Igneous rocks are rocks which were once molten inside the earth.There are over 700 different kinds of igneous rock. All of the many different kinds of igneous rock can be subdivided into one of two categories: intrusive rock and extrusive rock.
Intrusive rock hardens and forms below the earth’s surface. Intrusive rock is also called plutonic rock, named after Pluto, the Roman god of the Underworld (because Plutonic rock forms beneath the earth’s surface).
Extrusive rock is also called volcanic rock, named after Vulcan, the Roman god of fire, because volcanic rock is spewed onto the earth’s surface by volcanic activity which is also named after Vulcan. Many scientists believe that by using radiometric dating techniques they can determine approximately how long ago igneous rock cooled and formed.
The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.
These rates are usually expressed as the isotope's half-life--that is, the time it takes for one-half of the parent isotopes to decay.
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In general, color is a good clue to the silica content of lavas, with basalt being dark and felsite being light.
(The nucleus of an atom is made up of protons and neutrons.) For example, the element carbon, which always has six protons in its nucleus, has three isotopes: one with six neutrons in the nucleus, one with seven, and one with eight.
Some isotopes are stable, but some are unstable or radioactive.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.
Many elements have naturally occurring isotopes, varieties of the element that have different numbers of neutrons in the nucleus.
The idea behind this belief comes from the idea that when the rocks are molten their atomic clock resets. As radioactive elements within the rock decay into stable elements, the stable elements build up.