Types of rocks
Use rock and landscape vocabulary, including igneous, sedimentary, metamorphic, weathering, erosion, deposition, fossil, sediment, strata, permeable and impermeable, to explain how rocks form and how material moves
What a learner can do afterwards
- Correctly classify igneous, sedimentary, and metamorphic rocks and explain in one sentence how each type forms
- Use 'weathering', 'erosion', and 'deposition' correctly to describe rock breaking down, pieces moving, and sediment settling
- Explain how fossils form using 'sediment' and 'sedimentary rock' correctly
1 · Read
Rocks are grouped by how they form: igneous, sedimentary or metamorphic. Igneous rock forms when melted rock cools and becomes solid. Melted rock underground is magma; at the surface it is lava. Slow cooling usually allows larger crystals to grow. Fast cooling usually makes small crystals, or rock with no crystals.
Sediment includes loose rock grains, sand and mud. It can settle in layers called strata. Pressure compacts buried sediment, and minerals can cement grains together to make sedimentary rock. Some sedimentary rocks form from shells or minerals left by water. Permeable rock lets water pass through connected spaces. Impermeable rock lets very little water pass through.
Weathering breaks rock down where it is. Erosion moves rock pieces or soil. Deposition happens when moving material settles. A river may carry pieces that were already broken, so its journey need not include fresh weathering.
A dead fish may be buried by sediment in a lake. Burial can help protect bones or an imprint. More sediment adds layers called strata. Pressure compacts the sediment, and minerals can cement it into sedimentary rock. Evidence of the fish may remain inside as a fossil. Most dead living things do not become fossils.
Metamorphic rock forms when heat and pressure change an existing rock while it stays solid. Shale can change into slate this way. Melting followed by cooling makes igneous rock. Large crystals alone do not identify metamorphic rock: slowly cooled igneous rock can have them too.
Rock types describe how rocks form. Weathering breaks rock down, erosion moves material, and deposition is when it settles.
The lesson, questions and flashcards were written for this dataset.
The readings were written for this dataset. The linked rock cycle, weathering and erosion, and fossils videos are by Khan Academy.
More to read
A clue is not proof
A shiny rock is not automatically metamorphic. Igneous rocks can have shiny crystals too. Some metamorphic rocks have bands, while others do not.
Ask how the rock formed. Cooling melted rock makes igneous rock. Heat and pressure can change solid rock into metamorphic rock. Appearance gives clues, but one clue may fit more than one rock type.
Following a grain
Imagine a grain breaking from a rock beside a stream. That breakdown is weathering. Flowing water carries it downstream during erosion. When it settles, deposition has happened.
The grain may move and settle many times. More sediment can bury it. Pressure packs grains together, and minerals between them can act as cement. This is one route to sedimentary rock.
Water moving through rock
Tiny spaces between rock grains can connect. Water can travel through these paths in permeable rock. Impermeable rock lets very little water pass through.
Permeability describes water movement, not the rock's formation. A crack can also provide a water path, so a sample's condition matters when describing how water passes through it.
Checking rock layers
In sedimentary layers that have not been overturned, a lower layer formed before the layers above it. Later deposits added material on top.
Earth movements can tilt, fold or overturn layers. Geologists check for these changes before using position to compare ages. Fossils within the layers are preserved evidence of living things from the past.
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