with depth
result of weight of overlying rock
as pressure increases, melting point increases
Ex: rock that melts at 1100°C at Earth’s surface will melt at 1400°C at a depth of 100 km
water contentRocks and minerals contain small amount of water
As magma rises, pressure drops - gases are liberated from magma as bubbles - vesiculation
As water content increases, melting point decreases
TemperatureTemperature increases with depth
known as geothermal gradient
Ex: average geothermal gradient in crust is 25°C/
km, it drops sharply in mantle to as low as 1°C/km
mineral contentAs temperature increases, minerals begin to melt
minerals have different melting points
partial melting
Not all parts of a rock melt at the same temp
partial melting - process whereby some minerals melt at low temperatures while other minerals remain solid
As groups of minerals melt, different elements are added to magma - changing its composition
bowen’s reaction series
Bowen’s reaction series - illustrates relationship between cooling magma and formation of minerals that make up igneous rock
defines order in which minerals will crystallize from magma
as minerals form, their elements are removed from magma, and it becomes more concentrated in silica
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