Rocks Mineral mixtures
What are rocks? • All natural rocks are aggregates of minerals • The formation of rocks and their physical properties are linked
How and where do rocks form? • Many rocks originate from observable processes that take place at the Earth’s surface. • Rocks that do not appear to be like surface rocks give us clues to processes active within the Earth.
Rock Classification • Rocks are classified according to the processes that form them. • Formed by the solidification of magma • The breakdown of pre-existing rock • Solid state transformation of minerals in a pre-existing rock.
Three types of Rocks • Based on the processes of formation three main categories of rocks are recognized: • Igneous • Sedimentary • Metamorphic
Igneous Rocks • Crystallized material from molten magma. • Volcanic processes = volcanic rocks • “Beneath the crust” processes = plutonic rocks • Volcanic Rocks = extrusive • Plutonic Rocks = intrusive
Cooling of Magma • Rapid cooling of magma (extrusive) normally produces rocks with small mineral grains • Slow cooling of magma (intrusive) normally produces rocks with large mineral grains
Sedimentary Rocks • Deposition and precipitation of materials from the breakdown of older rocks • Occurs on Earth’s surface • Weathering can be: • Chemical – materials dissolve in water • Physical – broken fragments (clasts)
Deposition vs. Precipitation • Clastic sediments can be lithified (cemented together) to form clastic sedimentary rocks • Fragments are cemented with precipitates or simply by compaction • Materials dissolved in water will precipitate when conditions are right to form chemical sedimentary rocks
Bedding • Sedimentary rocks are recognized by distinctive bedding or layering • Bedding is a key feature in recognizing paleo-environments
Metamorphic Rocks • Either sedimentary or igneous rocks are transformed by processes occurring within the Earth’s crust. • Increased pressure • Increased temperature • Chemical reactions catalyzed by hot fluids (water most often)
http://www.beavton.k12.or.us/greenway/leahy/00-01/rocks/gneiss.jpghttp://www.beavton.k12.or.us/greenway/leahy/00-01/rocks/gneiss.jpg Types of Metamorphic Rocks • Regional Metamorphism • Affecting vast areas • Often at plate boundaries • Contact Metamorphism • Very localized • Occurs adjacent to volcanic or plutonic activity
3. Hydrothermal Metamorphism • Hot water flows through the rocks causing a chemical reaction and thus modifying the properties • Often causes the precipitation of metal-ore deposits
Minerals found in metamorphic rock depend on the minerals found in the parent rock. And the nature of the metamorphic processes. • Similarities in appearance to sedimentary rocks and igneous rocks • Bedding and banding at first glace look the same • Mineral assemblage appears similar to igneous – generally has more silicates