Rocks

Earth's crust began as molten rock. In the beginning all rock were igneous. Volcanoes deliver igneous rocks to the surface. Rock that forms underground is called intrusive rocks. Rocks that form from igneous rock fragments are called sedimentary rocks. Sand that is formed into rock is called sandstone. Layers of coal is formed from volcanic ash. Limestone is formed from skeletons of micro-organisms. Rocks that have been changed since they first formed are  called metamorphic rocks. Limestone turns into marble under a great amount of pressure. When metamorphic rock reaches the surface, it weathers and turns into sedimentary rocks.