Igneous rock

Definition

Igneous rock is a type of rock formed from the solidification of molten material called magma or lava. This process occurs when magma cools and crystallizes either beneath the Earth's surface, resulting in intrusive igneous rocks, or on the surface after a volcanic eruption, leading to extrusive igneous rocks. The formation of igneous rock plays a crucial role in the rock cycle, as it is one of the three main rock types alongside sedimentary and metamorphic rocks, highlighting its significance in understanding geologic processes.

5 Must Know Facts For Your Next Test

1. Intrusive igneous rocks, such as granite, form from magma that cools slowly underground, allowing larger crystals to develop.
2. Extrusive igneous rocks, like basalt, form from lava that cools quickly on the surface, resulting in smaller crystals or a glassy texture.
3. The composition of igneous rocks can vary widely based on the minerals present in the magma or lava, affecting their color and properties.
4. Igneous rocks can provide insights into the history of volcanic activity and the Earth's internal processes through their mineral content and texture.
5. The rock cycle illustrates how igneous rocks can be transformed into sedimentary or metamorphic rocks through processes like weathering, erosion, and heat and pressure.

Review Questions

• How do intrusive and extrusive igneous rocks differ in their formation processes and characteristics?
  • Intrusive igneous rocks form from magma that cools slowly beneath the Earth's surface, leading to the development of large crystals and a coarse-grained texture. In contrast, extrusive igneous rocks are formed from lava that cools quickly on the surface after a volcanic eruption, resulting in a fine-grained or glassy texture with smaller crystals. These differences are primarily due to the cooling rate of the molten material and significantly influence their physical properties.
• Discuss the significance of magma composition in determining the characteristics of igneous rocks.
  • The composition of magma plays a critical role in shaping the characteristics of igneous rocks. Different mineral compositions lead to variations in color, texture, and density among igneous rocks. For instance, basalt is rich in iron and magnesium and appears darker than granite, which is high in quartz and feldspar. Understanding magma composition not only helps geologists classify igneous rocks but also provides insights into volcanic activity and the tectonic environment where these rocks are formed.
• Evaluate how the rock cycle illustrates the transformation processes involving igneous rocks and their connection to other rock types.
  • The rock cycle showcases how igneous rocks interact with other rock types through processes like weathering, erosion, sedimentation, and metamorphism. For example, an igneous rock can break down into sediments through weathering and then become compacted and cemented to form sedimentary rock. Alternatively, if subjected to high heat and pressure, it can transform into metamorphic rock. This cyclical nature emphasizes the interconnectedness of Earth's materials and highlights how igneous rocks contribute to the dynamic processes shaping our planet's geology.

Related terms

Magma: Magma is a molten mixture of rocks, minerals, and gases located beneath the Earth's surface, from which igneous rocks are formed when it cools and solidifies.

Lava: Lava is molten rock that reaches the Earth's surface during a volcanic eruption and can solidify to form extrusive igneous rocks.

Crystallization:

Crystallization is the process by which atoms arrange themselves into a structured form, occurring as magma or lava cools and forming solid minerals in igneous rocks.