Final answer:
Intrusive igneous rocks form from magma that cools slowly inside the Earth, creating coarse-grained textures with large crystals, such as granite. Extrusive igneous rocks form when lava cools quickly on the Earth's surface, resulting in fine-grained textures with small crystals, such as basalt. The classification of igneous rocks is based on texture, mineral content, and environment of formation.
Step-by-step explanation:
Intrusive and Extrusive Igneous Rocks
Intrusive igneous rocks are formed from magma that cools slowly within the Earth. The slow cooling process allows large crystals to form, resulting in a coarse-grained texture. These rocks, also known as plutonic rocks, include well-known examples such as granite.
Extrusive igneous rocks, in contrast, are formed when lava reaches the Earth's surface and cools rapidly. This quick cooling does not allow large crystals to develop, creating a fine-grained or sometimes glassy texture. Common extrusive rocks include basalt and andesite.
The classification of igneous rocks is based partly on texture, with the crystal size indicating whether the rock is intrusive (large crystals) or extrusive (small crystals). The mineral content and environment of formation are also crucial for classification. For example, a coarse-grained rock with significant amounts of quartz and potassium feldspar might be classified as granite, while one with fine grains might be andesite.