- Rocks that have been cooled from magma.
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- Silicic igneous rocks have a composition rich in silica, of more than 66% and are light coloured. (leucocratic)
- Intermediate igneous rocks have a silica content of 66 to 52% and are grey (mesocratic).
- Mafic igneous rocks have a silica content of 52 to 45% and are dark (melanocratic).
- Ultramafic igneous rocks have a silica content of less than 45%.
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Felsic and Mafic Minerals
- Felsic minerals are light coloured and silica rich.
- Mafic minerals are dark coloured, silica poor and rich in magnesium and iron.
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Extrusive & Intrusive
- Extrusive is the general term for all igneous rocks that cool at the surface - both lava and pyroclasts.
- Intrusive is the term for igneous rocks that cool below the surface.
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Hypabyssal & Plutonic
- Hypabyssal is when igneous rocks form at relatively shallow depths below the surface.
- Plutonic is when igneous rocks form deep below the surface.
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- Divergent plate margins are where two plates are moving apart and magma is rising up between them.
- Convergent plate margins are where two plates are colliding and magma is formed above a subduction zone or deep in the crust. These can be ocean-ocean, ocean-continental orcontinental-continental.
- Hot spots are formed by a fixed mantle plume bringing magma to the surface.
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- Occurs where some of the minerals in a rock melt to form a magma.
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- Is any rock, whether sedimentary, metamorphic or igneous into which an igneous rock is intruded.
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Minor & Major Intrusions
- Minor intrusions cool at hypabyssal depth below the surface and include sills and dykes.
- Major intrusions are plutonic and cool deep the surface and include batholiths.
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- Is where the igneous rock meets the country rock.
- A chilled margin is where the igneous rock has cooled rapidly so it has fine crystals.
- A baked margin is in the country rock where it was heated by the intrusion and altered.
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Is a large area around a batholith where the rocks have been metamorphosed.
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- Is a vent at the surface of the Earth through which magma and other volcanic materials are ejected.
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- Pyroclast - An individual fragment ejected during an eruption.
- Pyroclastic - Describes all the fragmental materials formed by explosive eruptions, including bombs, blocks lapili and ash.
- Pyroclastic rocks - Formed when the pyroclastic material is compacted into rocks.
- Pyroclastic flow - A hot mixture of pyroclastic material and gas. A nuee ardente is a type of pyroclastic flow.
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- A line joining points of equal thickness of a deposit such as ash. The maps may be called isopach maps.
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- Mudflows of wet ash and volcanic debris that can flow rapidly down a mountainside.
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- A measure of a fluid's resistance to flow and controls the stickiness of a magma, which in turn depends on the silica content.
- Low viscosity is where magma is fluid and flows freely.
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- Have gentle slopes of less than 10 degrees and a roughly circular shape around a central vent.
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Active, Extinct and Dormant Volcanoes
- An active volcano has shown eruptive activity within recorded history. Currently there are about 600 active volcanoes and each year 50 to 60 actually erupt.
- An extinct volcano has not shown any historic activity and each year 50 to 60 actually erupt.
- A dormant volcano is one that has not shown eruptive activity with recorded history, but shows geological evidence of activity within the geologically recent past.
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- The term used to describe the fluid, non-explosive, basalt lavas.
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