The term “igneous” is the Latin word for fire. Igneous rocks form when hot liquid rock, called magma, cools and solidifies. The type of igneous rock that forms depends upon the composition of the magma and the time it takes to cool.

Igneous rock can form in one of three ways: When rock is heated to the point that it melts When pressure is released When rock changes its composition Magma “freezes” or solidifies around 1,000 0 C (1,800 0 F)

Change in Pressure When you remove the radiator cap from an over-heated car, the liquid water immediately changes to steam. By the same process, when pressure on very hot solid rock is released, it changes into a liquid. Pressure can be released as the rock approaches the earth’s surface, or when a fissure or other opening occurs.

Changes in Composition If fluids, such as water, combine with very hot rock, it may lower the melting point, allowing the solid rock to liquefy. This is the opposite of adding salt to ice cream mix to make it freeze.

Classification of Igneous Rocks

Felsic rocks are: Light-colored due to presence of quartz and feldspar Rich in silica (65%), aluminum, potassium, and sodium Began cooling at low temperatures May be intrusive or extrusive Coarse-grained: granite Fine-grained: rhyolite

granite On the right is coarse-grained granite rhyolite On the left is fine-grained rhyolite Grain size is due to cooling time; fast cooling results in fine grains, slow cooling results in larger crystals. Granite is an intrusive felsic igneous rock Rhyolite is an extrusive felsic igneous rock

Mafic rocks are: Dark-colored due to iron, magnesium Rich in calcium, iron, magnesium Began forming at high temps. May be intrusive or extrusive Coarse-grained: gabbro Fine-grained: basalt

Climbing intrusive basalt columns; Devil’s Tower, Wyoming

Basalt, a fine-grained, mafic igneous rock. Note the variation in in color: the dark area is typical basalt, while the lighter areas have undergone chemical reactions with air and water.

Gabbro, a coarse-grained mafic igneous rock

Obsidian is extrusive igneous rock that has cooled so quickly that no crystals can form. Is it Mafic or Felsic?

IntrusivePlutons Intrusive igneous rock formations are called Plutons; there are several different types: Batholiths Batholiths are the largest bodies of magma deposits- these measure at least 40 square miles or more for their upper surface, but can be as large as 15,000 square miles, about the size of Maryland and Connecticut combined

Halfdome in Yosemite National Park This is part of an exposed batholith

stock A stock is smaller than a batholith and typically represents the subsurface passage that fed molten material to a volcano or field of volcanoes over time. Sillsdikes Sills and dikes are layers of igneous rock that typically form along fault zones, fractures, or between and parallel to sedimentary layers.

Dome Mountains are formed from hot molten material (magma) rising from the Earth's mantle into the crust that pushes overlying sedimentary rock layers upward to form a "dome" shape. Unlike a volcano, the magma typically does not reach the Earth's surface. Instead, the magma cools underneath the surface and forms the core of the mountains. Navajo Mt.→

Note the dark intrusion of igneous rock in the lighter sedimentary rock. Is this a dike or a sill? Is this felsic or mafic igneous rock? Which rock type was there first? If this is an intrusive rock type, why is it now exposed?

Shiprock, New Mexico volcanic neck Shiprock is a volcanic neck Note the exposed dike

Extrusive igneous formations are created by lava flows, which can take many forms and produce a wide variety of rock formations.

Capitol Reef National Park, Utah Note the large, dark areas of igneous rock.

Pillow Lava

Lava tube in Hawaii

Lava tube cave near Flagstaff, Arizona Formed about 700,000 years ago

Sunset Crater National Monument, near Flagstaff, AZ

Giant’s Causeway in Ireland. How did this form? What type of rock is it formed of?