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Classifying Rocks

Classifying Rocks. Geologists observe: Mineral composition Color Texture a. Grain size b. Grain shape c. Grain pattern. Igneous Rocks. Classified according to their origin, texture, and mineral composition. ORIGIN:

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Classifying Rocks

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  1. Classifying Rocks Geologists observe: Mineral composition Color Texture a. Grain size b. Grain shape c. Grain pattern

  2. Igneous Rocks • Classified according to their origin, texture, and mineral composition. ORIGIN: Extrusive rock – igneous rock formed from lava that erupted onto Earth’s surface Intrusive rock – igneous rock formed when magma hardens beneath Earth’s surface

  3. Igneous Rocks (continued) • Texture depends on the size and shape of it’s mineral crystals. • Rapidly cooling lava forms fine grained igneous rocks with small crystals (extrusive rocks) • Slowly cooling magma forms coarse grained rocks with large crystals (intrusive rocks) • Mineral composition – low silica = dark color, no quartz / high silica = light colored rocks, hasquartz inside

  4. Examples of Igneous Rocks Extrusive rock – igneous rock formed from lava that erupted onto earth’s surface – Basalt Intrusive rock – igneous rock formed when magma hardens beneath earth’s surface - Granite

  5. Sedimentary Rocks(from sediment to rock) Sediment: small, solid pieces of material that come from rock or living things Erosion: running water, wind, or ice loosen and carry away fragments of rock Deposition: sediment settles out of the water or wind carrying it Compaction: process that presses sediments together Cementation: dissolved minerals crystallize and glue particles of sediment together

  6. Types of Sedimentary Rocks • Clastic Rocks: sedimentary rock that forms when rock fragments are squeezed together (Shale, Sandstone, & Conglomerate) • Organic Rocks: remains of plants and animals are deposited in thick layers (Coal & Limestone) • Chemical Rocks: when minerals that are dissolved in a solution crystallize (rock salt)

  7. Metamorphic Rocks Formed by heat and pressure Foliated Rocks: metamorphic rocks that have their grains arranged in parallel layers or bands – Slate & Gneiss Nonfoliated Rocks: mineral grains are arranged randomly, they do not split into layers – Marble & Quartzite

  8. Dating Rocks Relative age – the age compared to the ages of other rocks Absolute age – the number of years since the rock was formed Law of Superposition – the oldest rock layer is found at the bottom Unconformity: location where new rock meets older rock beneath the surface – shows a gap in the geological record

  9. Radioactive Decay Radioactive Decay: the atoms of one element break down to form atoms of another element * Occur naturally in igneous rocks Half-life in a radioactive element: the time it takes for half of the radioactive elements to decay Scientists use radioactive dating to determine the absolute ages of rocks

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