Igneous Rocks Form from the Cooling of Molten Rock
Igneous rocks are one of the three main rock types with the other two being sedimentary and metamorphic.
Igneous rocks form when hot molten rock (either magma or lava) cools and solidifies. These types of rocks can form with or without a crystalline structure and the process can take place above or below the Earth's surface.
When igneous rocks form deep below Earth's surface as magma cools, a process which can take thousands to millions of years, the rock that forms is known as an intrusive igneous rock. These rocks usually have large crystals because there was plenty of time for them to grow. A good example of an intrusive igneous rock is granite.
Igneous rocks that form above the Earth's surface often from lava which has been expelled from the vent of a volcano, are known as extrusive igneous rocks. With the lava cooling down much quicker due to the cooler environment crystals only have a limited amount of time to grow so are generally quite small. When the lava cools down very vast there's no time for atoms to bond together to form a crystalline structure. That means the rock has almost no crystals at all. Examples of this type of extrusive igneous rock include obsidian and pumice.
Rocks with a crystalline structure are known as minerals whilst those void of crystals are known as a mineraloid or an amorphous solid.