The Rock Cycle
Explain how igneous rocks form when molten rock cools, sedimentary rocks form through processes including compaction, cementation and mineral deposition, and metamorphic rocks form through heat and pressure without melting. Trace possible routes between rock types over geological time.
What a learner can do afterwards
- Describes how each rock type (igneous, sedimentary, metamorphic) is formed
- Gives an example of each rock type and its formation (e.g. granite, limestone, marble)
- Traces a rock sample through possible transitions in the rock cycle
- Explains geological time and why many rock-cycle changes are slow, while some stages can happen much faster
1 · Read
Rocks are grouped by how they formed. Igneous rock forms when molten rock cools and solidifies. Sedimentary rock can form when sediment is compacted and cemented, or when minerals come out of water. Metamorphic rock forms when heat and pressure change existing rock without melting it. The rock cycle describes possible changes between these groups. There is no required order.
Molten rock underground is called magma. At the surface it is called lava. Slow cooling underground gives crystals time to grow large, as in granite. Rapid cooling near the surface usually produces small crystals, as in basalt. Very rapid cooling can produce glass with no crystals. Crystal size can help you infer how an igneous rock cooled.
Weathering breaks down rock where it is. Erosion moves the material, and deposition leaves it in a new place. Buried sediment can be compacted by weight above it and cemented by minerals between its grains. Sand can become sandstone. Many limestones form from calcium carbonate shells and skeletons; others form when calcium carbonate comes out of water.
Limestone is sedimentary rock. Heat and pressure underground can cause its minerals to recrystallise into interlocking crystals, forming marble. Marble is metamorphic rock. It stays solid during this change. If limestone melts instead, it becomes molten material; cooling that material would make igneous rock, not marble.
Any rock type can weather into sediment. Any rock type can also melt under suitable conditions, and the molten material can cool into igneous rock. Further heat and pressure can change metamorphic rock again. There is no final rock type. For example, marble exposed at the surface can weather into sediment instead of melting.
Geological time covers Earth's long history, including spans of millions and billions of years. Building thick sediment layers, burial and mountain formation often take very long periods. Some stages are faster: erupted lava can solidify within a human lifetime. A rock has no fixed timetable, and a marble sample alone cannot tell you exactly how long its formation took.
Cooling forms igneous rock. Sedimentary processes form sedimentary rock. Heat and pressure change solid rock into metamorphic rock. Routes and timescales vary.
Lesson, questions and flashcards written for this dataset.
Choosing a possible rock pathway and Reading formation clues were written for this dataset. The linked video The rock cycle is credited to Khan Academy by the supplied source record; its publisher terms remain unverified.
More to read
Choosing a possible rock pathway
Imagine following material from a piece of marble. Its next change depends on its surroundings.
At the surface, weathering can break it down. Erosion can carry the material elsewhere. After deposition, burial can press sediment together, and minerals can cement its grains. This is one possible route towards sedimentary rock.
Another route requires melting. If conditions become hot enough, the marble's material can melt. When that molten material cools and solidifies, it forms igneous rock.
These routes connect surface processes with processes underground. Water and gravity help move sediment. Earth's internal heat supplies energy for changes underground. A rock-cycle diagram records possible pathways. It does not mean every rock must follow every arrow.
Reading formation clues
Large crystals in granite suggest that its molten material cooled slowly underground. Basalt usually has much smaller crystals because its lava cooled more quickly. Cooling can also be so rapid that glass forms without crystals.
Sedimentary rocks record other processes. Loose sand can be buried, pressed together and bound by cementing minerals to form sandstone. Limestone often contains calcium carbonate from organisms' shells and skeletons. It can also form when calcium carbonate comes out of water.
Marble forms when limestone changes while staying solid. Its minerals recrystallise under heat and pressure. Melting followed by cooling would produce igneous rock instead.
A formation clue does not tell you exactly how long formation took. Burial and mountain building can take very long periods, while lava can solidify during a human lifetime. Identifying marble alone cannot tell you the exact duration of its formation.
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