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.

Granite

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.

Gabbro

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

Diorite

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 & 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.