Igneous Rocks Formation and Definition

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.