Page 25 - Curriculum Visions Dynamic Book. To close the book, close the tab.

Intrusive rock, intrusion
Igneous rocks include extrusive volcanic rocks such as lava and ash and intrusive rocks formed underground such as dikes, sills, and batholiths.
(For other types of igneous rock see: Acid; Alkaline; Basic; Diorite; Gabbro; Pegmatite; Plutonic.)
Rock from volcanoes
Volcanic rocks—lava and ash—are made of crystals so tiny they
can often only be seen with a microscope. That is because lava and ash cool very quickly in the air, and there is no time for big crystals to form.
One of the most common features of volcanic rocks are small
Diorite
cavities. They are bubbles of gas that were trapped inside the rock as it cooled (see: Amygdule and Vesicle).
The most common volcanic rock is a black lava called basalt. Most of the world’s ocean floors are covered in basalt that has erupted from undersea volcanoes. In some places flood basalt has formed huge sheets of rocks thousands
of square kilometers in area and hundreds of meters thick. It is common in places like Hawaii, Iceland, and India. It flows from supervolcanoes during “supereruptions.”
Rock of dikes and sills
Some magma pushes into gaps between rocks but never reaches the surface. The sheets of magma
formed this way are called dikes and sills.
This magma cools in the ground, and so there is more time for crystals to form. The rocks are
still dark colored, but have bigger crystals in them.
Rock of magma chambers
When a magma chamber becomes extinct, it may take millions of years for the chamber to cool. There is plenty of time for large crystals to form. The most common of these rocks is called granite. It is usually pink and gray, and contains glassy minerals.
Incandescent
Glowing red, orange, yellow, or white as a result of high temperature. Incandescent lava can be seen during a fissure eruption, glowing avalanche, and pyroclastic flow.
Intensity
A number (written as a Roman numeral) that refers to the severity of an earthquake (see: Modified Mercalli scale).
(See also: Isoseismal.)
Intrusive rock, intrusion
Igneous 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 the largest intrusions. They are bodies of granite that were formerly magma chambers supplying volcanoes.
As magma forced its way from these chambers into weak areas in
the surrounding rock, it created many other kinds of intrusion. Sheets of igneous rock formed by prying the surrounding
rocks apart are called sills and laccoliths; those that cut across
the surrounding rocks and form wall-like sheets are called dikes.
Gabbro
 Igneous rock—Igneous rocks all have interlocking crystals. The main differences between them involve the size of the crystals and the proportion of dark- to light-colored minerals. Dark color and large crystals typify gabbro (middle), an igneous rock that forms underground in the oceans. Rocks of dikes and sills often have medium-sized or small crystals because they cooled quickly; this is diorite (top). Rocks formed from lava that flowed on the surface have bubbles (vesicles) and tiny crystals. This is andesite (bottom).
Andesite
25