Igneous Rocks

Dr. Pamela Gore
Georgia Perimeter College

1. Identify igneous rocks by their physical and chemical properties.
2. Tell which minerals are present in various types of igneous rocks.
3. List and describe the characteristics of intrusive igneous rocks. including textures.
4. List and describe the characteristics of extrusive igneous rocks, including textures.
5. Explain the origin of various types of igneous rocks, including cooling history.
6. Explain how igneous rocks are classified.

Origin of Igneous Rocks

Igneous rocks form by cooling and crystallizing from hot molten magma or lava.

• Magma - hot molten rock below the Earth's surface
• Lava - hot molten rock that erupts onto the Earth's surface through a volcano or crack (fissure)

Igneous rocks that cool and crystallize on the Earth's surface are called extrusive igneous rocks.
Another name for extrusive igneous rocks is volcanic igneous rocks (named for Vulcan, Roman god of the fire and forge).

Cooling Rates

• Quick cooling = fine grains.
  Extrusive igneous rocks tend to be fine grained or glassy.
  Sometimes they are vesicular with tiny holes formed by the release of trapped gases.
  Other extrusive igneous rocks have a fragmental or pyroclastic texture composed of pieces of volcanic rock and ash welded together by heat. Lava cools more quickly because it is on the surface.

• Slow cooling = coarse grains.
  Intrusive rocks tend to be coarse grained with intergrown crystals ranging from several millimeters to several inches in diameter.
  Magma cools more slowly because it is deep within the Earth where temperatures are high.

Igneous rocks are classified or named on the basis of their texture and their composition.

Igneous textures:

• Glassy - instantaneous cooling
• Obsidian = volcanic glass

Obsidian

• Aphanitic - fine grain size (< 1 mm); result of quick cooling

Rhyolite

• Basalt
• Rhyolite
• Andesite
• Phaneritic - coarse grain size; visible grains (1-10 mm); result of slow cooling

Granite - polished

• Granite
• Diorite
• Gabbro
• Pegmatitic - very large crystals (many over 2 cm)
• Granite pegmatite or pegmatitic granite
• Porphyritic- Mixture of grain sizes caused by mixed cooling history; slow cooling first, followed by a period of somewhat faster cooling.
• Terms for the textural components:
• Phenocrysts - the large crystals
• Groundmass or matrix - the finer crystals surrounding the large crystals. The groundmass may be either aphanitic or phaneritic.
• Types of porphyritic textures:
• Porphyritic-aphanitic
• Porphyritic-phaneritic
• Origin: mixed grain sizes and hence cooling rates, imply upward movement of magma from a deeper (hotter) location of extremely slow cooling, to either:
• a much shallower (cooler) location with fast cooling (porphyritic- aphanitic), or
• a somewhat shallower (slightly cooler) location with continued fairly slow cooling (porphyritic-phaneritic).
• Rock name = porphyry
• Granite porphyry or porphyritic granite (porphyritic-phaneritic) - phenocrysts usually potassium feldspar

Granite porphyry

• Andesite porphyry or porphyritic andesite (porphyritic-aphanitic) - phenocrysts usually hornblende (amphibole)



• Rhyolite porphyry or porphyritic rhyolite (porphyritic-aphanitic)
Rhyolite Porphyry

• Vesicular - contains tiny holes called vesicles which formed due to gas bubbles in the lava or magma. Very porous. May resemble a sponge. Commonly low density; may float on water.
• Vesicular basalt - basalt with a vesicles, which may be quite large. Sometimes lined with crystals to form geodes.

Vesicular basalt


Vesicular basalt with olivine phenocrysts, building stone at Hawaiian Volcano Observatory, Big Island of Hawaii

• Pumice - light in color; white to gray; may be glassy or dull. Fully riddled with holes. Very sponge-like. Floats. Used as an abrasive. (Pumice stone, Lava Soap).

Pumice

Pumice

• Scoria - dark in color; brown, black, or dark red; similar to vesicular basalt but is fully riddled with holes to form a spongy mass. (May find in barbecue grills as lava rock).

Scoria

• Pyroclastic or Fragmental - pieces of rock and ash come out of a volcano and get welded together by heat. May resemble rhyolite or andesite, but close examination shows pieces of fine-grained rock fragments in it. May also resemble a sedimentary conglomerate or breccia, except that rock fragments are all fine-grained igneous or vesicular.
• Tuff - made of volcanic ash
• Volcanic breccia - contains fragments of fine-grained igneous rocks that are larger than ash.

Composition of Igneous Rocks

Igneous rocks can be placed into four groups based on their chemical compositions:

1. Sialic (or granitic or felsic)
1. Dominated by silicon and aluminum (SiAl)
2. Usually light in color
3. Characteristic of continental crust
4. Forms a stiff (viscous) lava or magma
5. Rock types include:
1. Granite

Granite

2. Rhyolite

Rhyolite

6. Minerals commonly present include:
1. potassium feldspar (generally pink or white)
2. Na-plagioclase feldspar (generally white)
3. quartz (generally gray or colorless)
4. biotite
5. amphibole?
6. muscovite?
2. Intermediate (or andesitic)
1. Intermediate in composition between sialic and mafic
2. Rock types include:
1. Andesite (aphanitic)
2. Diorite (phaneritic)

Diorite

3. Minerals commonly present include:
1. plagioclase feldspar
2. amphibole
3. pyroxene
4. biotite
5. quartz
3. Mafic (or basaltic)
1. Contains abundant ferromagnesian minerals (magnesium and iron silicates)
2. Usually dark in color (dark gray to black)
3. Characteristic of Earth's oceanic crust, Hawaiian volcanoes
4. Forms a runny (low viscosity) lava
5. Also found on the Moon, Mars, and Venus
6. Rock types include:
1. Basalt (aphanitic)

Basalt

2. Gabbro (phaneritic)

Gabbro

3. Diabase - texture intermediate between basalt and gabbro; characteristic of Early Mesozoic dikes in eastern North America.
7. Minerals commonly present include:
1. Ca-plagioclase feldspar
2. pyroxene
3. olivine
4. amphibole
4. Ultramafic
1. Almost entirely magnesium and iron silicates (ferromagnesian minerals)
2. Rarely observed on the Earth's surface
3. Believed to be major constituent of Earth's mantle
4. Commonly found as xenoliths in basaltic lavas
5. Rock types include:
1. Peridotite (phaneritic)
1. dominated by olivine - the birthstone is Peridot, which gives its name to Peridotite

Peridotite

6. Minerals commonly present include:
1. Olivine is dominant. (Olivine is olive green).
2. may have minor amounts of pyroxene and Ca-plagioclase

Other types of igneous rock:

Syenite

A polished syenite called larvikite with centimeter- to inch-scale gray to blue plagioclase crystals. The industrial name for the rocks is "blue pearl". Photographed in an above-ground cemetery in New Orleans, LA

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This page created by Pamela J. W. Gore
Georgia Perimeter College, Clarkston Campus, Clarkston, GA
October 2, 1995
Modified January 24, 1997
Modified September 21, 1998
Modified July 17, 1999
Modified May 12, 2000
Modified May 30, 2000
Modified June 4, 2000