ROCKS AND THE ROCK CYCLE Objectives Identify the

ROCKS AND THE ROCK CYCLE Objectives: • Identify the three major types of rock and explain how each forms • Summarize the steps of the rock cycle • Summarize the factors that affect the stability of rocks

Rocks Rock is the material that makes up the solid parts of the Earth Rock can be a collection of two or more minerals, or solid organic matter

This slide show is intended to help you understand important types of rocks. The diagram in the next slide represents the ROCK CYCLE—a scheme that represents the processes of continuous changes that connect the three major groups of rocks: SEDIMENTARY IGNEOUS METAMORPHIC It also shows two other important parts of the “Rock Cycle” – SEDIMENTS and molten LAVA and MAGMA Source: http: //www. canadianrockhound. com/junior/rock_cycle. html

Igneous Rock Volcanic activity produces igneous rock Igneous is from a Latin term meaning “from fire” Igneous rock forms when magma, or molten rock, cools and hardens Magma is called lava when it is exposed at the Earth’s surface There are two main classes of igneous rock Extrusive- igneous rock that forms when lava cools and hardens at the Earth’s surface, fine (small) grains or crystals or glassy texture due to cooling quickly Intrusive- igneous rock that is under the Earth’s surface, course (large) grains or crystals due to cooling slowly

Formation of Magma forms when rock melts Chemical composition determines melting points The process by which different minerals in rock melt at different temperatures is known as partial melting

Textures Igneous rocks can come in different textures Course-Grained Igneous rocks that have large mineral crystals (grains) Formed when magma cools slowly Granite is one example The upper part of the continental crust is mainly made of granite Fine-Grained Igneous rocks that have small mineral crystals (grains) These are formed when magma is cooled rapidly Basalt and rhyolite are examples

IGNEOUS ROCKS Form by solidification (crystallization) of melted minerals

Other Igneous Rock Textures Some igneous rocks cool slow at first, but then cool rapidly when they reach the Earth’s surface This can cause large crystals embedded in a mass of smaller ones When thick magma cools quickly few crystals can grow, instead there is a glassy texture like obsidian

Composition of Igneous Rock Geologists divide igneous into three families: Felsic Rock- forms from magma that is high in silica and generally have light coloring. Examples include granite, rhyolite, obsidian, and pumice Mafic Rock- forms from magma that is low in silica, rich in iron and magnesium, has dark colored minerals. Examples are basalt and gabbro Intermediate Rock- contains less silica than felsic rock, but more than mafic. Examples are diorite andesite

Types of Intrusive Igneous Rocks Batholiths- (meaning “deep”) is referring to intrusive formations that spread over 100 km 2 when exposed to Earth’s surface Stocks- similar to batholiths but cover less than 100 km 2 Laccoliths- (meaning “lake of rock”) forms when magma flows b/w rock layers and creates a dome into the overlying rock Sills- when magma flows between rock layers and hardens Dikes- cut up through the rock layers and are common in areas of volcanic activity

Granite Light-colored, coarsegrained, no pattern Mostly quartz, feldspar, mica, and hornblende Often used for buildings and monuments http: //www. volcanoworld. org/vwdocs/vwlessons/Rocks/Rocks 10. html

Basalt Dark-colored, finegrained, extrusive Formed where lava erupted onto surface Most widespread igneous rocks Found locally in the Palisades along west shore of Hudson River, Connecticut River valley http: //www. volcanoworld. org/vwdocs/vwlessons/Slideshow/Igrocks/Igrock 2. html

Gabbro Dark-colored, coarsegrained intrusive Similar composition to basalt—plagioclase feldspar with some pyroxene and olivine http: //www. volcanoworld. org/vwdocs/vwlessons/Slideshow/Igrocks/Igrock 8. html

Obsidian Natural volcanic glass Forms when lava cools very quickly Usually dark, but small pieces may be clear Fractures along curved (conchoidal) surface Used as spear and arrow points, knives http: //www. volcanoworld. org/vwdocs/vwlessons/Slideshow/Igrocks/Igrock 7. html

Pumice and other igneous rocks Light colored, frothy (many air spaces) Same minerals as in granite, but finer in grain size For more about igneous rocks: http: //www. volcanoworld. org/vwdocs/vwlessons/Igrocks/Igrocks 1. html

