Igneous Rock

QUESTION: What is igneous rock?

ANSWER:

Igneous rock is one of three basic kinds of rock, along with sedimentary and metamorphic rock. The word igneous comes from the Latin word, ignis, which means fire. Igneous rocks are rocks which were once molten inside the earth.

There are over 700 different kinds of igneous rock. Two well known examples are granite and basalt. 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. Extrusive rock forms on 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. There it cools becoming extrusive rock.

Many scientists believe that by using radiometric dating techniques they can determine approximately how long ago igneous rock cooled and formed. The idea behind this belief comes from the idea that when the rocks are molten their atomic clock resets. When the rocks cool the clock starts. As radioactive elements within the rock decay into stable elements, the stable elements build up. Scientists estimate how long ago a rock cooled by how much stable “daughter” element was produced by the unstable radioactive “parent” element.

Radiometric dating is opposed by some scientists who believe that the underlying assumptions inherent to the dating scheme have been brought into question by various scientific studies.