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: 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
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, has quartz inside
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
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
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)
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
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
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
The Rock Cycle