Chapter 3
IGNEOUS ROCKS|
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Chapter 3 IGNEOUS ROCKS |
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Eruption of Kilauea's Pu'u O'o crater, 1983. |
Along with meteorite impacts, the eruption of molten material is one of the most common geological processes in the solar system. Volcanoes are found on Venus, Earth, Moon, Mars, Jupiter’s moon Io, and other moons.
Because igneous rocks such as granite and diorite are composed of tightly interlocking crystals, they are strong rocks and generally make good building materials.
| GRANITE |
22. ANORTHOSITE |
25. ANDESITE |
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20. DIORITE |
23. OBSIDIAN |
26. BASALT |
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21. GABBRO |
24. RHYOLITE |
27. PUMICE |
19. GRANITE
From the Latin word "granum" meaning "grain".
A coarse-grained (phaneritic) felsic intrusive igneous rock.
Common minerals: Quartz, Na-feldspar, K-feldspar, and muscovite.
Accessory minerals: Apatite and magnetite.
Light in color because it is composed of light-colored, felsic minerals.
Granite occurs exclusively as intrusive bodies.
Granite is the plutonic (intrusive) equivalent of rhyolite.
Chemically, granite is high in Si, Na, and K and low in Fe, Mg, and Ca.
From the Greek word "diorizein" meaning "to distinguish or to divide", in reference to the fact that the characteristic mineral, hornblende, is usually identifiable megascopically.
A coarse-grained intermediate intrusive igneous rock.
Intermediate color.
Common minerals: Feldspars and ferromagnesian (mafic) minerals; orthoclase and quartz may be present in small amounts.
Diorite tends to be equigranular and porphyritic types are rare.
Diorite is the plutonic (intrusive) equivalent of andesite.
Named after the town of Gabbro in Tuscany, Italy.
A coarse-grained, mafic, intrusive igneous rock.
Loosely used for any coarse-grained, dark igneous rock.
Common minerals: Ca-feldspar, olivine, augite, and hornblende.
Biotite and magnetite may occur in accessory amounts.
Dark in color because it is composed of dark-colored ferromagnesium (mafic) minerals.
Gabbro is the plutonic (intrusive) equivalent of basalt.
Chemically, gabbro is low in Si, Na, and K and high in Fe, Mg, and Ca.
From the Greek words "annot and orthos" meaning "straight".
A coarse-grained plutonic igneous rock.
Common minerals: More than 90% Na- and Ca-feldspar; the remainder is made up of ferromagnesian minerals.
No volcanic equivalent of anorthosite is known.
Anorthosite is a common rock in the San Gabriel Mountains.
An ancient name for volcanic glass.
Formed when lava chills on the Earth’s surface so quickly that crystals don’t have enough time to form.
Although obsidian can be of any composition, it is commonly felsic in composition, like granite and rhyolite.
Although felsic, most obsidian is black (red, green, and brown-colored obsidian is known too).
Obsidian has conchoidal fracture and a glassy luster.
Use: Sharp implements, jewelry, and art objects.
From the Greek word "rhyax" meaning "stream of lava".
A fine-grained to glassy, felsic volcanic rock.
Light to intermediate color.
Common minerals: Similar to granite usually in the form of phenocrysts. Minor amounts of ferromagnesians occur but are less obvious than in the corresponding plutonic rocks.
Rhyolite is the aphanitic (fine-grained) extrusive equivalent of granite.
Named after the Andes Mountains in South America, where andesite is a common volcanic rock.
A fine-grained (aphanitic) intermediate volcanic igneous rock.
Light to intermediate color.
Common minerals: Similar to diorite (feldspars and augite, hornblende, and biotite).
Porphyritic varieties are fairly common with both ferromagnesian (mafic) minerals and feldspars occurring as phenocrysts.
Andesite is the aphanitic (fine-grained) extrusive equivalent of diorite.
From the Latin word "basaltes" meaning "dark, hard marble".
A fine-grained mafic volcanic igneous rock.
More generally, any fine-grained, dark colored igneous rock.
Common minerals: Ca-feldspar, augite, with or without olivine, often as phenocrysts.
Magnetite is an important accessory, and quartz and hornblende are sometimes present.
Basalt is characterized by low amounts of Si, Na, and K and high amounts of Fe, Mg, and Ca.
Basalt is the aphanitic (fine-textured) extrusive equivalent of gabbro.
From the Latin word "pumex", meaning "pumice".
A vesicular, glassy volcanic rock, generally of rhyolite composition (felsic).
Often light in color.
Pumice is frozen volcanic froth. Its numerous vesicles make it light weight and pumice often floats on water.
Pumice is economically useful as lightweight aggregate and as an abrasive.
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