Rocks

What is a rock?• Which of the following are rocks:

IceWaterCoalDead PlantTable SaltProcessed SugarMined DiamondArtificial Diamond

Definition

• Solid• Naturally occurring• Combinations (mixtures!)• Minerals and/or organic materials

List

Rocks Not Rocks

Ice WaterCoal Dead PlantSalt SugarMined diamonds Artificial Diamonds

How are igneous rocks formed?

• Are made from magma that has been cooled down from liquid to solid

Difference between magma and lava?

Answer

• Magma is molten rock inside the Earth

• Lava is molten rock on the surface of the Earth

Rock Characteristics

• Use to tell history, origin of the rock

• Did it form inside or on the surface of Earth

• How long did it take to form?

Rock Characteristics

Rock Texture: Relates to size of the mineral crystals. The size of the crystal indicates the amount of time it takes to cool down and form a solid object. The larger the crystals, the longer the time of formation

Types of textures-Coarse Grained: Large crystals

Fine Grained : Small crystals. Will usually not be able to see crystals without microscope

Other texture types

• Glassy: No crystals, even under a microscope

– Indicates very rapid cooling of lava (seconds)

• Vesicular: Rocks with holes

– Holes indicate escape of gases during rapid cooling

Color (2nd Characteristic)

• Color indicates composition of the rock

• Light colored rocks: indicate rocks with high concentrations of Si, O, K and Na

• White, pinks and gray

– These rocks have a Felsic composition– Formed within continents

Color

• Dark colored: Color dominated by dark greens and black

• These rocks have high concentrations or Fe, Mg and Ca

• These rocks have a mafic composition• These rocks are formed within oceans

Nothing is black or white…

• A lot of rocks are combinations of felsic and mafic materials

• Color will be medium dark (usually dark gray)

• These have an intermediate composition

Felsic Rocks

Mafic Rocks

Density (3rd Characteristic)

• Mafic rocks are generally more dense than Felsic rocks

Rock Names• Match the name to your rock sample based on your

observations

• Basalt- fine grained , mafic• Gabbro- coarse grained, mafic• Granite- coarse grained, felsic• Obsidian- glassy, mafic• Pumice- vesicular (very small holes), felsic• Tuff- fine grained, felsic• Scoria – vesicular (larger holes), mafic

Types of volcanic rocks

• Obsidian: Mafic, Glass, no crystals, instant solid

• Tuff, pumice: Felsic, Tiny crystals, with holes, quick solidification

• Basalt: Mafic, tiny crystals, lava solidification

• Granite, Felsic, large crystals, magma solidification without exposure to air

What do the rocks tell about its history…

• Basalt, obsidian, pumice, tuff and scoria are examples of extrusive igneous rocks.

• These rocks were created from the material extruded from an volcanic eruption

• In general, extrusive rocks have no crystals or very small crystals: short cooling time

• Gabbro,and granite are examples of intrusive igneous rocks

• Formed when magma slowing cools inside the earth before exposed through erosion

• Formed when magma is given a chance to slowly cool over millions of years, without being exposed to the surface of the earth

Why do intrusive rocks grow such big crystals?

• Because the rocks in the interior of the Earth are poor conductors of heat, it takes a long time for the hot magma to be cooled. Thus, the intrusive igneous rocks have a coarse-grained texture due to their slow cooling.

Devils tower