When it comes to igneous rocks, there are two main types: intrusive and extrusive. Both types have distinct characteristics that set them apart from each other. In this article, we will be focusing on the characteristics common to extrusive igneous rocks.
What Are Extrusive Igneous Rocks?
Extrusive igneous rocks are formed when molten lava is expelled from a volcano and rapidly cools on the Earth’s surface. This rapid cooling prevents the formation of large mineral crystals, resulting in the fine-grained texture commonly associated with extrusive rocks.
Extrusive rocks are also known as volcanic rocks, as they are often associated with volcanic activity. These rocks can be found in areas where volcanic eruptions have occurred, such as the mid-ocean ridges, ocean basins, and volcanic islands.
Characteristic Common to Extrusive Igneous Rocks
There are several characteristics that are common to extrusive igneous rocks. These characteristics are a result of the rapid cooling process that the lava undergoes when it reaches the Earth’s surface. Let’s take a closer look at each of these characteristics:
1. Fine-Grained Texture:
One of the most common characteristics of extrusive igneous rocks is their fine-grained texture. This is a direct result of the rapid cooling of the lava on the Earth’s surface. As the lava cools quickly, there is not enough time for large mineral crystals to form, resulting in a fine-grained texture. This texture is often smooth and feels like sandpaper to the touch.
| Extrusive Igneous Rocks | Texture |
|---|---|
| Basalt | Fine-grained |
| Andesite | Fine-grained |
| Rhyolite | Fine-grained |
2. Porous Nature:
Another common characteristic of extrusive igneous rocks is their porous nature. The rapid cooling process traps gas bubbles within the rock, creating a porous structure. These gas bubbles are often visible to the naked eye and can give the rock a frothy appearance. The porosity of extrusive rocks can vary depending on the amount of gas trapped during the cooling process.
3. Glassy Surface:
Some extrusive rocks, such as obsidian, have a glassy surface due to their rapid cooling process. This glassy appearance is a result of the lack of crystal formation in the rock, giving it a shiny and reflective surface. The glassy nature of these rocks makes them highly valued for decorative purposes.
4. High Silica Content:
Extrusive igneous rocks often have a high silica content, which contributes to their fine-grained texture and overall composition. The high silica content also gives these rocks a high viscosity, making them more resistant to flow. This high silica content is a result of the magma’s composition, which is rich in silica and other elements.
5. Light Coloration:
Many extrusive igneous rocks have a light coloration, often ranging from white to grey. This light coloration is attributed to the composition of the minerals present in the rock, which are often rich in silica and feldspar. The light coloration of these rocks makes them easily distinguishable from other types of igneous rocks.
Examples of Extrusive Igneous Rocks
There are several examples of extrusive igneous rocks that exhibit these common characteristics. Some of the most well-known extrusive rocks include:
- Basalt: A fine-grained extrusive rock with a dark coloration, often found in volcanic areas and ocean basins.
- Andesite: Another fine-grained extrusive rock, typically found in volcanic island arcs and continental margins.
- Rhyolite: A light-colored extrusive rock with a high silica content, commonly found in continental rift zones and volcanic regions.
- Obsidian: A glassy extrusive rock with a dark coloration, prized for its decorative and ornamental uses.
- Pumice: A porous extrusive rock with a frothy appearance, often used in skincare products and lightweight construction materials.
Conclusion
Extrusive igneous rocks have several characteristics that are common to them, including their fine-grained texture, porous nature, glassy surface, high silica content, and light coloration. These characteristics are a result of the rapid cooling process that the lava undergoes when it reaches the Earth’s surface. By understanding these common characteristics, we can better identify and appreciate the unique properties of extrusive igneous rocks.
Whether it’s the smooth texture of basalt, the porous nature of pumice, or the glassy surface of obsidian, extrusive igneous rocks have a diverse range of characteristics that make them a fascinating subject of study in the field of geology.
