- Igneous rocks are also called primary rocks because they are formed when molten lava solidifies. Sedimentary rocks and metamorphic rocks are formed from other rocks.
- Sedimentary rocks which are organically formed consist of remains or forms of sea plants and animals which have been buried and preserved in these rocks for a long time. They may have fossils trapped in their layers while formation of sedimentary rocks.
- The core of the Earth is in a semi—solid state because of extreme pressure which is 3 million tonnes greater at core than at the surface.