For an explanation, see the main splats page
Rocks and rock cycles
- The material that we call rock goes through cycles, being melted, weathered, eroded, buried and eventually heated and compressed until it melts again.
- Rocks are mostly made of minerals or their weathering products, which were, at one stage of their existence, crystalline, and may still be crystalline.
- Rocks erode and re-form in the rock cycle. The process involves chemical and mechanical weathering, erosion, transport, deposition and compaction
- Rocks and soil erode. Water transports sediments downstream, and the sediment particle size in a stream depends on the speed of the water flow.
- Sedimentary rocks which are buried under a sufficient load of more recent sediment may be compressed and heated so that over time they form metamorphic rocks.
- Metamorphic and sedimentary rocks may be melted to form magma that later becomes an igneous rock, but usually they are metamorphosed long before they melt.
- The properties of rocks are determined by the minerals in them, and the minerals in rocks often reveal their origins, even the depths at which they formed.
- Most minerals in rocks are present as crystals: the size of the crystals in igneous rocks shows how quickly they cooled, with slow cooling giving big crystals.
- The hardness of minerals can be used to distinguish them by finding what they can and cannot scratch, and what will scratch them, using Mohs' scale of hardness.
- Rocks that cooled slowly at first may contain phenocrysts, because when the cooling speeds up, the phenocrysts may be surrounded by smaller crystals.
- Rocks weather chemically, with minerals changing chemically, often to soluble material which leaches away, resistant particles like quartz, and clay minerals.
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