文档介绍：Chapter 4: Igneous Rocks: Product of Earth’s Internal Fire
Introduction: What Is an Igneous Rock?
Igneous rocks vary greatly.
Some contain large mineral grains.
Others contain grains so small they can barely be seen under a high power microscope.
Igneous rocks also vary greatly in color.
All igneous rocks are formed through the cooling and solidification of magma.
Intrusive Versus Extrusive Igneous Rocks
Intrusive igneous rocks form when magma cools within existing rocks in Earth’s crust.
Extrusive igneous rocks form when magma cools on Earth’s surface, where they have been “extruded.”
Texture In Igneous Rocks
The two most obvious textural features of an igneous rock are the size of its mineral grains and how the mineral grains are packed together.
Sizes of mineral grains:
Intrusive rocks are coarse-grained.
Magma that solidifies in the crust cools slowly and has sufficient time to form large mineral grains.
Extrusive rocks are fine-grained
Magma that solidifies on the surface usually cools rapidly, allowing insufficient time for large crystals to grow.
Coarse-grained igneous rock is called a phanerite 显晶岩(from the Greek word meaning visible).
Texture In Igneous Rocks
Igneous rock that contains unusually large mineral grains (2cm or larger) is called a pegmatite 伟晶岩.
Fine-grained igneous rock is called an aphanite隐晶岩(from the Greek word meaning invisible).
Texture In Igneous Rocks
Texture In Igneous Rocks
The isolated large grains are phenocrysts 斑晶.
A porphyry is an igneous rock in which 50% or more of the rock is coarse mineral grains scattered through a mixture of fine mineral grains.
Texture In Igneous Rocks
Glassy rocks.
Atoms lack time anize themselves into minerals.
A mineraloid forms instead (mineral-like solid that lacks either a crystal structure or a position or both).
Extrusive igneous rocks that are largely or wholly glassy are called obsidian 黑矅石.
They display a distinctive conchoidal 贝壳状fracture (smooth, curved surface).
Glassy R