Sedimentary Rock When rocks, mineral crystals, and organic matter have been broken into fragments they are known as sediments When sediment deposits are compressed, cemented together, and harden, sedimentary rock has formed Three Classifications of Sedimentary Rock Biologic (organic)-forms from the remains of living things Clastic- small fragments that are compacted and cemented into solid rock Chemical (non-clastic)-forms when the ions from dissolved minerals precipitate out of water

Metamorphic Rock Forces and processes such as high pressure, extreme heat, and chemical processes can change to form of existing rock The rock that forms when existing rock is altered is called metamorphic rock Metamorphic means “changed form”

Metamorphic Rocks Formed by heat and pressure changing existing rocks REGIONAL METAMORPHIC affects a large area and results from plate tectonics CONTACT METAMORPHISM affects rocks on a local scale, such as “baking” sedimentary rocks next to magma or lava For more information: http: //www. volcanoworld. org/vwdocs/vwlessons/Metrocks/Metrocks 1. html

http: //www. indiana. edu/~geol 116/week 6/wk 6. html

Here is another version of the Rock Cycle http: //www. volcanoworld. org/vwdocs/vwlessons/Metrocks/Metrocks 2. html

http: //www. volcanoworld. org/vwdocs/vwlessons/Rocks/Rocks 8. html

Properties of Rocks Chemical Stability The rate at which a mineral breaks down is called is based on its chemical stability Chemical stability is a measure of the tendency of a chemical compound to maintain its original chemical composition rather than break down to form a different chemical Basically the minerals that are most stable will form in low temperatures and NOT breakdown quickly Minerals formed in the highest temperatures are the least stable and WILL breakdown more quickly

Physical Stability Rocks have natural zones of weakness that are determined where rocks form Sedimentary rock tends to break between the layers that make them Igneous rocks have evenly spaced weakness zones called joints that form when the rock cools and contracts

Beware of Rock!!

Sedimentary Rocks Sedimentary rocks may be made of rock fragments— sediments—or by chemical reactions. The classification of sediments is shown below. http: //www. volcanoworld. org/vwdocs/vwlessons/Sedrocks/Sedrocks 6. html

Clastic rocks–made of cemented sediments—are classified by their grain sizes. http: //www. volcanoworld. org/vwdocs/vwlessons/Sedrocks/Sedrocks 9. html

Non-clastic rocks form by chemical precipitation (settling out from a solution. ) Limestone is made from calcite, chert from quartz, and halite is rock salt. http: //www. volcanoworld. org/vwdocs/vwlessons/Sedrocks/Sedrocks 10. html

Biologic sedimentary rocks come from the remains of organic matter. The most important of these is coal. Anthracite coal results from the greatest pressure and releases the most energy when burned. Other varieties are bituminous and lignite. “Petrified” (permineralized) wood is another organic rock. http: //www. volcanoworld. org/vwdocs/vwlessons/Sedrocks/Sedrocks 11. html

More about sedimentary rocks Shale is the most common sedimentary rock Sedimentary rocks cover about threequarters of the land surface For more about sedimentary rocks: http: //www. volcanoworld. org/vwdocs/vwlessons/Sedrocks/Sedrocks 1. html

“Foliated” rocks contain much mica and other rocks that produce layering or banding Gneisses and schists are common in New York City and Westchester. http: //www. volcanoworld. org/vwdocs/vwlessons/Metrocks/Metrocks 5. htm l

Non-foliated metamorphic rocks include marble, which comes from limestone, and quartzite, which comes from sandstone http: //www. volcanoworld. org/vwdocs/vwlessons/Metrocks/Metrocks 12. html

A. Evidence of Metamorphism Shale E. g. , Sedimentary Rocks Made of minerals derived from weathering of a parent rock Stable at atmospheric temperatures and pressures (low T & P) Originally horizontal, continuous and uniform layers 1. Bent (deformed) layers Gneiss

Evidence of Metamorphism 2. Flattened Pebbles Conglomerate Differential pressure “squashes” rock and included features Metaconglomerate

Evidence of Metamorphism Quartz Sandstone Quartzite 3. Crystalline Texture Minerals tightly interlocking due to recrystallization under pressure

Evidence of Metamorphism 4. New mineral assemblages E. g. , Shale: Clay minerals (some quartz) Metamorphism (Mid-grade) Forms Schist: Mica, Feldspar and other silicate minerals

Additional Resources There are many web sites that can provide you with more information about rocks. Most of these slides come from “Volcano World. ” You can learn more from their slide show at http: //www. volcanoworld. org/vwdocs/vwlessons /lessons/Slideshow/Slideindex. html