Rocks Tell Stories Records of Geologic Past
Rock Classification – 3 major groups based on their methods of formation or origin • Igneous rocks … from cooling and solidification of lava or magma • Sedimentary rocks … from compacted and cemented sediments, or chemical precipitates or evaporites • Metamorphic rocks … meta (change) morphic (form) … rocks changed by heat and pressure –but remain solid
Formation of Igneous Rocks • COOLING AND SOLIDIFICATION OF MAGMA/LAVA • INTERGROWN OR INTERCONNECTED SILICATE CRYSTALS IN A RANDOM PATTERN – NOT ORGANIZED
MAGMA VS. LAVA Magma – molten material BELOWthe surface of the Earth Lava – molten material AT OR ABOVE earth’s surface
ENVIRONMENT OF FORMATION COOLING HISTORY
IGNEOUS ROCKS ARE CLASSIFIED BASED ON 2 MAIN CHARACTERISTICS • TEXTURE – MINERAL GRAIN SIZE • MINERAL COMPOSITION (COLOR)
Classification of Igneous Rocks by Texture • WHAT IS TEXTURE? It is NOT how it feels! It’s the SIZE OF MINERAL CRYSTALS
What controls texture? slower cooling coarser crystals deep in ground How fast an igneous rock cools. What controls how fast an igneous rock cools? Where the rock is formed.
ESRT – TOP OF THE IGNEOUS ROCK SCHEME TELLS THE TEXTURE / GRAIN SIZE
EXTRUSIVE – VOLCANIC - LAVA • When lava cools extremely fastcrystals have no time to form – JUST FROZEN IN PLACE RANDOMLY WHEN LAVA ENTERS ATMOSPHERE OR WATER • GLASSY – NON-CRYSTALLINE Non-vesicular Vesicular gas pockets
EXTRUSIVE – VOLCANIC - LAVA Lava that cools fastat or near the Earth’s surface Produces small crystals FINE-GRAINED TEXTURE not easily seen with the naked eye.
INTRUSIVE – PLUTONIC - MAGMA Magma that cools slowly deep underground produces large crystals COARSE-GRAINED TEXTURE easily seen with the naked eye.
INTRUSIVE – PLUTONIC - MAGMA EXTREMELY SLOW COOLING HAS ABNORMALLY LARGE CRYSTALS AND IS VERY COARSE.
CHARACTERISTICS OF IGNEOUS ROCKS IN THE MIDDLE OF THE IGNEOUS ROCK CHART ARE CHARACTERISTICS BASED ON WHICH MINERALS ARE IN EACH ROCK. NOTICE 2 NEW WORDS FELSIC AND MAFIC
Continental mixed oceanic mantle FELSIC MAFIC ULTRAMAFIC
THE BOTTOM OF THE IGNEOUS ROCK SCHEME TELLS YOU THE COMPOSITION TEXTURE + COMPOSITION CAN NAME ROCK
FELSIC COMPOSITION COLUMN + COARSE TEXTURE ROW
INTRUSIVE EXTRUSIVE Granite Rhyolite Figure 4.7 A MINERAL COMPOSITION THE SAME – TEXTURE ONLY DIFFERENCE Intrusive vs. Extrusive
INTRUSIVE EXTRUSIVE Andesite MINERAL COMPOSITION THE SAME ---- TEXTURE ONLY DIFFERENCE Intrusive vs. Extrusive Diorite Figure 4.7 B
INTRUSIVE EXTRUSIVE Basalt Gabbro MINERAL COMPOSITION THE SAME – TEXTURE ONLY DIFFERENCE Intrusive vs. Extrusive
Sedimentary Rocks • 80 % of Earth’s surface is covered with sediments and Sedimentary Rocks (only a thin layer on top)
What is sediment???? • Broken pieces of rocks called Sediments • Chemical precipitates ( dissolved solids come out of solution) • Organic fragments – shells, decayed plant matter – this is called BIOCLASTIC
Source of Sediment • When Igneous, Metamorphic Rocks, and Sedimentary Rocks are uncovered and exposed to the atmosphere and hydrosphere they start to break down • Weathering is the general term for all actions that break down or wear away rock • Erosion is the moving/transport of the sediment
Sedimentary Rocks form when sediments are transported to large bodies of water and are …
Sedimentary Rocks • Sediments that are eroded settle to the bottom of the rivers, lakes, and oceans • Layer after layer of eroded earth is deposited on top of each other
Most Sedimentary Rocks form under large bodies of water, such as lakes, seas, and oceans
Classification of Sedimentary Rocks • We classify sedimentary rocks depending whether the sedimentsare LAND DERIVED (came from) CHEMICALLY - ORGANICALLY DERIVED
Sediments are pebbles, cobbles, and boulders rounded Breccia angular
Silt (0.006 to 0.0004 cms. SILTSTONE Sand (0.2 to 0.006 cms.) SANDSTONE Clay (less than 0.0004 cms.) SHALE
2 TYPES OF TEXTURE • CRYSTALLINE FOR CHEMICALLY FORMED SEDIMENTARY ROCKS • BIOCLASTIC FOR ORGANICALLY FORMED SEDIMENTARY ROCKS • HOW USEFUL IS GRAIN SIZE????
CHEMICAL Sedimentary RocksSediment From Solution • All liquid water contains dissolved minerals. • These minerals precipitate (dissolved solids come out of solution/water) • Usually happens because of evaporation
Characteristics of Chemically Formed Sedimentary Rocks • They are monominerallic – consisting of intergrown crystals of just 1 mineral. • Form in shallow seas
ROCK SALT Intergrown crystals of just 1 mineral DOLOSTONE ROCK GYPSUM