IGNEOUS ROCKS

Igneous Rocks Originate in Lava and Magma

Extrusive igneous rocks can be in the form of:

lava flows

ash

Intrusive rocks can be: batholiths

dykes

sills

laccoliths

Classification of Rocks

Classification of Rocks

Rocks are aggregates of minerals. Geologists divide rocks into three groups: igneous, metamorphic, and sedimentary.

Igneous rocks crystallize from magma. Metamorphic rocks form by the deformation and/or

recrystallization of pre-existing rock by changes in temperature, pressure, and/or chemistry.

Sedimentary rocks form by weathering and erosion of preexisting rock to make sediment, which is lithified into rock.

ROCK CYCLE

Classifying Igneous Rocks

If the magma erupts at the surface, it is called an extrusive igneous, or volcanic rock

Volcanic rocks are classified by the size, abundance, and type of crystals. If no crystals are visible, geologists call the rock aphanitic. If crystals are abundant, the rock is called porphyritic.

CLASSIFYING IGNEOUS ROCKS

Intrusive igneous rocks are coarse-grained because they cooled slowly inside the Earth.

Extrusive (volcanic) igneous rocks are fine-grained because they cooled quickly at the Earth's surface.

Igneous Rocks

Granite Rhyolite

Larger Grains (Cooled slowly) Small Grains (cooled fast)

Granite and rhyolite contain quartz, orthoclase and plagioclase feldspar,

(light colored minerals) and minor amounts of ferromagnesian (dark

colored) minerals.

GRANITE, GRANITIC ROCKS

Granite comes from silicic magma which cooled slowly underground forming large grains.

BASALT

Basalt is made from mafic magmathat cooled rapidly aboveground and so is fine grained.

Igneous Rocks

Gabbro Basalt

Larger Grains, Slow Cooled Small Grains, Rapid Cooling

Gabbro and basalt contain abundant ferromagnesian minerals (dark

colored) and lesser amounts of plagioclase feldspar. Note the basalt

has gas bubbles (called vesicles by geologists).

Basaltic Columns

OBSIDIAN

Igneous RocksObsidian is a black or

dark-colored volcanic glass, usually of rhyolite composition, characterized by conchoidal fracture. It is sometimes banded.

Obsidian is made from lava that was quenched, often in water.