Geology Ch.4

Mafic

(from magnesium and ferrum, the Latin name for iron)

Igneous Rocks

(ignis = fire) form as molten rock cools and solidifies. solid mass of interlocking silicate minerals.

Magmas consist of 3 materials:

- a liquid component - a solid component - gaseous component

As magma cools and solidifies, these elements combine to form 2 major groups of silicate minerals, what are they?

-The dark (or ferromagnesian) silicates are rich in iron and/or magnesium and comparatively low in silica. -The light (or nonferromagnesian) silicates contain greater amounts of potassium, sodium, and calcium rather than iron and magnesium.

what 3 factors influence the textures of igneous rocks?

-the rate at which molten rock cools - the amount of silica present - the amount of dissolved gases in the magma.

Ultramafic

A compositional group of igneous rocks containing mostly olivine and pyroxene.

The effect of cooling on rock textures is fairly straightforward:

Slow cooling promotes the growth of large crystals, whereas rapid cooling tends to generate small crystals.

Basaltic composition

a compositional group of igneous rocks indicating that the rock contains substantial dark silicate minerals and calcium-rich plagiocase feldspar.

Andesitic composition or Intermediate composition

a compositional group of igneous rocks that contains at least 25 percent dark silicate minerals. The other dominant mineral is plagioclase feldspar.

Volatiles

are the gaseous components of magma and are materials that vaporize (form a gas) at surface pressures.

Aphanitic (fine-grained) Texture

composed of crystals that are too small for the individual minerals to be identified without a microscope.

Phaneritic (coarse-grained) Texture

composed of mineral grains that are large enough to be identified without a microscope.

Porphyritic Texture

composed of two distinctly different crystal sizes.

Glassy Texture

composed of unordered atoms and resembles dark manufactured glass. (obsidian is a natural glass that usually forms when highly silica-rich magmas solidify.)

Pegmatites

exceptionally coarse-grained igneous rocks.

Vesicular Texture

extrusive rock containing voids left by gas bubbles that escape as lava solidifies. (pumice is a frothy volcanic glass that displays a vesicular texture.)

What is the most abundant mineral group?

feldspars.

Igneous rocks that form at the surface or as small intrusive masses within the upper crust where cooling is relatively rapid exhibit a _______

fine-grained texture termed aphanitic (a = not, phaner = visible).

Extrusive igneous rocks

form when molten rock solidifies at the surface. *are also called volcanic rocks -- after Vulcan, the Roman fire god.

Intrusive igneous rocks

formed when magma crystallizes at depth, also known as plutonic rocks -- after Pluto, the god of the lower world in classical mythology.

Crystallization

in the process called crystallization, cooling reverses the events of melting. As the temperature of the liquid drops, ions pack more closely together as their rate of movement slows.

Melt

is the liquid portion of magma and is composed mainly of mobile ions of the eight most common elements found in Earth's crust - silicon and oxygen, along with lesser amounts of aluminum, potassium, calcium, sodium, iron, and magnesium.

Magma

is the parent material for igneous rocks and is formed by partial melting that occurs at various levels within Earth's crust and upper mantle to depths of about 250 kilometers (about 150 miles).

where is Mount Rushmore National Memorial located at?

located in the Black Hills of South Dakota, and is carved from intrusive igneous rocks.

Pyroclastic (fragmental)Texture

produced by the consolidation of fragments that may include ash, once molten blobs, or large angular blocks that were ejected during an explosive volcanic eruption.

Pegmatitic Texture

rocks of this type, in which most of the crystals are larger than 1 centimeter in diameter, are described as having a pagmatitic texture.

what are the most abundant constituents of igneous rocks?

silicon and oxygen. These 2 elements, plus ions of aluminium (Al), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), and iron (Fe), make up roughly 98 percent, by weight, of most magmas.

Texture

used to describe the overall appearance of a rock based on the size, shape, and arrangement of its mineral grains -- not how it feels to touch.

most common volatile found in magma are __

water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2), which are confined by the immense pressure exerted by the overlying rocks.

Coarse-grained texture described as phaneritic (phaner =visible)

when large masses of magma slowly crystallize at great dept, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic (phaner = visible).

Lava

when molten rock reaches Earth's surface.