This Earthcache brings you to a slate quarry, with some examples of intrusions.
An intrusion is a body of igneous (volcanic) rock, also known as magma, that penetrates other existing rock within the crust of the planet, then cools and solidifies.
Intrusive rocks are always formed underground, as opposed to extrusive rocks that are formed when the magma cools at or close to the surface of the earth, eg during a volcanic eruption.
Intrusive rocks tend to have larger grain sizes than extrusive rocks. This is because being underground, the molten magma cools very slowly, allowing lots of time for large crystals to grow. Extrusive rocks cool very quickly at the earth's surface and tend to be fine grained.
Some common intrusive rocks include: diorite, granite, pegmatite, gabbro.
Some common extrusive rocks include: basalt, pumice, obsidian, scoria.
Types of Intrusions
Intrusions come in many forms. Here are some examples:
- Batholith: This is a huge mass of igneous rock, greater than 100 square kilometers in area. The You Yangs in southern Victoria are an example of a granitic batholith.
- Dyke: This is a thin sheet of igneous rock that forms when magma is pushed into fractures in existing rock, and are often seen to be vertical or near-vertical as they cut through the surrounding rock.
- Sill: This is similar to a dyke, except that the fracture is between existing layers of rock, therefore the sill will more likely run parallel to the other rock layers.
- Vein: A vein that contains igneous rock is very similar to a sill, although veins tend to be more irregular and discontinuous. They don't always contain igneous rock however, but may contain crystallized minerals that were transported in water that infiltrated the fractures. Examples of such crystallized deposits include quartz, feldspar, amphibole, mica and chlorite. Sometimes these deposits can be a repository for precious metals or gemstones, for example gold, amethyst, garnet, moonstone, diamond and topaz.
- Volcanic Neck: This is the remains of a cooled magma intrusion that has filled the vent of a volcano before it reached the earth's surface. Over millions of years, erosion of the surrounding rock reveals the intrusion. The Glasshouse Mountains in QLD contain examples of volcanic necks.
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Questions:
Standing at the posted coordinates, look around (particularly towards the easterly direction) to see an isolated rocky outcrop with some intrusions, including an obvious band of quartz.
Q1. From the "Types of Intrusions" listed above, what type of intrusion do you think this quartz band might be and why?
Q2. Please describe the intrusion in your own words, including colours and textures of rocks in the band, and estimate (in centimetres) the width of the band.
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