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	Types of Volcanoes	The Ring of Fire

ice age An ice age is a time lasting thousands of years during which the Earth is very cold and largely covered by ice and glaciers.

igneous intrusion An igneous intrusion (also called a laccolith or a plutonic formation) is a formation in which magma (molten rock) is trapped beneath the surface of the Earth and pushes the rock located above it into a dome shape. It has a flat base and a convex upper surface. The magma cools and solidifies, and eventually, it is exposed (as the fractured sedimentary rock above it erodes away).

igneous rock When molten rock cools, igneous rock is formed. Intrusive rocks are igneous rocks that have formed (and cooled slowly) beneath the surface of the Earth. Extrusive rocks are igneous rocks that have formed (and cooled more quickly) at the surface of the Earth (like obsidian). Some igneous rocks include granite, obsidian (volcanic glass), basalt, and andesite porphyry.

ignimbrite Ignimbrite (meaning "fire rain stone") is a type of igneous rock that is formed in the pyroclastic flow (the pumice flow deposit) of violently explosing volcanoes. Ignimbrite forms as molten lava and ash fuse; this rock is streaked with parallel bands of black glass. The texture of ignimbrite varies from hard to soft.

impact crater Impact craters are the remains of collisions between an asteroid or meteorite and the Earth.

index fossil Index fossils are commonly found fossils that are limited in time span. They help in dating other fossils. For example: trilobites were common during the Paleozoic, but not found before the Permian period, and ammonites were common during the Mesozoic Era, but not found after the Cretaceous period.

intertidal zone The intertidal zone is where the sea meets the land.

intrusive igneous rock Intrusive rocks are igneous rocks that have formed (and cooled slowly) beneath the surface of the Earth. These rocks usually have a coarse-grained or medium-grained texture, like granite.

ion An ion is an atom that is missing one or more electrons; ions have an electrical charge.

Iridium anomaly Iridium is an element that is rare on the Earth's surface, but abundant on chondritic meteors. The presence of excess Iridium at the K-T layer (the Iridium anomaly) supports the Alvarez asteroid theory.

ISOTOPE An isotope of an element is another form of the same element, that has a different number of neutrons in the nucleus (giving it a different atomic weight).

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	Types of Volcanoes	The Ring of Fire

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