Rocks Adapted from: Dr. Michael J. Passow

Sedimentary Rocks Consist mainly of weathered rock debris which have become compacted and cemented Cover about ¾ of Earth’s surface

Sediment Sizes Gritty in your teeth

Clastic texture - made of cemented (lithified) sediments, classified by their grain sizes Ex. Shale – the most common sedimentary rock

Chemical sedimentary rocks form by chemical precipitation (evaporation of water) Ex. - Limestone is made from calcite, chert from quartz, and rock salt is halite.

Biochemical/Organic sedimentary rocks – come from the remains of organic matter Lignite, Bituminous, Anthracite coal results from the greatest pressure and releases the most energy when burned. Coquina forms from sea shells Fossiliferous limestone formed from shell fragments

IGNEOUS ROCKS Form from crystallization of magma EXTRUSIVE – formed from lava –Fine-grained (small) crystals –Glassy (no crystals) INTRUSIVE - formed from magma –Coarse-grained (large) crystals

Aphanitic Texture - Crystals are uniformly fine- grained - Individual crystals invisible to the unaided eye - Magma cooled rapidly Ex. - Basalt

Porphyritic Texture - Large crystals (phenocrysts) are surrounded by an aphanitic matrix (groundmass) - 2 rates of cooling Ex. – Diorite porphyry

Vesicular Texture - Vesicles are holes produced by gases escaping from lava as it solidifies - Cooled rapidly Ex. - Scoria

Glassy Texture - Very rapid cooling - Volcanic glass Ex. - Obsidian

Phaneritic Texture - Large crystals - Cooled slowly, underground - Individual minerals are evident Ex. - Diorite

Granite Light-colored, coarse- grained, no pattern Mostly quartz, feldspar, mica, and hornblende Often used for buildings and monuments

Basalt Dark-colored, fine-grained, extrusive Formed where lava erupted onto surface Most widespread igneous rocks

Gabbro Dark-colored, coarse- grained intrusive Similar composition to basalt—plagioclase feldspar with some pyroxene and olivine

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

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: cks1.html cks1.html

Metamorphic Rocks Formed by heat and pressure changing existing rocks REGIONAL METAMORPHISM 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

“Foliated” texture show layering or banding

Non-foliated texture rocks include marble, which comes from limestone, and quartzite, which comes from sandstone

Rock Cycle