Rocks, Rocks and More Rocks! Igneous – “FIRE ROCKS” (for now) Extrusive Igneous Rocks Notes: Extrusive Igneous Rocks form when lava erupts from a volcano or “bubbles up” from tectonic plates moving apart - onto Earth’s surface. Ex: Basalt – the most common extrusive rock on Earth; forms most of Earth’s ocean floor. Ex: Obsidian – black volcanic glass Earth's Most Abundant Bedrock: Basalt Basalt forms more of Earth's surface than any other rock type. Most areas in ocean basins are basalt. May be on land from lava flows, too. Igneous Rocks Igneous Rock - Obsidian Obsidian was used to make knives, arrow heads, spear points, scrapers and many other weapons and tools How igneous rocks form… Extrusive When a volcano erupts or when magma bubbles to the surface Intrusive when magma cools slowly inside earth Intrusive Igneous Rocks Notes: forms when magma hardens beneath Earth’s surface (INSIDE). Ex: Granite – the most abundant intrusive rock on Earth’s continents. Forms the core of many mountain ranges. Ex: Porphyry- looks like jello w/fruit inside it Intrusive Igneous Rocks Granite is used to make many objects Granite is also well-known from its many worldfamous natural exposures. These include: Stone Mountain, GA; Mount Rushmore, SD Intrusive Igneous Rock Porphyry Igneous Rocks Notes: Texture – fine grained Rapidly cooling lava forms a finegrained rock with small crystals. Ex: Basalt - crystals too small to be seen without a microscope. Ex: Obsidian –smooth, shiny texture like thick glass & cooled without forming crystals “Volcanic Glass” Igneous Rocks Obsidian Obsidian on the side of a volcano Igneous Rocks Notes: Texture: coarse-grained Slowly cooling magma forms coarsegrained rock with large crystals Ex: granite and porphyry Igneous Rocks – Mineral Composition Notes: When magma is high in silica lightcolored rocks form - like granite. When lava is low in silica darkcolored rocks form - like basalt High Silica vs. Low Silica Uses of Igneous Rock Granite – Statues, Bridges and old public buildings, Paving streets with cobblestone, floors, kitchen countertops (thin polished sheets of Granite) Uses of Igneous Rock Basalt - Gravel for construction Obsidian - Sharp tools for cutting and scraping (Native Am!), Starting vegetable gardens (fertilizer) – it’s called Perlite, which is formed from heating Obsidian Pumice is cool! It can float! Pumice polishing – used for cleaning and Uses of Igneous Rock Statues, Public Buildings - Granite Uses of Igneous Rocks Cobblestone streets, kitchen countertops – Granite Questions… What processes form Igneous Rocks? How are the 2 types of Igneous Rock different? How can we use Igneous Rocks? How have they been used in the past? Can you describe the most common Intrusive Ign. Rock? Where is it commonly found? Can you describe the most abundant Extrusive Ign. Rock? What area on Earth is mostly made of this type?