Geology Review 2
Chapter 4- Igneous Rocks
-a body of molten rock found at DEPTH, including any dissolved gases and crystals
-magma that reaches earth’s surface
-igneous rock formed below Earth’s surface
-igneous activity that occurs at earth’s surface
-igneous rocks that form at depth
-pertaining to the activities, structures, or rock types of a volcano
-an igneous rock composed almost entirely of light-colored silicates, Phaneritic and felsic
-dark grey intermediate igeous rock, phaneritic
-dark, coarse grained rock, phaneritic and mafic
- igneous, extrusive volcanic rock, aphanitic and felsic
- aphanitic, mafic
Components of magma
Liquid portion-melt, mostly silicon and oxygen
Solid component-silicate minerals that crystallized in the melt
Gaseous phase-volatiles, H2O and CO2, SO2
-igneous rocks that form at the surface, rapid cooling=fine-grained texture
-igneous rocks that form below the surface, slow cooling=coarse-grained texture
-large crystals imbedded in a matrix of smaller crystals
-voids left by gas bubbles that escape a lava as it solidifies
-vesicular texture, produced by basaltic magma
Relationship between rate of cooling, silica, gas content, and igneous texture
Amount of Silica
More Silica=slower cooling, (higher cooling temperature, lower melting point)
Silica component of melt becomes enriched as magma evolves
Water and gas can lower melting point, causes magma to rise
When gas evaporates can leave a vesicular texture
- dark colored, contains substantial dark-silicate minerals and calcium-rich plagioclase feldspar
Intermediate- 25% dark silicate minerals, other dominant mineral plagioclase feldspar
- rock composed
almost entirely of light-colored silicates
Igneous rocks form by crystallization of magma.
As temperature of the liquid drops, the ions
pack closer and closer together as they slow their rate of movement.
Silicon and oxygen
molecules link together first. SO2 tetrehedra join with other ions to create an igneous rock.
Igneous rocks that form below earth’s surface exhibit a coarse-grained texture because more time
is allowed for crystallization.
Igneous rocks that form above earth’s surface exhibit a fine-
grained structure because they crystallize very quickly.
Texture is the size, shape, and distribution of the particles that collectively constitute a rock.
Factors that contribute to the texture of an igneous rock include, rate of cooling, amount of silica,
amount of dissolved gasses.