NSSS5.2

toc =5.2 Igneous Rocks=

Vocabulary

 * Extrusive Rock**: Igneous rock made from hardened lava
 * Intrusive Rock**: Hardened magma beneath the Earth's surface
 * Porphyritic texture**: Large crystals scattered with a background of smaller crystals

Characteristics of Igneous Rock

 * Igneous rocks were the first rocks that hardened on the Earth
 * Igneous comes from the Latin word //ignis// meaning fire
 * Igneous rocks are classified by their origin, texture, and minerals
 * Origin
 * There are two classifications of igneous rocks
 * Extrusive rock
 * Basalt is the most common of the extrusive rocks
 * Basalt makes up most of the crust, including lava plateaus, oceanic crust, and shield volcanoes
 * Intrusive rock
 * Granite is the most common intrusive rock
 * Granite batholiths are the core of many dead volcanoes
 * Texture
 * An igneous rock's texture are usually fine or coarse grained, or glassy or porphyritic
 * Intrusive and extrusive have different textures
 * Intrusive rocks have a porphyritic texture or larger crystals in front of smaller ones
 * Extrusive rocks have a glassy texture and it contains crystals too small to be seen
 * Mineral Composition
 * Magma, high in silica, create granite which can have many different shades
 * Granite, rich in hornblende and mica, is light gray with dark specks while granite rich in feldspar is speckled pink

Uses of Igneous Rocks

 * Since igneous rocks are hard, dense, and durable, people used this rock for tools and other building materials.
 * Native Americans have used obsidian for making very sharp tools
 * Granite has been used for more than 3,500 years.
 * Egyptians used it for statues
 * The Incas used granite and other igneous rocks to make a fortress
 * Even today, granite is used to make buildings and bridges
 * Pumice, basalt and obsidian have been also been used for important purposes
 * Crushed basalt is used to make gravel
 * Pumice's rough surface is used for polishing
 * Perlite is mixed in soil for starting vegetable seeds