Ch. 6 Rocks Chapter 6 Rocks
What is a Rock? • Rocks = Naturally-occurring mixtures of minerals, Mineraloids, glass or organic matter. • Mineraloid=opal, Amber, coal, etc.
What is the difference between a rock and a mineral? Rocks are made up of ONE or MORE minerals. A rock can have once-living material in it.
Rocks are continually changed by many processes, such as weathering, erosion, compaction, cementation, melting, and cooling. • Rocks can change to and from the three types: 1. Igneous 2. Sedimentary 3. Metamorphic
What is the process through which rocks change? • The Rock Cycle—the process through which earth materials change back and forth among the different types of rocks.
IGNEOUS SEDIMENTARY Weathering, Erosion, Compaction, Cementation Heat and Pressure Melting, Solidification Melting, Solidification Weathering, Erosion, Compaction, Cementation METAMORPHIC Heat and Pressure
What are the types of rocks? • IGNEOUS • SEDIMENTARY • METAMORPHIC • Rocks are divided into 3 groups based on how they were formed
Igneous Rocks • “Ignis” = Latin for “fire”. • Formed from the cooling of either magma or lava. • The most abundant type of rock. • Classified according to their origin and composition.
ORIGIN—Where rocks are formed • 1. Below ground = from magma- forms intrusive igneous rock. • Usually have LARGE crystal grains (they cooled slowly)
2. Above ground = from lava- forms extrusive igneous rock • Usually have SMALL or NO crystals (they cooled too quickly)
3. Porphyritic = Formed Both Above and Below Ground- have large & small crystals
Sedimentary Rocks • Formed from sedimentsthat are pressed or cemented together. • Sedimentary rocks represent 7% of the Earth’s crust, • but they cover 70% of the Earth’s surface.
Sediments are Rock fragments, mineral grains, plant & animal remains. • Water or wind breaks down rock into sediments.
These sediments are moved by wind, water, ice or gravity. • Then they are deposited (dropped)
What turns sediments into solid rock? • Compaction=The heavy sediments press down on the layers beneath causing the sediments to fuse together.
Cementation=Dissolved minerals flow between the particles and cement them together.
Metamorphic Rocks • Rocks that have changed due to intense temperature and pressure • “Meta” means “change” and morphosis means “form” in Greek • Igneous, sedimentary and other metamorphic rocks can change to become metamorphic rocks
What occurs in the Earth to change these rocks? • Pressure from overlying rock layers • High heat, but not enough to melt the rock • Rocks may be flattened or bent or atoms may be exchanged to form new minerals.
How are metamorphic rocks classified? • Foliated—mineral grains are flattened and line up in parallel bands • Example: gneiss formed from rearrangement of minerals in granite into bands
How are metamorphic rocks classified? • Non-Foliated—No bands are formed • Example: marble formed from limestone
Where do metamorphic rocks usually form? • Where magma intrudes relatively cool rock • Near colliding plates (near mountain ranges) • Places that are covered miles thick with other rock causing pressure • When hot water intrudes rock • Where a meteorite strikes Earth (rare) • Where lightning bolts strike rocks (rare)