Geology 1403 (Igneous rocks)

Igneous rock

Cooling and solidification of magma, melting of existing rock

Lava

Magma that reaches the surface

Extrusive

Solidifies at the surface (volcanic)

Intrusive

Solidifies below the surface (plutonic)

How does one classify an igneous rock?

Based on texture and mineral composition

Igneous texture

size, shape, and arrangement of mineral grains, it is determined mainly by the cooling history

What are the different type of igneous rock textures?

Phaneritic, aphanitic, porphyritic phaneritic, porphyritic aphanitic, glassy, vesicular, pyroclastic, pegmatitic

Phaneritic

Crystals large enough to see with naked eye, slow cooling, intrusive origin

Aphanitic

Some crystals to small to been seen with naked eye, fast cooling, extrusive origin

Porphyritic

Crystals of certain minerals in rock are larger than others, period of slow cooling followed by rapid cooling

Phenocryst

Larger crystals in a porphyritic rock

Groundmass

Smaller crystals in a porphyritic rock

Glassy

lava cooled to quickly for any crystals to form

Vesicular

Has air holes due to gases escaping from lava

Pyroclastic

Composed of rock fragments ejected from a volcano

Pegmatitic

Crystals all larger than a centimeter

Mineral composition

The composition of magma determines what types of rocks may form from it.

What is the component of all magma?

Silica

What determines the melting temperature of a rock to form magma?

Heat, pressure, amount of water in the rock

Mafic

Igneous Rock containing large amounts of Iron and Magnesium, dark in color

Felsic

Describes magma or igneous rock that is rich in feldspars and silica and that is generally light in color.

Geothermal gradient

Rate at which temperature increases with depth in the Earth

Decompression melting

melting due to a drop in confining pressure that occurs as rock rises

What are the types of igneous rocks that characterize the continental crust?

Granite and rhyolite

What are the types of igneous rocks that characterize the oceanic crust?

Basalt and gabbro?

What is the most common igneous rock formed at a divergent plate boundary?

Basalt

Partial melting

Incomplete melting of rock produces a magma from minerals with the lowest melting temperatures

Magmatic differentiation

The formation of one or more secondary magmas from a single parent magma

Plutons

Bodies of igneous rock that crystallized underground

What are the different intrusive rock structures?

Batholith, dike, sill

Batholith

Large, irregularly shaped body of igneous rock that cuts across the layering of the rock into which it intrudes

Dike

Tabular pluton thats nearly vertical, often occurring where magma solidified in a network of fractures

Sill

Horizontal tabular pluton that solidified between layers of horizontal rock

Why are magmas that form in subduction zones more felsic than those forming oceanic crust at divergent plate boundaries?

Due to the consumption of crust as it moves upward

Whats the effect of pressure on the melting temperature of a dry rock?

High pressure equals high melting temperature

Whats the effect of pressure on the melting temperature of a wet rock?

High pressure equals low melting temperature