Intrusive Vs. Extrusive Igneous Rocks
Igneous rocks are formed through the cooling and solidification of molten rock, or magma, either beneath the Earth’s surface or at the surface. Igneous rocks are classified into two main types based on where they form: Intrusive (Plutonic) Igneous Rocks and Extrusive (Volcanic) Igneous Rocks.
Intrusive Igneous Rocks (Plutonic Rocks)
Intrusive igneous rocks, also known as plutonic rocks, are formed when magma cools and solidifies beneath the Earth's surface. This process occurs when magma from the Earth's mantle rises into the crust but does not reach the surface. Instead, it accumulates in large underground chambers, called magma chambers, where it cools and solidifies over time.
Characteristics of Intrusive Igneous Rocks
Formation Location: Intrusive igneous rocks form beneath the Earth's surface when magma cools and solidifies slowly within the Earth’s crust. This often happens in large bodies like magma chambers.
Cooling Rate: Since they are formed deep underground, where temperatures are higher and cooling is slow, intrusive rocks cool gradually over thousands or even millions of years.
High density: Intrusive rocks are generally denser than their extrusive counterparts due to the tightly packed crystals.
Texture: Typically phaneritic (coarse-grained), The slow cooling allows the minerals to grow into large, visible crystals, making the texture coarse-grained. This means individual mineral grains can be seen with the naked eye.
Occurrence Locations: Intrusive rocks are often found in mountain ranges, where deep erosion has exposed them. They are also found in batholiths (large masses of intrusive rock), dikes, sills, and laccoliths (underground intrusions).
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| Examples of Intrusive Igneous Rocks (Plutonic Rocks) |
Examples of Intrusive Igneous Rocks
Granite: A common intrusive rock, consisting mainly
of quartz, feldspar, and mica. It has a light color and a coarse-grained
texture. It forms deep beneath the Earth's surface in large bodies (batholiths). Widely used in construction and monuments, it's found in mountain ranges like the Sierra Nevada.
Gabbro: A dark-colored intrusive rock with a coarse-grained texture and a mixture of pyroxene, plagioclase, and olivine minerals, mafic in composition. It’s commonly found in the oceanic crust.
Diorite: An intermediate rock between granite and gabbro in terms of mineral composition, with less quartz, diorite contains feldspar, hornblende, and plagioclase.
Peridotite is a dense, ultramafic rock primarily composed of olivine and pyroxene. It forms in the Earth's mantle and is brought to the surface through tectonic processes. Peridotite is the primary source of olivine and diamonds.
Pegmatite is an intrusive igneous rock characterized by extremely coarse-grained crystals, often several centimeters or more in size. It forms from the last stages of magma differentiation, where volatile components promote the growth of large crystals. Common minerals include quartz, feldspar, and mica, but pegmatites can also contain rare minerals and gemstones such as tourmaline, beryl, and others rich in lithium, cesium, and tantalum.
Types of Intrusive Bodies (Plutons)
Batholith: A large mass of intrusive igneous rock, usually covering hundreds of square kilometers.
Sill: A horizontal or near-horizontal intrusion that occurs between layers of sedimentary rock.
Dike: A vertical or steeply inclined intrusion that cuts across existing rock layers.
Laccolith: A dome-shaped intrusion where the magma pushes the overlying rock upward.
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| Types of igneous Bodies (Plutons) |
Extrusive Igneous Rocks (Volcanic Rocks)
Extrusive igneous rocks, also known as volcanic rocks, are formed when magma reaches the Earth's surface and cools rapidly. This process occurs when magma from the Earth's mantle rises to the surface through volcanic vents or fissures, resulting in volcanic eruptions.
Characteristics of Extrusive Igneous Rocks
Formation Location: Extrusive igneous rocks form on or near the Earth's surface from lava that erupts from volcanoes or fissures and cools quickly.
Rapid Cooling: Lava cools quickly when it reaches the surface, either in air or under water, leading to fine-grained or glassy textures because there's not enough time for large crystals to grow.
Crystal Size: Rapid cooling prevents the formation of large crystals, resulting in a fine-grained or aphanitic texture. Often, crystals are too small to be seen without magnification. Some extrusive rocks cool so fast that no crystals form, creating glassy textures.
Lower density: Extrusive rocks are generally less dense than intrusive rocks due to the presence of vesicles (gas bubbles) and smaller crystal sizes.
Occurrence Locations: Extrusive igneous rocks are predominantly found in areas associated with volcanic activity, such as mid-ocean ridges where new crust is created, hot spots like Hawaii where magma plumes rise through the lithosphere, and subduction zones where plates converge, leading to explosive or effusive eruptions that form structures like lava flows, volcanic cones, and extensive ash deposits.
