Rocks What are rocks Rocks are a mixture

Rocks

What are rocks? • Rocks are a mixture of minerals. • Some of the rock forming minerals include mica, hornblende, quartz

The three groups are based on how they are formed • Igneous • Sedimentary • Metamorphic

Igneous • Igneous (means fire) formed from molten rock that cools and then hardens. • Molten rock under the surface is called magma: molten rock on or very near the surface is called lava.

Sedimentary • Sedimentary rocks formed from sediments becoming pressed or cemented together or when sediments precipitate out of solution

Metamorphic • Metamorphic (means to change) formed from changes in igneous, sedimentary or other metamorphic rocks.

MAGMA & g on n i l i Coo dificat soli IGNEOUS ROCK Me ltin g on SEDIMENTS Heat & pressu re osi er & ng eri ath We METAMORPHIC ROCK Compaction /Cementation SEDIMENTARY Lithification ROCK

Igneous Rocks • What determines the type of minerals? • The type of minerals that develop depend on the type of magma. There are 2 basic types of magma: – Basaltic – Granitic

Granitic Magma • Granitic magma is viscous (like oatmeal), high % of silica and low % of oxygen. • When it cools, light colored minerals crystallize, producing felsic colored rocks. • These minerals have a relatively low density. • Most of the rocks that form the continents are made form this type of magma.

Basaltic Magma • Basaltic magma is fast moving (like hot syrup), high % of oxygen and low % of silica. • When it cools, dark colored minerals crystallize, producing mafic colored rocks. • These minerals have a relatively high density. • Most of the rocks that form the ocean floor are made of this type of magma.

So now lets talk about volcanoes • http: //www. pbs. org/wnet/savageearth/anim ations/volcanoes/index. html

How are volcanoes classified? • The type of volcano determines…. – If the volcano erupts quietly or explosively – What type of magma is produced • There are three types of volcanoes

Cinder Cone • Explosive – Lava, gas, and ash flow high in the air • Steep sided • Example: – Mt. Saint Helens

Shield Volcano • Quiet • Basaltic magma • Broad volcano with sloping sides • Example: – The Hawaiian islands

Composite volcano • They vary – They can be quiet or loud

What determines the size of crystals in igneous rocks? Molten rock can cool in two ways, deep within the earth or on/near the surface. • Intrusive – Cooled slowly deep within the earth. – Intrusive rocks have large crystals. – Granite and gabbro are intrusive rocks. • Extrusive – Cooled quickly on/near surface of earth. – Extrusive rocks have small crystals or no crystals. – Basalt, rhyolite, scoria, obsidian, and pumice are all extrusive rocks.

Texture The size of crystal is referred to as the texture of the rock. • Intrusive – Intrusive igneous rocks have larger crystals and a coarse grained texture. • Extrusive – Extrusive igneous rocks have either: small crystals and a finegrained texture or if they cooled too quickly for crystals to form they have a glassy texture. – Rocks with a glassy texture wither look truly glassy (obsidian), or full of air hole (such as pumice or scoria). – Pumice is actually so full of air hole it will float in water

Igneous Rock Features • Intrusions – a body of igneous rock that has crystallized from molten magma below the surface of the Earth

Batholith • The largest type of intrusion • Magma cools before reaching the surface. • Sometimes they are exposed due to erosion.

Laccolith • They are flat at the bottom and domed at the top.

Dike • Magma is squeezed into vertical cracks that cut across the rock layers.

Sill • Magma is squeezed into horizontal cracks that cut across the rock layers.

Sedimentary Rocks • Sedimentary rocks are made from loose material such as rock fragments, mineral grains, and bits of plants and animal remains that have been moved by wind, water, ice or gravity. • Sedimentary rocks are usually made from only 1 mineral.

There are two types of Sedimentary Rock • Clastic • Non-clastic

Clastic sedimentary rocks They are formed from fragments of existing rocks. They are also called detrital which means “to wear away”. Particle Size Rock Type smallest Clay Shale Siltstone Sandstone largest Gravel Conglomera te

How are sediments held together? • Particles are held together by compaction or cementation. • Rocks are weathered, transported, compacted and cemented. • Lithification is the process of compaction and cementation.

Non- clastic Chemical Sedimentary Rocks • From dissolved minerals in evaporating seas. • Examples are limestone and rock salt.

Non-clastic Organic Sedimentary Rocks • From the remains of living organisms • Shell fragments, leaves, coral, ect. • Examples are: – Coal from plants – Coquina from whole shell fragments – Limestone from microscopic animals with Ca. CO 3 skeletons

Metamorphic Rocks • Metamorphic rocks have changed form due to temperature and pressure increases or they undergo changes in the composition of a metamorphic rock.

What causes metamorphic rocks? Two processes…. . • Heat and pressure is involved in both of these processes. • Contact Metamorphism (most met. rocks form this way) – Rocks undergo changes because they come in contact with a heat source, usually magma. This “cooks” the rock • Regional Metamorphism – Associated with increased temperature and pressure associated with the formation of mountains – This pressure causes foliation

Metamorphic Rocks Type of textures • Foliated • Non- foliated

Foliated • They have layers of colored bands • Foliated rocks include slate, schist, gneiss.

Non-foliated • They do not have layers or colored bands. • Usually this type of rock only has one type of mineral. • Non-foliated rocks include marble and quartzite.

Weathering

Weathering is the process that breaks down rocks into smaller and smaller fragments.

Mechanical Weathering • Breaks rocks apart without changing their chemical composition. • Each fragment has the same composition as the original rock. • Causes – – Plants Ice wedging Gravity Abrasion

Chemical Weathering • The chemical composition of the rock changes. • Causes – Water – Acids • Plant acid • Sulfuric acid • Carbonic acid – Reacts with limestone – Oxygen • Oxidation which causes “rust”

Climate and Weathering • Climate affects the rate of weathering. • Mechanical weathering occurs more in cold climates. • Chemical weathering occurs more in warm climates.

Karst Topography • What is topography? • Karst topography is developed in areas underlain by calcium carbonate rocks such as limestone and dolomite.

How does Karst topography occur? • It forms when limestone is slowly dissolved away by slightly acidic groundwater.

What does it look like? • When the limestone is weathered away it forms features such as – Caves – sinkholes

Where do we have karst topography in VA? • The valley and ridge province is abundant in limestone, therefore it exhibits karst topography. • Examples – Luray Caverns – Dixie Caverns – Natural Bridge Caverns The weathering creates stalagtites (hold tight to the top of the cavern) and stalagmites (at floor of the cavern)

Erosion • The process by which Earth materials are transported. • Agents of erosion – water – ice – wind

Deposition • The process by which Earth materials carried by wind, water, or ice settle out and are deposited

River Deposition • The major factors that affect the rate of deposition are – – particle size Shape Density the velocity of the transporting stream • The smaller particles settle more slowly than the larger particles, due to the pull of gravity. The smaller particles tend to stay in suspension for longer periods of time

River Deposition and Erosion

Rocks Igneous Sedimentary Igneous Intrusions Volcanoes Shield Cinder Composite Intrusive Laccolith Extrusive Batholith Formation Dike Sill Weathering Erosion Metamorphic Foliated Deposition Clastic Non-clastic Non foliated