Igneous rocks that usually contain large crystals are known as intrusive or plutonic rocks. These rocks form when magma cools slowly beneath the Earth’s surface. The slow cooling process allows for the growth of large mineral crystals, which can be easily seen with the naked eye.
One of the most common examples of an intrusive igneous rock is granite. Granite is composed mainly of quartz, feldspar, and mica, and it often has a coarse-grained texture due to its large crystals. Other examples include diorite and gabbro, which also have large crystals due to their slow cooling process.
In contrast, extrusive or volcanic igneous rocks, such as basalt, form when lava cools quickly on the Earth’s surface. This rapid cooling results in small crystals or even a glassy texture, as there isn’t enough time for large crystals to form.
Understanding the difference between intrusive and extrusive igneous rocks can help in identifying the type of rock and its formation process. Intrusive rocks, with their large crystals, are a clear indication of a slow cooling history deep within the Earth’s crust.