Fine grained igneous rocks, also known as aphanitic rocks, form from the rapid cooling of molten rock material, or magma, when it erupts onto the Earth’s surface as lava. This quick cooling process prevents the formation of large crystals, resulting in a smooth and often glossy texture.
When magma rises to the surface, it can flow out during a volcanic eruption or seep out from fissures. As the lava comes into contact with the cooler atmosphere or water, it solidifies quickly. The rapid loss of heat inhibits crystal growth, leading to the fine-grained texture characteristic of these types of rocks.
Examples of fine-grained igneous rocks include basalt and rhyolite. Basalt, which is typically dark and dense, forms mainly from the basaltic lava flows common at mid-ocean ridges and hotspot volcanoes. Rhyolite, on the other hand, is usually light-colored and forms from more viscous lava associated with explosive volcanic eruptions.
In summary, the key to the formation of fine-grained igneous rocks lies in the speed at which lava cools after being erupted, allowing for the development of a fine-grained texture due to the formation of small crystals.