Chapter 5-Igneous Rocks

Lecture notes- to be put in notebook

What are igneous rocks?

• Temperature generally increases with depth in Earth’s crust.

• Pressure also increases with depth, a result of the weight of overlying rock.

What are igneous rocks?• As pressure on rock

increases, its melting point also increases.

• Different minerals have different melting points, not all parts of a rock melt at the same time.

• Partial melting -Some minerals melt at low temperatures while other minerals remain solid.

• As each group of minerals melts, different elements are added to the magma “stew”

• Fractional crystallization-When magma cools, it crystallizes in the reverse order of partial melting.

• As each group of minerals crystallizes, it removes elements from the remaining magma instead of adding new elements.

Mineral composition-

• Felsic rocks such as granite are light-colored, have high silica contents,

–contain quartz and the feldspars orthoclase and plagioclase.

GraniteGranite

Mineral composition- • Mafic rocks such as

gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium. – contain plagiocide, biotite,

amphibole, pyroxene, and olivine.

GabbroGabbro

• Diorite is a good example of an intermediate rock with moderate amounts of biotite, amphibole, and pyroxene.

DioriteDiorite

• Ultramafic rocks-have low silica contents and very high levels of iron and magnesium–Peridotite and dunite –Formed by the crystallization

of olivine and pyroxene

Peridotite & DunitePeridotite & Dunite

• Cooling rates-–Extrusive igneous rocks such as

obsidian have no visible mineral grains (cooled quickly)

–Intrusive igneous rocks such as gabbro may have crystals larger than 1 cm (cooled slowly)

• Cooling rates- Continued• Geologists make thin sections of

minerals so that they can see the shapes of the grains

• Interlocking edges form when the grains crystallize and grow together

• During fractional crystallization, the minerals that form early in the process float in a liquid and have space to grow distinct shapes

• Porphyritic rocks have grains of two different sizes

–Caused by slowly cooling magma which suddenly begins to cool rapidly

• The interlocking grain textures of igneous rocks help to give them strength.

• Many of the minerals found in igneous rocks are resistant to weathering.

• Granite is among the most durable of igneous rocks.

Ore Deposits

• Valuable ore deposits are often associated with igneous intrusions.

• These ore deposits are found within igneous rock, such as the layered intrusions.

Ore Deposits

• Veins-Metallic elements are released at the end of magma crystallization.–Veins of extremely large-

grained minerals are called pegmatites.

• Diamonds are found in ultramafic rocks known as kimberlites.

• Diamond can only form under very high pressures.