Igneous Rocks Chapter 10.2

Igneous Rocks Igneous Rocks Igneous rocks form when magma cools and hardens. Igneous rocks are categorized into two groups: Intrusive & Extrusive Intrusive igneous rocks form when magma from deep below the earth’s crust cool and harden. We call them intrusive because the magma that forms them intrudes, or enters, into other rock masses beneath the earth’s surface and slowly cools. Extrusive igneous rocks are formed from the rapid cooling of lava, or melted rock on the earth’s surface.

Intrusive (left) & Extrusive (right)

Igneous Rocks Intrusive and extrusive igneous rocks differ mainly in the size of their crystalline mineral masses or grains which is called texture. The texture of igneous rocks is determined by the cooling rate of the magma or lava that formed the rock.

Texture of Igneous Rocks Since intrusive igneous rocks are formed by slow cooling and hardening of magma underground, there is plenty of time to form large, well developed crystalline grains. These rocks are formed of large mineral grains and have a coarse grained texture. An example is granite.

Granite

Texture When extrusive igneous rocks cool and harden rapidly on the surface of the earth there is not much time for large crystalline grains to form, and therefore produce fine grained rock. An example of this type of rock is basalt.

Basalt

Texture Sometimes igneous rocks form from magma that cools slowly at first and then rapidly as it nears the earth’s surface. This type of cooling produces large crystals embedded within a mass of smaller crystals. Porphyritic texture is described by large and small crystals.

Porphyritic texture

Texture When highly viscous, silica rich magma cools rapidly, the resulting rock has no crystals. An example of this type of rock is obsidian or volcanic glass. When magma that contains a large proportion of dissolved gases cools rapidly, the gases may become trapped in the rock and produce a rock full of holes. An example of this type of rock is pumice.

Obsidian & Pumice

Composition of Igneous Rocks The mineral composition of an igneous rock is determined by the chemical composition of the magma from which the rock develops.

Composition Igneous rocks have been divided into three families based on mineral composition: felsic, intermediate, and mafic. Felsic rocks form from magmas that are high in silica. Examples of rocks in the felsic family include granite, rhyolite, and obsidian.

Rhyolite

Composition Mafic rocks form from magmas that are low in silica but rich in iron and magnesium. Examples of rocks in the mafic family include basalt and gabbro. Intermediate igneous rocks contain less quartz than rocks in the felsic family and have a color that falls between felsic and mafic rocks. Examples of rocks in the intermediate family include diorite and andesite.

Gabbro, Diorite, & Andesite

Igneous Rock Structures Igneous rock structures that form underground are called intrusions. Batholiths are the largest of all intrusions and have at least 100 square kilometers of surface exposure. The word batholith means “deep rock.” Batholiths form the cores of many major mountain ranges.

Rock Structures A stock is an intrusion similar to a batholith but covers less than 100 square kilometers. A laccolith forms when magma flows between rock layers and spreads upward pushing the overlying rock into an arc. Laccoliths can sometimes be identified by small dome shaped mountains.

Rock Structures A sill is formed when a sheet of magma flows between the layers of rock and hardens. A sill will lie parallel to the rock layers surrounding it, even if the layers are tilted. A dike is formed when magma forces its way through rock layers by following existing vertical fractures or by creating new ones and solidifies.

Rock Structures

Rock Structures Igneous rock structures that form on the surface are called extrusions. A volcanic neck forms when a dormant volcano weathers down to only the hard, solidified rock in the vent. A lava plateau forms from lava that flows out of long cracks in the earth’s surface, spreads out over a vast area, and fills in valleys covering hills. Once the lava is hardened it forms a plateau.

Lava Plateau & Volcanic Neck