Which of the following are true with regard to igneous rocks?
1. They are formed only inside the earth, under great pressure
2. They do not occur in layers.
3. They are mostly crystalline in nature .
4. Granite and Gabbro are intrusive igneous rock.
II, III, and IV
Igneous means "fiery", and igneous rocks are rocks that were made by heating them up in a fire. Usually inside the earth, radioactivity creates very high temperatures of about 1000 degrees Celsius. Because it's so hot, any rocks that are down in that core are liquids, not hard as they would be on the surface of the Earth.
Sometimes these liquid rocks shoot up to the surface when volcanoes suddenly erupt. Then tons of liquid rock rockets out of the top of the volcano and lands on the surface. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of existing rocks in either a planet's mantle or crust.