Igneous Rocks

What is a rock? Rocks are composed of one or more minerals. Rocks composed of only one mineral are monominerallic. Limestone is an example of a monominerallic rock. Most rocks are polyminerallic – made of more than one mineral. Granite is a polyminerallic rock composed of quartz, feldspar, hornblende, and mica. Pure coal and organic limestone are common rocks that do not contain minerals.

Chemical Composition of Rocks Most rocks have a number of minerals in common. There are 2500 different minerals on Earth; Several minerals appear in numerous varieties. Only 10-15 minerals (rock- forming minerals) make up 90% of Earth’s crust.

Types of Rocks Rocks are classified into three groups according to how they are formed – igneous, sedimentary, and metamorphic. Texture and mineral content are also used to distinguish rock types. Igneous rocks form from the solidification of molten rock. Sedimentary rocks form by compaction and sedimentation of sediments. Metamorphic rocks form through recrystallization of existing rock.

Formation of Igneous Rocks Igneous rock form due to the crystallization of molten rock. Molten rock beneath Earth’s surface is called magma. When magma reaches the surface, it is called lava. Most igneous rocks are polyminerallic. Crystal size is proportional to cooling time – the longer it takes, the bigger the crystals Crystal type depends on the temperature of the mineral when its solidifies, and the pressure when the crystals form.

Texture Texture (grain appearance) depends on the size of the crystals and their arrangement in the rock Texture is dependent on the rate of cooling.

Conditions of Igneous Rock Formation Pressure and temperature deep beneath Earth’s surface result in a slow rate of cooling. Slow cooling produces rocks with large crystals:

Fast cooling leads to small crystals….

… or no crystals at all

Igneous rocks are classified by their texture Rocks that cooled slowly have large crystals – they are coarse grained. Rocks that cooled quickly have small crystals – they are fine grained. Rocks that cooled very quickly have no grains – they have a glassy texture Igneous rocks are also grouped by their chemical composition. There is a reference table on the ESRT that you will learn to use…

Features of Igneous Rocks Felsic rocks have lots of potassium feldspar, quartz; they are light in color. Granite forms from felsic rocks. Felsic rocks are more common in the continents. Felsic rocks are rich in silicon and aluminum.

Mafic rocks are found in ocean basins. Mafic rocks are dark in color. Mafic rocks are rich in iron and magnesium. Mafic rocks are more dense than felsic rocks. Gabbro is a classic mafic rock – also used for tabletops.

Igneous Intrusions When igneous rocks solidify below Earth’s surface, they are called intrusions. Intrusive igneous rocks have coarse grains, large crystals over 1 mm in size. The core of many mountains are formed of intrusive rock.

Extrusive Igneous Rocks When rocks solidify at Earth’s surface, they are called extrusive igneous rocks, or volcanic rocks. Lava always forms extrusive rock. Basalt is the most common form of extrusive igneous rock.

The ESRT Classifies Igneous Rocks as Intrusive or Extrusive

Volcanos Magma that reaches the surface through openings forms a volcano. Cinder cones are steep, explosive volcanos. Magma from cinder cones contains gases – carbon dioxide, sulfur dioxide, and water vapor. Some rocks reflect this fact – they are vesicular – they have holes where the gas used to be.

Vesicular Igneous Rocks

Shield Volcanos When lava flows quietly, without explosions, shield volcanos are produced. When lava flows for many miles, a lava plateau is formed.

Structures of Igneous Rocks When magma doesn’t reach the surface, it can force its way into fractures and harden. Large masses of intrusive rocks called plutons are formed. Common types of plutons include sills, dikes, laccoliths, and batholiths

The Palisades Sill

Another Palisades Sill (Grand Canyon)

Dikes

Volcanic Neck

Laccolith

Batholith

Igneous Rock Lab

1 2 3 4 5 6 7 8 9 10 Dark Coarse Intrusive Ocean # Color Texture Formation (Extrusive, Intrusive) Location (Ocean, Land, Intermediate) Minerals present Composition (Felsic/Mafic) Rock Name 1 Dark Coarse Intrusive Ocean Pyroxene, Olivine, P-Feldspar Mafic Gabbro 2 Light Land K-Feldspar, Quartz, Biotite, Amphibole Felsic Granite 3 Intermediate Shore P-Feldspar, Biotite, Amphibole Medium Diorite 4 Fine, vesicular Extrusive P, K Feldspar, Quartz, Biotite, Amphibole Pumice 5 Fine Rhyolite 6 Fine w/large crystals Andesite 7 Black Glassy Obsidian 8 P-feldspar, Pyroxene, Olivine Vesicular Basalt 9 Basalt 10 Brown Scoria