Which Characteristic Is Common To Extrusive Igneous Rocks

Extrusive igneous rocks, born from the fiery embrace of volcanoes and rapidly cooled at the Earth's surface, possess unique characteristics that set them apart. One of the most defining features they share is their fine-grained texture. This shared trait is a direct consequence of their rapid cooling process, which inhibits the growth of large crystals. Let's delve deeper into the world of extrusive igneous rocks and explore the factors that contribute to this common characteristic.

What are Extrusive Igneous Rocks?

Extrusive igneous rocks, also known as volcanic rocks, are formed when magma erupts onto the Earth's surface as lava and cools quickly. This rapid cooling occurs due to the significant temperature difference between the molten rock and the surrounding atmosphere or water. Examples include:

Basalt: A dark-colored, fine-grained rock that forms the oceanic crust and is commonly found in lava flows.
Rhyolite: A light-colored, fine-grained rock with a similar composition to granite, but with much smaller crystals.
Andesite: An intermediate-colored, fine-grained rock commonly found in volcanic arcs above subduction zones.
Obsidian: A volcanic glass formed from extremely rapid cooling, resulting in a glassy texture.
Pumice: A light-colored, porous rock formed from gas-rich lava, resulting in a frothy texture.
Tuff: A rock composed of volcanic ash and other fragmented volcanic debris.

The Defining Characteristic: Fine-Grained Texture

The defining characteristic common to virtually all extrusive igneous rocks is their fine-grained texture, also known as aphanitic texture. This means that the individual mineral crystals that make up the rock are so small that they are difficult or impossible to see with the naked eye. To see these crystals, you would typically need a microscope.

Why Fine-Grained Texture? The Science Behind Rapid Cooling

The formation of fine-grained texture in extrusive rocks is governed by the principles of crystal growth and nucleation. When magma cools, atoms and molecules begin to bond together to form mineral crystals. The size of these crystals depends on two key factors:

Nucleation Rate: The rate at which new crystal nuclei (the initial seeds for crystal growth) form.
Crystal Growth Rate: The rate at which existing crystal nuclei grow in size.

When magma cools rapidly, the nucleation rate is very high. This means that many crystal nuclei form simultaneously throughout the cooling magma. However, because the magma is cooling so quickly, there is not enough time for these nuclei to grow into large, well-developed crystals. The crystal growth rate is significantly limited.

The result is a rock composed of numerous, tiny crystals packed tightly together. This is what we observe as a fine-grained texture. Imagine baking cookies: if you rapidly cool the dough, you'll end up with a lot of very small, underdeveloped cookies instead of a few large, well-formed ones.

In contrast, intrusive igneous rocks (formed when magma cools slowly beneath the Earth's surface) have a coarse-grained texture (also known as phaneritic texture). This is because the slow cooling allows for fewer nucleation sites and more time for crystal growth, resulting in larger, visible crystals.

Factors Influencing Grain Size in Extrusive Rocks

While fine-grained texture is the hallmark of extrusive rocks, the exact grain size can vary depending on several factors:

Cooling Rate: The faster the cooling rate, the finer the grain size. Extremely rapid cooling, as in the case of obsidian, can prevent crystal formation altogether, resulting in a glassy texture.
Magma Composition: The chemical composition of the magma can influence the rate of crystal growth. Some elements and compounds may promote faster crystal growth than others.
Volatile Content: The presence of volatiles (dissolved gases) in the magma can also affect crystal growth. Volatiles can lower the viscosity of the magma, allowing for faster diffusion of atoms and molecules, potentially leading to slightly larger crystals.
Depth of Extrusion: Although they are classified as extrusive, some lavas might cool slightly slower if they solidify just below the surface, leading to slightly coarser textures than those that cool in open air or water.

Exceptions to the Rule: Porphyritic Texture

While fine-grained texture is the most common characteristic of extrusive rocks, there are exceptions. One notable exception is porphyritic texture.

