Chapter 31

1. Chapter 31 Rocks Presented by April Senger

2. Igneous Rocks • Formed by the cooling and crystallization of hot, molten rock called magma • Igneous means “formed by fire” • 95% of the entire crust is igneous • Examples: Basalt and granite • The most common igneous rocks on the continents is granite and andesite • The most common igneous rocks on the ocean floors is basalt Presented by April Senger

3. Sedimentary Rocks • Formed from pieces of other rocks (sediments) carried by water, wind or ice • The most common rocks in the upper part of the Earth’s crust are sedimentary • 2/3 of the surface are covered by sedimentary rock • Examples: Sandstone, shale, and limestone Presented by April Senger

4. Metamorphic Rocks • Metamorphic rocks are formed from older, pre-existing rocks that are transformed by high temperature, high pressure or both without melting back into magma • Metamorphic means “change in form” • Examples: Marble and slate Presented by April Senger

5. Extrusive Rocks • Extrusive means “pushed out of” and are rocks that are formed either at or below the Earth’s surface • Magma that is high in silicon flows very slow or is viscous • Basalt (low in silicon) flows quickly Presented by April Senger

6. Lava and Volcanoes • Magma that reaches the surface is called lava • Lava can reach the surface through cracks, fractures or vents called volcanoes • Eruptions from fissures are more common than volcanic eruptions • Basaltic lava is common at fissures because of its fast flow • Most basaltic flows occur in the ocean but the Columbia Plateu in the Pacfic NW and Ceccan Plateau in India are extensive flood basalts Presented by April Senger

7. Concept Check • Is it correct to say that lava and magma with a high silicon content are more viscous than those with lower silicon content? • Yes…Viscous means resistance to flow Presented by April Senger

8. 3 Kinds of Volcanoes • Shield volcanoes are gradual and slightly sloping (Mauna Loa, HI). • Composite volcanoes are layers of ash, cinders and lava caused by trapped gases in the lava exploding upward. Mount Fuji (Japan), Mount St. Helens, Rainier, & Hood are composites. • Cinder cones are small and common. They have lots of trapped gases and have violent eruptions. They are only active for a short time. Paricutin in Mexico is the most famous Presented by April Senger

9. Concept Check • How are the lavas that form shield volcanoes different from those that form composite cones? • Shield is low silicon content and flow easily to form wide and sloping sides • Composite is high silicon content and is viscous • They have explosive eruptions of thick lava and ash that don’t flow away and builds a steep composite cone Presented by April Senger

10. Craters and Calderas • A crater is formed above the central vent of an erupting volcano • The walls of the crater often collapse after an eruption making the vent larger • Craters can grow to over 1 km in diameter • Large craters are called caldera (5-30 km) Presented by April Senger

11. Cool, Rather Hot Occurrences • When the walls of the vent collapse sometimes the entire magma chamber collapses • 7000 years ago, Mount Mazama erupting shaking the entire NW US • Crater Lake (OR) formed when the volcano blew its top nd rain water filled the caldera 9 km wide and 590 meters deep • Yellowstone is considered a hot spot caldera 45 km wide and is remnant from a volcano 600,000 years ago Presented by April Senger

12. Intrusive Rocks • Rocks that form beneath the Earth’s surface are intrusive which means pushed into • Large intrusive rocks are called plutons • The only way to study them is for them to reach the Earth’s surface Presented by April Senger

13. Dikes are long fractures of magma that cut across layers of existing rocks They are old channel ways for rising magma to occur near volcanic vents Note the volcanic necks at Shiprock, NM Sills are horizontal fractures of magma Laccoliths are mushroom shaped dikes that push layers up rather than cutting through them Batholiths are many plutons that push even mountain ranges upward The Sierra Nevada batholith is growing faster than erosion is tearing it down Plutons & Other Features Presented by April Senger

14. Concept Check • Why is it incorrect to say that igneous rocks may form from the intrusion of lava? • No - Intrusion means it occurs inside the Earth and lava is the term that means outside the Earth • Is it correct to say that igneous rocks may form from the extrusion of lava? • Yes - Extrusive means outside and lava is on the surface Presented by April Senger

15. Mechanical & Chemical Weathering and Erosion • Mechanical weathering is physically breaking rocks into smaller pieces of rock • Chemical weathering consist of chemical reactions that involve water and decomposed rock into smaller pieces • Erosion is the process by which weathered rock particles are removed and transported by water, wind or ice Presented by April Senger

16. Sediments • Sediments composed of small fragments of other rock are called clastic sediments • Sediments produced by chemical means are called chemical sediments • After transport, particles become rounded and deposition and sedimentation begin Presented by April Senger

17. Water transport is most powerful at the source Larger sediments drop out more quickly than smaller sediments in a process called sorting Ice is poorly sorted where wind rounds grains into smaller sediments and is well sorted Sedimentation is either compaction or cementation Weight from overlying layers compacts sediments together Water “glues” sediments together with dissolved silicon dioxide or iron oxide Iron cause red or orange stain such as Bryce Canyon Deposition & Sedimentation Presented by April Senger

18. Clastic Rocks • Clastric rocks are classified by grain size • Fine grained is like shale • Shale most likely formed in calm waters and if it is black it has high organic content • Medium grained is like sandstone • Large grained is conglomerate (pebbles and even boulders) Presented by April Senger

19. Fossils • Fossils are found in sedimentary rocks • They often contain the remains of preexisting life on Earth • Some are whole organisms, parts of the organisms or impressions Presented by April Senger

20. Concept Check • Why is it incorrect to say that clastic sediments are precipitated out of water? • The solid left behind in precipiation is a chemical sediment and clastic is not chemical • Is it correct to say that evaporites are deposited by water? • Yes, because the word deposit is a term that is applied to all types of sediment Presented by April Senger

21. Metamorphic Recrystallization & Nonmechnical Deformation • Recrystallization occurs when minerals in a rock change because the rock has been exposed to high temps and pressure • Nonmechanical deformation occurs when a rock is subjected to physical stress and may or may not have high temperature Presented by April Senger

22. Contact Metamorphism • Contact is when a body of rock is intruded by magma • The change can be cm to several hundred meters • Contact areas are dry and high temp and produce garnet and pyroxene • Further away it is cooler and moist and you find muscovite and chlorite Presented by April Senger

23. Regional Metamorphism • Regional is the kind of change that rock undergoes when it is subjected to physical stress and heat over a large area • They are found at the major mountain belts • The large areas of heat and pressure are well known for producing gems • They have a layered appearance Presented by April Senger

24. Foliated vs. Nonfoliated • Foliation is a prominent visual feature of regional metamorphosed rocks and is very different than the layering seen in sedimentary rock • Basically sheets form from the pressure in rocks such as slate, schist and gneiss • Nonfoliated rocks may not have the right amount of sheet minerals, heat or pressure • Examples are marble and quartzite Presented by April Senger

25. Rock Cycle • It is important to recognize that the rock cycle is not a linear set path • Magma, igneous rocks, weathering, erosion, sediements, cementation, compaction, heat, pressure, etc can rotate through the stages through many paths (Page 567) Presented by April Senger