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.
Igneous – “FIRE 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
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.
Obsidian was used to make knives, arrow heads, spear points, scrapers and many other weapons and tools
When a volcano erupts or when magma bubbles to the surface
Intrusive when magma cools slowly inside earth
Ex: Granite – the most abundantintrusive rock on Earth’s continents.
Forms the core of many mountain ranges.
Ex: Porphyry- looks like jello w/fruit inside it
is used to make many objects
Granite is also well-known from its many world-famous natural exposures.
These include: Stone Mountain, GA; Mount Rushmore, SD
Rapidly cooling lava forms a fine-grained 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”
Obsidian on the side of a volcano
Slowly cooling magma forms coarse-grained rock with large crystals
Ex: granite and porphyry
When magma is high in silica light-colored rocks form - like granite.
When lava is low in silica dark-colored rocks form - like basalt