Porphyritic texture is characterized by the presence of large, well-formed crystals (phenocrysts) embedded in a fine-grained matrix (groundmass). This texture indicates a two-stage cooling history.

In the first stage, the magma cools slowly at depth, allowing for the formation of large phenocrysts. Then, the magma is erupted onto the surface, where it cools rapidly. The remaining liquid solidifies quickly, forming the fine-grained groundmass around the pre-existing phenocrysts.

Porphyritic texture is a valuable tool for geologists because it provides clues about the magma's journey and the conditions under which it formed.

Examples of Extrusive Rocks and Their Textures

Let's examine some common extrusive rocks and their typical textures:

Basalt: Typically fine-grained, but can be porphyritic if it contains phenocrysts of olivine or pyroxene. The rapid cooling on the earth's surface prohibits the formation of large crystals, hence the fine grains.
Rhyolite: Usually fine-grained or glassy. If it contains phenocrysts (e.g., quartz or feldspar), it is classified as porphyritic rhyolite. Its high silica content influences its viscosity and thus its cooling rate.
Andesite: Typically fine-grained. Like basalt and rhyolite, it can also exhibit porphyritic textures. Andesite's intermediate composition situates its texture between the extremes of basalt and rhyolite.
Obsidian: Glassy texture. The extremely rapid cooling prevents any crystal formation. Its smooth, conchoidal fracture is distinctive.
Pumice: Vesicular (porous) and glassy texture. The abundant gas bubbles give it a lightweight, frothy appearance. The rapid cooling and depressurization trap gas within the solidifying lava.
Tuff: Variable texture, depending on the size and type of volcanic debris. It can range from fine-grained (ash tuff) to coarse-grained (lapilli tuff). Tuff is a product of explosive volcanic eruptions.

Distinguishing Extrusive Rocks from Intrusive Rocks

The texture of an igneous rock is a primary factor used to distinguish between extrusive and intrusive rocks. As previously mentioned, intrusive rocks cool slowly beneath the Earth's surface, resulting in a coarse-grained texture. Examples of intrusive rocks include:

Granite: A light-colored, coarse-grained rock that is a major component of the continental crust.
Diorite: An intermediate-colored, coarse-grained rock with a composition between granite and gabbro.
Gabbro: A dark-colored, coarse-grained rock that is the intrusive equivalent of basalt.
Peridotite: An ultramafic, coarse-grained rock composed primarily of olivine and pyroxene, found in the Earth's mantle.

The difference in cooling rates between extrusive and intrusive environments leads to a striking difference in crystal size. In general, if you can easily see the individual mineral crystals with the naked eye, the rock is likely intrusive. If the crystals are too small to see without magnification, the rock is likely extrusive.

Why is Understanding Texture Important?

The texture of an igneous rock provides valuable information about its origin and the geological processes that shaped it. By studying the texture, geologists can:

Determine the cooling history of the magma: Was it a rapid eruption or a slow, gradual cooling process?
Infer the depth of formation: Did the rock form at the surface or deep within the Earth?
Understand the composition of the magma: The texture can provide clues about the chemical composition of the magma and the minerals that crystallized from it.
Reconstruct past volcanic events: The texture of volcanic rocks can help geologists understand the nature and intensity of past volcanic eruptions.
Classify and identify rocks: Texture is a key characteristic used in the classification and identification of igneous rocks.

The Economic Significance of Extrusive Rocks

Extrusive rocks play a significant role in various economic activities:

Construction Materials: Basalt and other extrusive rocks are used as aggregate in concrete and asphalt, as well as for building stone.
Geothermal Energy: Volcanic regions with abundant extrusive rocks are often sources of geothermal energy, which can be harnessed for electricity generation and heating.
Mineral Resources: Some extrusive rocks are associated with valuable mineral deposits, such as gold, silver, and copper.
Soil Formation: The weathering of extrusive rocks contributes to the formation of fertile soils, which are essential for agriculture.
Tourism: Volcanic landscapes with unique extrusive rock formations are popular tourist destinations, generating revenue for local communities.

