Download unit 7: igneous Rocks

unit 7: igneous Rocks
Student information
Igneous rock, known as"fire rock,"is rock formed by the cooling of melted material, such as magma inside the earth and lava above the ground. Igneous rocks are formed as a result of activity at plate boundaries: volcanoes and
sea-floor spreading. Obsidian is an example of igneous rock. Obsidian forms
when lava cools quickly above ground.
Obsidian
Volcanoes are the result of converging boundaries crashing together. The heat and pressure
becomes so great that rock melts within the upper mantle, or asthenosphere, layer of the earth.
The molten rock is forced to the surface, forming mountains of rock. If enough pressure builds up,
the molten rock may be forced out of the earth in an explosive eruption. Molten rock is known as
magma and is converted into lava as it is extruded (forced through an opening in the earth) onto
the earth's surface to cool. This process forms extrusive igneous rocks. Basalt is an example of an extrusive igneous rock. Magma that cools inside the earth forms intrusive igneous rocks. Granite is
an example of an intrusive igneous rock.
Physical properties of igneous rocks are largely determined by the rate at which the rocks
cool and the manner in which they are extruded. Slowly cooling lava results in larger, more definitive
crystals, whereas rapidly cooling crystals are often so small they can't be seen with the naked eye.
Geologists use crystal size and arrangement as clues to identify the conditions under which igneous
rocks were formed.
The mineral composition of igneous rocks depends on minerals in the magma. Igneous rocks
are divided into three families depending on the minerals they contain.
felics—high in silica content (+65%); lightweight,
main minerals include orthoclastic feldspar, mica, and
quartz; rock examples include rhyolite and granites.
Rhyolite
Granite
mafics—high in iron and magnesium; lower in silica
content (45-55%); usually darker in color; main minerals include plagioclastic feldspar, olivine, and pyroxene;
rock examples include basalts and gabbros.
Basalt
Gabbro
intermediates—contain 55-65% silica; medium color;
main minerals include hornblende, biotite mica, and
pyroxene; rock examples include andesite and diorite.
Andesite
Diorite
Magma is composed of silicates, or combinations of other chemical elements with silicon
ancfoxygen. The amount of silica controls the viscosity (the resistance of a substance to flow). Magmas with high viscosity tend to contain higher amounts of silica and commonly form lighter-colored
igneous rocks. The highly viscous magma often builds up under intense pressure and is associated
with spectacular volcanic eruptions. Low viscosity basalts, which contain less silica, are commonly
associated with sea-floor spreading as thin magma oozes through the earth's surface.