All rocks on Earth fit into three groups based on how they were formed. Igneous rocks form when hot melted rock (called magma or lava) cools and hardens. Sedimentary rocks form when small pieces of rock, sand, or mud pile up in layers and get pressed together over time. Metamorphic rocks form when existing rocks are changed by extreme heat and pressure deep underground. Every rock you find belongs to one of these three groups.
Provide samples of each rock type for hands-on examination. Show granite (igneous -- cooled from melted rock, visible crystals), sandstone (sedimentary -- visible sand grains pressed together), and marble (metamorphic -- limestone changed by heat and pressure). Use simple formation stories: "This rock was once melted lava," "This rock is made of ancient sand pressed together," "This rock used to be a different rock but heat and pressure changed it."
Pick up a rock. It might look ordinary, but it has a story -- a story about how it was made. Every rock on Earth was formed in one of three ways, and knowing which way tells you which of the three rock groups it belongs to.
Igneous rocks are born from fire (the word "igneous" comes from the Latin word for fire). Deep inside the Earth, it is so hot that rock melts into a thick, glowing liquid called magma. When magma cools down -- either underground over thousands of years or on the surface after a volcanic eruption -- it hardens into solid igneous rock. If it cools slowly underground, the rock has big, visible crystals you can see, like in granite. If it cools quickly on the surface from lava, the crystals are tiny or invisible, like in basalt.
Sedimentary rocks are built from pieces. When wind, water, and ice break down rocks into tiny pieces -- sand, mud, pebbles -- those pieces are called sediment. Rivers carry sediment to lakes and oceans, where it settles in layers on the bottom. Over millions of years, the layers pile up, and the weight of the layers on top presses the lower layers together. Minerals dissolved in water act like glue, cementing the pieces into solid rock. Sandstone (made from sand) and limestone (made from tiny sea creature shells) are common sedimentary rocks. You can often see the layers in these rocks.
Metamorphic rocks were once igneous or sedimentary rocks that got changed. "Metamorphic" means "changed in form." When a rock gets buried deep underground, extreme heat and pressure can change its minerals and structure without melting it. Limestone, a sedimentary rock, becomes marble under heat and pressure. Shale becomes slate. The original rock is transformed into something new and often harder and more beautiful.
This is a foundational topic with no prerequisites.
No prerequisites — this is a starting point.