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