In Conclusion: The Fine-Grained Story

The fine-grained texture is a unifying characteristic of extrusive igneous rocks, a testament to their rapid cooling at the Earth's surface. This rapid cooling inhibits the growth of large crystals, resulting in rocks composed of numerous, tiny mineral grains. While there are exceptions to this rule, such as porphyritic textures, the fine-grained nature of most extrusive rocks serves as a valuable indicator of their volcanic origin. Understanding the texture of igneous rocks is crucial for geologists to unravel the mysteries of Earth's geological past and for utilizing these rocks for various economic purposes. From the towering basalt columns of the Giant's Causeway to the lightweight pumice stones used in personal care, extrusive rocks are a ubiquitous and economically important part of our planet.

Frequently Asked Questions (FAQ)

Why are extrusive rocks called volcanic rocks? Extrusive rocks are called volcanic rocks because they are formed from lava that erupts from volcanoes.
What is the difference between lava and magma? Magma is molten rock beneath the Earth's surface, while lava is molten rock that has erupted onto the Earth's surface.
Are all dark-colored rocks extrusive? No, not all dark-colored rocks are extrusive. Some dark-colored intrusive rocks, such as gabbro, also exist. However, dark color combined with a fine-grained texture is a strong indicator of an extrusive rock like basalt.
Can I find fossils in extrusive rocks? It is rare to find fossils in extrusive rocks because the high temperatures of lava typically destroy any organic material. Fossils are more commonly found in sedimentary rocks.
What tools do geologists use to study the texture of rocks? Geologists use various tools to study rock textures, including hand lenses, petrographic microscopes, and scanning electron microscopes (SEMs). A hand lens is useful for examining macroscopic features, while microscopes are necessary for observing the fine details of crystal size and shape.
How does the color of an extrusive rock relate to its composition? Generally, light-colored extrusive rocks (like rhyolite) are rich in silica and feldspar, while dark-colored extrusive rocks (like basalt) are rich in iron and magnesium.
What are some uses of pumice in everyday life? Pumice is used in various applications, including exfoliating skin, polishing materials, and as an abrasive in cleaning products. Its porous texture makes it an effective scrubbing agent.
How do volcanic eruptions affect the texture of the rocks they produce? The style of a volcanic eruption (effusive vs. explosive) can influence the texture of the resulting rocks. Effusive eruptions, which involve the slow flow of lava, tend to produce fine-grained or glassy textures. Explosive eruptions, which involve the violent ejection of volcanic ash and debris, can produce fragmental textures in rocks like tuff and volcanic breccia.
Is obsidian a mineral? Obsidian is not a mineral because it lacks a crystalline structure. Minerals are defined as naturally occurring, inorganic solids with a definite chemical composition and a crystalline structure. Obsidian is a volcanic glass, which is an amorphous (non-crystalline) solid.
How does the viscosity of lava affect the texture of the rock it forms? The viscosity of lava plays a crucial role. Low-viscosity lava, like that of basaltic composition, allows gases to escape easily and tends to form smooth, fine-grained textures. High-viscosity lava, like that of rhyolitic composition, traps gases, which can lead to the formation of vesicular textures (like in pumice) or contribute to explosive eruptions that produce fragmental textures.
What is a volcanic bomb, and how does it relate to extrusive rocks? A volcanic bomb is a blob of molten rock ejected from a volcano during an explosive eruption. As it flies through the air, it cools and solidifies, often developing aerodynamic shapes. Volcanic bombs are classified as extrusive rocks because they solidify from lava that has been erupted onto the Earth's surface. Their texture can vary depending on the cooling rate and composition of the lava.

By understanding the characteristics of extrusive igneous rocks, we gain valuable insights into the dynamic processes that shape our planet.