Slide 1
Rock categories describe how rocks formed
Igneous, sedimentary and metamorphic rocks are classified by how they form. The rock cycle links the processes that can transform one into another.
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The diagram is a network of possible paths, not a compulsory sequence. A rock can undergo repeated alteration or remain in one setting for a long time without completing a neat circular tour.
Slide 2
Cooling molten material creates igneous rock
Molten material can solidify into igneous rock. Cooling conditions influence the size and arrangement of the resulting crystals.
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Slow cooling often allows larger crystals to grow, whereas rapid cooling can create tiny crystals or glass. Composition and the history of crystallization also matter, so grain size alone is not a complete account.
Slide 3
Weathering and transport supply sediment
Weathering breaks down or chemically alters exposed rock. Erosion and transport move products that can later accumulate as sediment.
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Burial can compact grains and mineral-rich fluids can cement them together. Some sedimentary rocks instead form through chemical precipitation or biological accumulation, so not all begin as broken fragments.
Slide 4
Metamorphism changes rock while it remains largely solid
Metamorphism changes a rock’s minerals and texture while it remains largely solid. Melting would begin a different route toward igneous formation.
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New minerals can be stable under the changed pressure and temperature. Directed stress may align platy minerals, giving the rock a fabric that records deformation as well as mineral replacement.
Slide 5
Uplift exposes deep rocks to a new cycle
Tectonic movement and erosion can expose rocks formed at depth. Their next history depends on the new environment.
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A rock’s grains may preserve older events even after the rock changes category. Geologists therefore distinguish the age of individual minerals from the age of deposition, deformation or later heating.
Sources
- The Rock Cycle
US Geological Survey
- Geology: USGS Bulletin 2195
US Geological Survey