Find out how different types of rocks can be classified depending on how they were formed. Material processes - Classifying materials - Rocks Learning Points Sedimentary rock is formed from compressed sediment at the bottom of seas, rivers and lakes. Metamorphic rock is formed underground through extreme heat and pressure. Igneous rock is formed when magma or lava solidifies. A Twig Curriculum Film. Delivering key learning points. Get straight to the facts in just three minutes – directly linked to core learning, Twig's curriculum films will fit neatly into your lesson plans.