Final answer:
Igneous rocks form from the cooling of magma (intrusive) or lava (extrusive), with extrusive igneous rocks being fine-grained due to rapid cooling on the Earth's surface and formed by volcanic activity influenced by plate tectonics.
Step-by-step explanation:
Igneous rocks are classified based on whether they form from the cooling and solidification of magma (intrusive igneous rocks) or lava (extrusive igneous rocks). When magma rises to the Earth's surface and erupts from a volcano, it is known as lava. As lava flows onto the surface, it cools rapidly, forming extrusive igneous rocks with very fine grains, often with a dull appearance due to the small crystal sizes. Unlike their extrusive counterparts, intrusive igneous rocks form when magma cools slowly beneath the surface, leading to larger crystal formations.
The distribution of igneous rocks across the landscape, such as the volcanic rocks in North America, is closely related to the processes of plate tectonics, which include volcanic eruptions. These eruptions can range from gentle flows of basaltic lava that travel over 20 kilometers to explosive events with global climate impact. The Catalina Mountains near Tucson offer a glimpse into the history of igneous rocks, with granite sills formed approximately 50 million years ago as an example of intrusive igneous rock formations.