Igneous rocks pt. 3

Some minerals that make up rocks

feldspar

Very common in the earth’s crust.

feldspar

biotite

hornblende

olivine

plagioclase

granite

granite

rock type: igneous

distinguishing features: completely crystallized, large, well formed interlocking crystals visible to the naked eye, no preferred crystal orientation, fairly light in color (white, gray, pink).

main minerals quartz, feldspar, biotiteweathering behavior : physical weathering can

break the rock along its crystal boundaries

granite is an igneous rock that forms by slow cooling of silica-rich magma deep within the earth

basalt

basalt

Basalt is an igneous rock that forms through rapid cooling at the surface of the earth. The island of hawaii, for example is almost entirely made from basaltic lava flows.

weathering of iron minerals can turn the surface tan or reddish brown (see image below

basalt

Obsidian

Volcanic glass of the granite family. Cleavage or fracture? (conchoidal)

Rhyolite

also in the granite family

rhyolite

rhyolite

Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals.

So what makes a granite rock?

Rocks are made of minerals.

Granite rocks are made of quartz, feldspar, and at least one other mineral.

pumice

basalt

Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene.

Basalt is also abundant on the moon!

basalt

gabbro

Gabbro is a coarse-grained, dark colored, intrusive igneous rock that contains feldspar, augite and sometimes olivine.

diorite

diorite

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz.

PORPHYRITIC ROCKS

Extrusive AND intrusive!

Porphyritic rocks

Rocks that have two distinctly different textures: large crystals AND fine-grained crystals.

Porphyritic rock

Porphyritic rocks

A theory of how porphyritic rocks are formed:

Magma cools slowly deep inside the earth. Large crystals are able to form.

THEN, the remaining liquid magma is pushed upward until close to the surface of the earth.

Forms fine crystals.

Porphyritic rock

Porphyritic granite