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Hydrothermal
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 includes the sticky material that will later become a volcanic eruption, and many more 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 worked for metals such as gold, silver, copper, zinc and lead (see: Galena).
I
Igneous rock
Rock formed when magma cools and solidifies. Igneous rocks are found in volcanic rocks such as lava and ash and once-buried rocks such as dikes and batholiths. Granite is the most common igneous rock on land. It forms when magma cools
in a batholith. The most common igneous rock of all is basalt. It
is mainly found on the floors of the world’s oceans, but also makes dikes and sills on land.
(For types of igneous rock see: Acid rock; Andesite; Diorite; Extrusive rock, extrusion; Feldspar; Gabbro; Granite; Plutonic rock; Porphyry, porphyritic rock; Rhyolite.)
Illuviation
The process of depositing clays washed from the topsoil into the subsoil. It tends to occur in slightly acid soils and where there is too little calcium or humus in the
soil to hold the clay particles
together (preventing flocculation). Soils with lots of clay in the subsoil tend to waterlog easily and be difficult to work. (See also: Acid brown soil and Eluviation.)
Impermeable
A rock or soil that will not allow a liquid to pass through it.
Combinations of permeable and impermeable rocks produce springs and trap water and petroleum.
A band of impermeable rock
is called an aquiclude (while a band of permeable rock is called an aquifer). An impermeable rock above an aquifer can trap water underground. If a well is drilled through an impermeable rock, water will gush up from the underlying aquifer.
If an impermeable rock lies above a permeable rock containing natural gas and oil, the petroleum will be trapped. The impermeable rock is then called a cap rock. If a well is drilled through the cap rock, natural gas and crude oil will gush to the surface.
If an impermeable rock lies below an aquifer, then the water will be stored above the aquiclude and a spring will be formed wherever the edges of the rocks are exposed at the surface. Spring lines thus show approximately where permeable and impermeable rock meet.
Impurities
Small amounts of elements or compounds in a mineral.
Infiltration
The process of water sinking through the surface of the soil. Once the water has entered the soil, it spreads through the soil (see: Permeable soil).
If the soil surface has been trampled or crushed by heavy
machinery, the infiltration of water may be low, even though the permeability of the soil below the crushed layer may be high. Soils with a surface crust formed by splashing rain will also have low infiltration rates.
Inorganic matter
The mineral matter in a soil.
Intrusive rock, intrusion
Rocks that have formed from cooling magma that has forced its way through other rocks.
The main types of intrusion are batholiths, bosses, laccoliths, sills and dikes.
Batholiths are huge bodies of granite that were formerly magma chambers supplying volcanoes. As magma forced its way from these chambers into weaknesses in the surrounding rock, it created many kinds of intrusion. Sheets of igneous rock formed by prising the surrounding rocks apart are called sills; those that cut across the surrounding rocks and form wall-like sheets are called dikes.
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 Latosol – This is a latosol soil from Queensland in Australia. It is very deep and the red colour indicates that it is enriched with iron. On exposure to air, some latosols become very hard and form laterites.
 Laterite – Laterites can contain iron nodules.