Igneous Rocks

Devil's Tower National Monument

In Wyoming is an example of an igneous rock.

Igneous rock

Rocks formed by the cooling and solidification of molten magma.

Igneous rocks

Can have many different compositions, depending on the magma they cool from. They can also look different based on their cooling conditions.

Igneous rocks are grouped according to

• Chemical composition
• Mode of occurrence
• Mineralogy
• Geometric setting of the igneous structure
• Texture

Felsic igneous rocks

Containing an excessive silica content, more than 63% SiO₂ (examples: granite and rhyolite)

Intermediate igneous rocks

Containing among 52–63% SiO₂ (examples: andesite and dacite)

Mafic igneous rocks

Have low silica (45–52%) and typically high iron–magnesium content material (examples: gabbro and basalt)

Ultramafic igneous rocks

Rocks with much less than 45% silica (examples: picrite, komatiite, and peridotite)

Alkalic igneous rocks

With 5–15% alkali (K₂O + Na₂O) content or with a molar ratio of alkali to silica extra than 1:6 (examples phonolite and trachyte)

Extrusive rocks

Are formed on the surface of the Earth from lava, which is magma that has emerged from underground.

Intrusive rocks

Are formed from magma that cools and solidifies within the crust of the planet.

Intrusive rocks

Also called plutonic rocks, cool slowly without ever reaching the surface. They have large crystals that are usually visible without a microscope. This surface is known as a phaneritic texture.

Intrusive Igneous Rock

Perhaps the best-known phaneritic rock is granite.
One extreme type of phaneritic rock is called pegmatite, found often in the U.S. state of Maine.
Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand.

Extrusive

Or volcanic, igneous rock is produced when magma exits and cools above (or very near) the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere.

Extrusive igneous rock

Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture.

Extrusive Igneous Rock

Magma comes out as lava and cools on the surface

Intrusive Igneous Rock

Magma cools off beneath the surface

Aphanitic (fine-grained)

Rapid rate of surface cooling results in microscopic crystals
The top sample is rhyolite, which has the same compostion as granite
Aphanitic rocks may exhibit a secondary vesicular texture, like this basalt, as gas escaped from the lava

Glassy texture

This texture forms when lava from a volcanic eruption cools very rapidly such that no crystallization occurs.
This results in an amorphous glass that has little or no crystals. Obsidian and pumice rocks have this type of texture.

Pegmatitic texture

This type of texture is formed when magma cools and some minerals increase in size extensively. The sizes may range from some centimeters to quite a number of meters. Pegmatite displays this texture.

Phaneritic texture

This texture is seen in plutonic igneous rocks, which underwent slow crystallization underneath the surface of the Earth.
When magma cools at a slow pace, the minerals are able to increase in size and have large crystals.
The crystals can be seen and distinguished by the naked eye.
Diorite, gabbro and granite possess this type of texture.

Porphyritic texture

This texture is caused by the rapid change of conditions as the magma continues to cool down.
Can also form when magma is crystallized under a volcano but eruption occurs before the crystallization is complete. As a result, the lava formed crystallizes much faster with smaller-sized crystals.

Pyroclastic texture

Form when violent volcanic eruptions throw the lava into the atmosphere, creating fragmental and glassy materials. These materials eventually fall to the surface as lapilli, volcanic ash and volcanic bombs.

Mayon Volcano

Andesite rock is an igneous rock found on the Mayon Volcano, which is in the Philippines.
Rhyolite is another type of igneous rock found on stratovolcanoes (like Mount Mayon). In color, rhyolite is light or almost white, with a lot of silica.