igneous rocks

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

bowen’s reaction series feldspars

iron & magnesium

rich

fractional crystallization

Fractional crystallization - layering of mineral crystals that occurs as magma cools

first minerals that crystallize from magma are last minerals that melt