CHAPTER 3 - VOLCANIC ROCKS (P. 49-68)

MAGMA - molten rock and dissolved gas (in the subsurface).
LAVA - magma that reaches the Earth's surface before cooling (most gasses have escaped).
Magma is like seltzer: It contains "Volatile gases" - H20, CO2, SO2, N2, CH4 and other dissolved gases.

TWO GENERAL CLASSES OF IGNEOUS FEATURES

• EXTRUSIVE or VOLCANIC (surface)
• INTRUSIVE or PLUTONIC (subsurface)

CRYSTALLIZATION - the formation and growth of a crystalline solid from a liquid or a gas; crystals are the orderly arrangement of atoms in a framework. In IGNEOUS ROCKS, crystal size is a reflection of:

• slow cooling: large crystals (deep underground)
• fast cooling: small crystals (near surface)
• very rapid cooling: glass (without Volatile compounds like CO2, H2O)

in a confined space, such as IN lava or magma, crystals are interlocking. The crystals we see in museums typically grew on the walls of underground caverns or empty spaces left by fluids (water or gas) in the earth.

IGNEOUS ROCK TEXTURES

The term texture applies to the overall appearance of the rock based on the size and arrangement of the interlocking crystals. Textural terms include:

• APHANITIC TEXTURE - Igneous rocks that form on the earth's surface have very fine-grained texture because the crystals are too small to see without magnification.

• PHANERITIC TEXTURE - Igneous rocks with large, visible crystals because the rock formed slowly in an underground magma chamber.

• PORPHYRITIC TEXTURE - an igneous rock in which PHENOCRYSTS (large crystals) are surrounded by a fine groundmass (very small crystals). This shows that the large crystals had a chance to form in a magma while it was migrating and cooling.

• GLASSY TEXTURE (caused by rapid cooling); fresh fractures display a conchoidal fracture like broken glass. An igneous rock with glassy texture is called OBSIDIAN.

• PYROCLASTIC TEXTURE - igneous rock formed from consolidated fragments of cooling magma blown out of a volcano comes in various shapes: BLOCKS (chunks of rock), BOMBS (molten blobs), CINDERS, and fine ASH. These materials may be "welded" together by heat, or cemented together by other processes later. CINDERS have lots of VESICLES (holes created by escaping gases at the lava cools). PUMICE is a type of glassy cinder that has so much gas trapped in the rock that it will float on water.

MINERAL COMPOSITION OF IGNEOUS ROCKS

BOWEN'S REACTION SERIES explains how the composition of magma changes at it cools. HIGH TEMPERATURE (Mafic) MINERALS form first and SETTLE OUT (by sinking to the bottom of the magma chamber). As the magma continues to cool all the high temperature minerals form first, leaving a cooler, but still molten rock (magma). As this magma cools to form rock the last minerals to crystallize are the LOW TEMPERATURE (Felsic) MINERALS. This process changes the composition of the magma as it cools: this process is called MAGMATIC DIFFERENTIATION.

The MANTLE (which is ultimately the source of heat rising in the earth's CRUST) is nearly homogeneous in composition, Consists of an ULTRAMAFIC rock called PERIDOTITE composed of the minerals OLIVENE, PYROXENE, PLAGIOCLASE (Ca- Feldspar), plus other minerals and dissolved gases.

MAFIC ROCKS

Where magma from the MANTLE reaches the surface, such as along mid-ocean ridges and large, hot, igneous plumes rising under certain volcanos, like HAWAII, the composition of the lava has not changed very much. The magma is very MAFIC in composition. MAFIC IGNEOUS ROCKS include:

• GABBRO - phaneritic (large crystals) of mafic minerals: olivine, pyroxene, etc.
• BASALT - aphanitic (small crystals) of mafic minerals: olivene, pyroxene, etc.

Mafic rocks (basalt and gabbro) are very dark in color, and very dense because of their high iron content. Mafic rocks are abundant in the earth's crust beneath the oceans.

FELSIC ROCKS

Magma that has undergone a high degree of MAGMATIC DIFFERENTIATION has a composition consisting of FELSIC MINERALS (low temperature igneous minerals, such as quarts, K-spar, Na-spar, and mica. Felsic rocks include:

• GRANITE - phaneritic rock (consisting of large crystals of K-spar, Quartz, Na- spar, and Mica) - usually pink to white, most common in continental crustal rocks (examples of places were granite is abundant on the earth's surface: Rocky Mountains, Appalachian, Canadian Shield region.

• RHYOLITE - aphanitic (small crystals) of felsic minerals with the same composition of granite.

  ANDESITIC ROCKS

  ANDESITE is a medium gray, fine grained rock of volcanic origin, named after the Andes Mountain, but is abundant in all volcanic regions associated with subduction zones (where crust is sinking and partially remelting - example: volcanos associated with the Pacific "Ring of Fire"). Andesite is INTERMEDIATE in composition between MAFIC and FELSIC rocks. The mixture of mafic and felsic minerals gives the rock a "salt and pepper" appearance.

  • DIORITE - phaneritic rock (large crystals) of both mafic and felsic minerals.

  • ANDESITE - aphanitic rock (small crystals) of both mafic and felsic minerals.

  SUMMARY:
  Intrusive: Gabbro Diorite Granite
  Extrusive: Basalt Andesite Rhyolite
  

  SUBSURFACE (INTRUSIVE) IGNEOUS FEATURES

  HYDROTHERMAL SOLUTIONS - "volatilize gases" that escape from magma (water, CO2, Methane) plus "dissolved" minerals.

  HYDROTHERMAL DEPOSITS - precipitates from solutions escaping from a magma chamber, form VEIN DEPOSITS. Often form economic deposits of gold, copper, zinc, silver, and other metals.

  PEGMATITE - very large crystals of igneous minerals form near the top of a gas saturated igneous intrusion.

  FEATURES ASSOCIATED WITH INTRUSIVE IGNEOUS ACTIVITY

  

  NOTE: The PALISADES, a long rocky cliff in NJ along the western side of the Hudson River in the vicinity of the George Washington Bridge is an excellent example of a SILL, a large volcanic intrusion that squeezed in between sedimentary rock layers that were at one time buried deep beneath the earth's surface.

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