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Igneous Rocks


Glossary
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Magmatic Stoping

Magma that cools deep below the Earth's surface forms plutonic rock. Granite, diorite, and gabbro are examples of plutonic rocks. Plutonic rocks come in many different shapes and sizes. Magma that solidifies in small cracks forms intrusions only centimetres wide whereas magma that solidifies in huge underground magma chambers creates intrusions measuring tens of kilometres in width. Large bodies of plutonic rock, called batholiths or plutons, formed deep underground millions of years ago. Today these ancient rocks can be seen at the surface because all of the rock above them has been worn away.

Dykes or sills are intrusive rocks that form when magma cools and hardens in fractures near the surface. Dykes form flat, vertical slabs that cut across the layers of the enclosing rock. Magma flowing through the crust between the layers creates flat-lying sills. A sill is like a horizontal dyke. Because dykes and sills cool very quickly they are are usually fine-grained.

Dyke Sill

For many years geologists argued about how these plutons were formed. It is generally accepted that magma, flowing into a region of crust that has become weakened or softened, cools and crystallizes to form a pluton. But how does the magma make room for itself? One answer might be assimilation. Hot magma, rising as a large blob, melts the surrounding rock making room for more magma. As a large cavern or magma chamber is formed, pieces of rock falling from the roof or sides of the chamber become enclosed by magma and begin to melt. This makes the cavern larger. The process continues until the heat of the magma is no longer sufficient to melt any more of the surrounding rock.

Igneous Intrusion

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    Last Modified: 2005-04-26