Igneous Rocks Formation and Definition

Study Notes

Igneous Rock Definition

Igneous rocks are formed when molten material called magma cools and solidifies.
There are three types of rocks: igneous, sedimentary, and metamorphic rocks.

Igneous Rock Formation

Magma forms in the upper mantle, where the temperature is between 600 to 1300 degrees Celsius.
Rocks can melt if the temperature is at least near the temperature in the upper mantle, the pressure decreases, or volatiles like water are present.
Magma can rise to the Earth's surface through boundaries between tectonic plates or hot spots.

Plutonic vs. Volcanic Rocks

Plutonic rocks are formed when magma is trapped underneath the surface and slowly cools.
Volcanic rocks are formed when magma is extruded onto the Earth's surface through volcanic vents and fissures as lava.

Volcanic Rocks

Volcanic rocks are formed when the molten material is extruded onto the Earth's surface or near the surface.
The rocks formed are also called extrusive rocks.
Volcanic rocks have a fine-grained or glassy texture and may contain vesicles.
Examples of volcanic rocks include pumice.

Pumice

Pumice is formed when lava from a violent eruption quickly cools and solidifies.
It is an example of extrusive igneous rock.
Pumice is characterized by its light-colored, low density, and vesicle-rich appearance.
It is primarily composed of silicon dioxide and some amounts of aluminum oxide.

Plutonic Rocks

Plutonic rocks, also called intrusive rocks, are formed from magma that did not reach the Earth's surface and are trapped underneath the crust.
Plutonic rocks cool at a very slow rate and may take thousands or millions of years to solidify.
They have a coarse-grained texture and can form large crystals that can be easily observed.
Examples of plutonic rocks include granite.

Granite

Granite is formed by a viscous felsic magma that slowly cools underneath the Earth's surface.
It is characterized by its coarse-grained, light-colored crystalline texture.
Its light color originates from its mineral composition, which includes feldspar, quartz, mica, and amphibole.
Granite is an example of intrusive igneous rock.

Plutons

Pluton is a general term for any large igneous body that cools and solidifies near the Earth's surface.
Plutons can fill and widen existing gaps, forming batholiths, dikes, and sills.
They are only exposed to the surface when the overlying rocks are eroded.

Batholiths

Batholiths are irregularly shaped plutons that can reach up to hundreds of kilometers long and several kilometers across.
They are typically composed of several smaller plutons with the same composition.
Examples of batholiths include the Sierra Nevada Batholith in California and Pikes Peak granite in Colorado.

Sills and Dikes

Dikes are formed when magma intrudes on a pre-existing rock body and starts to crystallize as sheet intrusions.
Dikes are characterized by high angle or near-vertical orientation.
Sills are formed when magma intrudes horizontally and is parallel to the beds and foliations in the rock.
Examples of dikes include the Mackenzie dike swarm in the Northwest Territories, Canada and the Spanish Peaks volcanic dikes in Southern Colorado.
Examples of sills include the Dolerite sills located in Karroo, South Africa.

Description

Learn about the definition and formation of igneous rocks, including how they are created from molten magma and cool down to solidify. Understand the characteristics that distinguish igneous rocks from other types of rocks.