Цикл порід: вивітрювання, осадове та метаморфічні перетворення

Three rock families, one loop

Every rock on Earth's surface belongs to one of three families, and the rock cycle is the set of processes that convert each family into the others, driven by heat from Earth’s interior, gravity, and the water cycle at the surface. Igneous rock crystallises directly from molten rock — magma underground (intrusive, coarse-grained, like granite) or lava at the surface (extrusive, fine-grained or glassy, like basalt or obsidian, which cools too fast for large crystals to grow).

The Rock Cycle: A Continuous Process

Rocks are constantly being transformed through a cycle of processes known as the rock cycle. This cycle involves three main types of rocks: igneous, sedimentary, and metamorphic.

Igneous rocks form from the cooling and solidification of magma or lava. Magma is molten rock beneath the Earth's surface, while lava is molten rock that has erupted onto the surface. As magma cools, it forms intrusive igneous rocks like granite, which have large crystals due to slow cooling deep within the Earth.

Sedimentary rocks form from the accumulation and cementation of sediments – fragments of other rocks, minerals, or organic material. Weathering and erosion break down existing rocks into smaller pieces, which are then transported by water, wind, or ice. These sediments eventually settle in layers, forming sedimentary rocks like sandstone (made of cemented sand) or limestone (often formed from the remains of marine organisms).

Metamorphic rocks form when existing igneous or sedimentary rocks are transformed by heat, pressure, or chemical reactions. This transformation occurs deep within the Earth's crust, where conditions are different than at the surface. For example, shale (a sedimentary rock) can become slate under high pressure and temperature, while limestone can become marble.

The rock cycle is a continuous process with no real beginning or end. One type of rock can transform into another through various geological processes.

Re-establishing Solid Rock from Loose Sediment

Where sediment accumulates and is buried under more sediment, two processes convert it into solid sedimentary rock: compaction, where the weight of overlying layers squeezes out water and air, packing grains tighter, and cementation, where minerals dissolved in groundwater — commonly calcite, silica or iron oxide — precipitate in the pore spaces and glue the grains together. Together these processes are called lithification. Sandstone (cemented sand grains), shale (compacted clay) and limestone (compacted shell and coral fragments, or precipitated calcium carbonate) are the classic products.

Heat and pressure without melting: metamorphism

Bury any rock — igneous, sedimentary, or already metamorphic — deep enough, or place it near a body of intruding magma, and rising heat and pressure will recrystallise its minerals into new, more stable forms without fully melting the rock. This is metamorphism. Shale becomes slate, then phyllite, then schist, then gneiss as temperature and pressure climb; limestone becomes marble; sandstone becomes quartzite. Directed pressure often aligns platy minerals into visible banding, called foliation, which is why gneiss shows stripes and slate splits into flat sheets.

Closing the loop: melting

Push temperature and pressure past a rock's melting threshold — commonly near subduction zones, where one tectonic plate is forced under another and drags water and crust down into the mantle, lowering the melting point of surrounding rock — and any rock type, igneous, sedimentary or metamorphic, melts into magma. That magma can cool underground into new intrusive igneous rock, or erupt and cool at the surface, restarting the cycle from the top.

magma/lava --cools & crystallises--> igneous rock igneous/sedimentary/metamorphic rock --weathering & erosion--> sediment sediment --compaction & cementation--> sedimentary rock any rock --heat & pressure, no melting--> metamorphic rock any rock --melting--> magma (loop closes) No step is guaranteed or one-way — a rock can cycle through weathering and lithification repeatedly without ever melting, or melt and re-solidify without ever being buried and metamorphosed. The loop describes possible pathways driven by plate tectonics and surface processes operating over anything from thousands to hundreds of millions of years, not a fixed schedule any individual rock follows.

magma/lava  --cools & crystallises-->        igneous rock
igneous/sedimentary/metamorphic rock
            --weathering & erosion-->        sediment
sediment    --compaction & cementation-->    sedimentary rock
any rock    --heat & pressure, no melting--> metamorphic rock
any rock    --melting-->                     magma  (loop closes)

Frequently asked questions

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