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| Examples of Extrusive Igneous Rocks (Volcanic Rocks) |
Examples of Extrusive Igneous Rocks
Basalt: The most common extrusive rock, Basalt is A dark-colored extrusive rock with a fine-grained texture and a mixture of pyroxene, plagioclase, and olivine minerals. Very common on the ocean floor and in volcanic areas.
Rhyolite: Light-colored, fine-grained volcanic rock rich in silica, often found in continental volcanic settings. It is chemically similar to granite but extrusive.
Pumice: Formed during explosive volcanic eruptions, pumice is a light-colored, highly vesicular volcanic rock with such a low density due to its high gas content that it can float on water. Its texture is frothy, resulting from rapidly cooling lava filled with gas bubbles.
Scoria: While akin to pumice in its vesicular texture, scoria is denser and darker, typically reddish-brown or black, reflecting its mafic composition. It forms from gas-rich basaltic or andesitic lava, cooling quickly to trap numerous bubbles of gas. Scoria often emerges as cinders from volcanic vents and is popularly used in landscaping and as drainage material.
Obsidian: A natural volcanic glass formed when lava cools so rapidly that no crystals have time to form. It has a smooth, glassy appearance and is often black or dark brown.
Extrusive Igneous Rocks Textures
Aphanitic Texture: Fine-grained texture where individual crystals are too small to be seen without magnification. This results from relatively quick cooling. Examples include basalt, andesite, and rhyolite when they cool rapidly.
Glassy Texture: When lava cools extremely quickly, there's no time for crystal formation, resulting in a glass-like texture. Obsidian is the prime example of this texture.
Vesicular Texture: Contains numerous cavities known as vesicles formed by gas bubbles escaping from the lava as it cools. Pumice (which can float on water due to its high vesicle content) and scoria are typical examples.
Pyroclastic Texture: Composed of volcanic fragments that can range from fine ash to larger bombs or blocks, ejected during explosive eruptions. Textures can be quite varied, from tuff (consolidated ash) to breccia (angular fragments cemented together).
Porphyritic Texture: Features two distinct sizes of crystals, with larger crystals (phenocrysts) embedded in a finer-grained or glassy matrix. This indicates a two-stage cooling process where some crystals grew slowly underground before the magma was erupted and cooled quickly.
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| Extrusive Igneous Rocks Textures: Aphanitic, glassy, Vesicular, and Pyroclastic Texture. |
Volcanic Structures Associated with Extrusive Rocks
- Lava Flow: A sheet of lava that flows over the ground during an eruption and solidifies into rock.
- Volcanic Dome: A rounded, dome-shaped mass formed by the slow extrusion of viscous lava.
- Pyroclastic Flow: A fast-moving current of hot gas and volcanic materials (ash, pumice, and rock fragments) that solidifies into a pyroclastic rock like tuff.
- Lava Plateau: Extensive, flat areas formed by repeated lava flows over time.
Intrusive and Extrusive Equivalents: A Tale of Two Textures
Many intrusive and extrusive rocks share similar chemical compositions but differ significantly in texture due to their contrasting cooling histories. For example:
Granite (Intrusive) and Rhyolite (Extrusive): Both are felsic rocks rich in silica and feldspar, but granite has a coarse-grained texture while rhyolite is fine-grained.
Gabbro (Intrusive) and Basalt (Extrusive): Both are mafic rocks rich in magnesium and iron, but gabbro is coarse-grained while basalt is fine-grained.
Differences Between Intrusive and Extrusive Rocks
| Property | Intrusive Igneous Rocks | Extrusive Igneous Rocks |
|---|---|---|
| Location of Formation | Beneath the Earth’s surface | On or near the Earth’s surface |
| Cooling Rate | Slow | Rapid |
| Crystal Size | Large, visible crystals (coarse-grained) | Small or no crystals (fine-grained) |
| Texture | Phaneritic (coarse-grained) | Aphanitic (fine-grained), glassy, or vesicular |
| Examples | Granite, Gabbro, Diorite | Basalt, Rhyolite, Obsidian |
| Silica Content | Varies, but often higher in silica | Varies, often lower in silica, except for rhyolite |
| Occurrence | Found in plutons, batholiths, dykes | Found in lava flows, volcanoes, and ash deposits |
In summary, Intrusive and extrusive igneous rocks are both formed from the cooling of magma or lava, but their formation locations lead to distinct characteristics. Intrusive rocks, which cool slowly beneath the Earth's surface, have a coarse-grained texture due to the ample time for crystals to grow. Extrusive rocks, on the other hand, cool rapidly at the surface, resulting in fine-grained or even glassy textures. Both types play important roles in the Earth's geology and have various practical applications.