Igneous intrusions, also known as plutonic formations, occur when magma cools and solidifies beneath the Earth's surface. These formations vary in size and shape and can be categorized into several major types:
Major Types of Igneous Intrusions
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Batholiths
- Description: Batholiths are large, deep-seated intrusions that cover extensive areas, often exceeding 100 square kilometers. They typically form from the accumulation of multiple plutonic events, with the magma cooling slowly over millions of years.
- Illustration: Imagine a vast underground dome of granite rising from the Earth's crust, sometimes exposed at the surface through erosion.
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Stocks
- Description: A stock is similar to a batholith but smaller in size, covering less than 100 square kilometers. Stocks often appear as isolated or elongated masses that intrude surrounding rock layers.
- Illustration: Visualize a smaller dome or irregular body of granite peeking out from the surrounding sedimentary rock.
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Sills
- Description: Sills are horizontal or gently inclined sheets of igneous rock that form when magma intrudes between existing rock layers, cooling and solidifying in that position.
- Illustration: Picture a layer of granite or diorite interspersed laterally between sedimentary rocks, resembling a flat sandwich in a geological layer cake.
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Dykes
- Description: Dykes are vertical or steeply inclined intrusions that cut across existing rock layers. They form when magma forces its way through cracks and fissures, solidifying in a column-like shape.
- Illustration: Imagine a stair-step fracture in the surrounding rocks filled with dark-colored basalt or andesite.
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Laccoliths
- Description: Laccoliths are mushroom-shaped intrusions that form when magma pushes up the layers of overlying rock but does not break through to the surface. They often have a domed top and a flat bottom.
- Illustration: Envision a pustule bulging out from beneath the surface layers, often causing the surrounding rocks to arch upward.
Textural Differences Between Intrusive and Extrusive Rocks
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Intrusive Rocks (Plutonic Rocks)
- Cooling Process: Intrusive rocks cool slowly beneath the Earth's surface, allowing crystals to grow larger.
- Texture: Coarse-grained (phaneritic) texture due to the slow cooling processes, resulting in visible crystals. The most common types include granite, diorite, and gabbro.
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Extrusive Rocks (Volcanic Rocks)
- Cooling Process: Extrusive rocks cool rapidly once they erupt onto the Earth's surface, preventing large crystal growth.
- Texture: Fine-grained (aphanitic) texture or glassy due to the rapid cooling, making the individual crystals too small to see without a microscope. Examples include basalt, rhyolite, and obsidian.
In summary, the key difference in texture arises from the rate of cooling: slow cooling leads to large visible crystals in intrusive rocks, while rapid cooling results in small crystals or glassy textures in extrusive rocks.