The origination of igneous rocks is by lava and magma below or above ground. These rocks form when the magma or lava cool and become a solid form. As lava cools on the surface of the earth, it quickly becomes a solid due to the water or air being cooler. As lava cools on the earth’s surface due to cooler water and air, crystals to not have time to be formed. These are known as volcanic rocks. These rocks have fine grains and textures which are not able to been with the naked eye. These rocks also have a glassy look to them. As magma cools underground, it cools slowly which gives crystals enough time to develop. These rocks are called plutonic rocks. These rocks are rough on the surface and have grains and textures which are able to be seen with the naked eye.
There are two categories of igneous rocks. These categories are intrusive and extrusive. Intrusive rocks are created beneath the surface of the earth which cool slowly and form crystals. Intrusive rocks have a rough or course surface. So0me examples of intrusive rocks are gabbro, granite, and diorite. Extrusive rocks are created on the surface of the earth and rapidly cool. These may have small crystal if any since the cooling time is quick. They also have a smooth surface and may even have a glassy look to them. Some examples of extrusive rocks are basalt, rhyolite, andesite, and obsidian. Since Igneous rocks form from a fluid state to solidification they have an equigranular texture. In many igneous rocks mineral crystals are formed due to the length of cooling time and the load of minerals.
As a child when I lived in LaPine Oregon I remember going with my grandfather to outside Bend Oregon to what he called the lava beds. I remember seeing different colors of obsidian. Some of these rocks were brown with dark spots through them and some were all blank. They all had a glassy look to them. Remembering this