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Hornblende Hornblende
A dark-green silicate mineral containing sodium, potassium, calcium, magnesium, iron and aluminium. Hornblende is a common mineral in rocks like basalt and gabbro.
Hot spot
A place where a fixed mantle magma fountain, or plume, reaches the surface. One of the best examples is the Hawaiian Islands.
Hydrothermal
A change brought about in a
rock or mineral due to the action of superheated mineral-rich fluids, usually water.
When molten rock forces its way to the surface, it makes up
the sticky material that will later become a volcanic eruption and is also a part of other liquid mixtures that can easily force
their way into small cracks in
the neighbouring rocks. They are called hydrothermal (hot water) liquids. They contain rich concentrations of metals. When they later cool and solidify in the surrounding rocks, they form rich mineral deposits that can be mined for metals such as gold, silver, copper, zinc and lead.
Hypocentre
Another word for the focus of an earthquake. It is the point within the Earth where an earthquake rupture starts. (See also: Epicentre.)
I
Icelandic-type eruption
A name given to a type of fissure eruption common in Iceland.
Kauai (oldest)
Oahu
Maui
Pacific Plate moves
Hawaii (youngest)
Hot spot
Igneous rock
Rock formed when magma cools and solidifies. The word igneous means fire baked. All igneous rocks either began in magma chambers or volcanoes.
There is a wide variety of igneous rocks. The most common are basalt and granite. Basalt is the most common form of lava that flows from volcanoes. Granite is the cold remains of extinct magma chambers.
Dike
 Hot spot – The hot spot below the Pacific Ocean has produced the Hawaiian chain of volcanoes. There is probably another hotspot under the Pacific Northwest, still marked by Yellowstone National Park.
 Igneous rock – The diagram shows the main forms of intrusive and extrusive rocks in their position of formation.
Plug of lava
Layers of lava and ash
Sill
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Magma chamber. When it has cooled, it will be a batholith.