Types of Rock
Know the three main rock types: igneous forms from cooling melted rock, many sedimentary rocks form from pieces pressed and stuck together, and metamorphic rock changes through heat and pressure without melting; fossils are usually found in sedimentary rock
What a learner can do afterwards
- Name the three main rock types: igneous, sedimentary, metamorphic
- Explain that igneous rock forms when lava or magma cools and hardens
- Explain that fossils are usually found in sedimentary rock because sediment can bury and preserve remains or marks of past living things
1 · Read
Rocks form in different ways. Scientists group them into three main types: igneous, sedimentary, and metamorphic. Learning how a rock formed helps you decide which group it belongs to.
Ben sees lava flowing in a volcano video. When it cools and hardens, it becomes igneous rock. Melted rock underground is called magma. At the surface, it is called lava. Cooling magma also forms igneous rock, even if it never reaches the surface.
Sand, mud and small rock pieces are called sediment. Many sedimentary rocks form as sediment settles in layers. Over time, the pieces get pressed and stuck together. Sediment can bury remains or marks of past living things, such as shells or footprints. These can become fossils. Fossils are usually found in sedimentary rock, but many sedimentary rocks have none.
Metamorphic rock forms when heat and pressure change an existing rock without melting it. Pressure means pushing or squeezing. Some metamorphic rocks have bands. Bands alone do not prove a rock's type. If rock melts and later cools, it forms igneous rock instead.
Igneous rock forms from cooling melted rock. Many sedimentary rocks form from joined pieces and can hold fossils. Metamorphic rock changes through heat and pressure without melting.
Readings and flashcards written for this dataset. Rock and fossil references supplied facts and design ideas, not copied or closely paraphrased text.
More to read
Time for crystals to grow
Crystals can grow as melted rock cools. A crystal is a solid with an orderly pattern inside.
Slow cooling gives crystals more time to grow. Magma cooling underground often forms crystals large enough to see. Lava at the surface often cools faster, leaving smaller crystals. Very fast cooling can produce a glassy texture.
Crystal size gives a clue about cooling. It does not identify every rock by itself.
Changed without melting
A solid rock can change through heat and pressure. Its minerals and their arrangement can change while the rock stays solid. This forms metamorphic rock. Slate and marble are examples.
Some metamorphic rocks have bands; others do not. Shine and stripes alone are not enough to name a rock's type.
Melting changes the process. If the melted material later cools and hardens, the new rock is igneous.
Choosing where to look for fossils
A fossil search starts with clues about the rock. Sedimentary rock is a useful place to look because sediment can bury and preserve remains or marks of past living things.
For a particular animal, hunters often start with sediment laid down when that animal lived. But an older fossil can wash out of older rock and be buried again in younger sediment. The fossil is then older than its new layer.
Exposed rock is easier to examine than rock hidden beneath soil or plants. Even suitable rock may have no fossils. These clues guide a search; they do not promise a find.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.