How the Rock Cycle Changes Earth's Materials

Igneous rocks form when molten material cools and solidifies. Magma is molten rock below Earth's surface, while lava is molten rock that reaches the surface. As the material cools, minerals crystallise and lock together.

Slow cooling underground usually gives crystals more time to grow, producing coarse-grained rocks such as granite. Rapid cooling at or near the surface produces smaller crystals, as in many basalts. Extremely fast cooling can even create volcanic glass with very little crystal structure.

Igneous rocks can later be exposed by uplift and erosion. Once at the surface, they are no longer protected from changing temperature, water, ice, wind and living organisms. Their next part in the cycle begins with breakdown.

Weathering breaks existing rock into smaller pieces or changes its minerals chemically. Erosion then transports sediment through water, wind, ice or gravity. When transport slows, sediment is deposited in layers.

Over time, new layers add weight. Compaction squeezes grains closer together, and minerals carried by groundwater can cement the grains into solid sedimentary rock. Sandstone, shale and many limestones form through different versions of this process.

Sedimentary rocks can preserve fossils, ripple marks and other clues about past environments. Their layers may record ancient rivers, deserts, seas or lakes. The rock is therefore both a material and a piece of geological evidence. Sedimentary layers can later be tilted, folded or lifted by tectonic forces, placing once-horizontal deposits high above the environments where they originally formed.

Metamorphic rocks form when existing rocks are changed by heat, pressure and chemically active fluids without fully melting. Minerals can recrystallise, new minerals may form and the texture can become strongly layered.

Slate can form from shale, while marble can form from limestone. If a metamorphic rock becomes hot enough to melt, it returns to molten material and can later form new igneous rock. Uplift can also expose it to weathering instead, sending it towards sedimentary processes.

The important pathways include:

• •Cooling and crystallisation form igneous rock.
• •Weathering, deposition and lithification form sedimentary rock.
• •Heat and pressure form metamorphic rock.
• •Melting returns solid rock to magma.
• •Uplift can expose any rock type to weathering.

It can help to follow a single mineral grain. Imagine a quartz crystal forming as granite cools slowly deep underground. Millions of years later, uplift and erosion expose the granite, and weathering breaks down the softer minerals around the quartz, which is hard and chemically resistant. A river carries the freed grain to the sea, where it settles with countless others as sand. Buried and cemented, the sand becomes sandstone. If that sandstone is later heated and squeezed during mountain building, the grains recrystallise and fuse, forming the hard metamorphic rock quartzite.

The idea that such changes need immense time owes much to James Hutton, an eighteenth-century Scottish geologist. At Siccar Point on the east coast of Scotland, he saw nearly vertical layers of rock cut off and covered by gently tilted layers of a younger sandstone. To explain this, the lower layers had to be deposited, hardened, tilted, raised and eroded before new sediment could be laid on top. Hutton argued that ordinary slow processes, repeated over vast periods, could account for it. A surface like this, recording a long gap in time, is called an unconformity.

You can watch weathering at work without visiting mountains. Old churchyards and cemeteries are a good place to look, because gravestones are made from different rocks and many carry carved dates. Marble and limestone are made mainly of calcium carbonate, which slowly reacts with slightly acidic rainwater. On older marble stones the lettering often looks blurred and sugary, and the edges become rounded, even within a century or two, especially in towns with a history of smoky air.

Granite headstones of similar age usually keep sharp lettering, because their main minerals, such as quartz and feldspar, resist chemical attack far better. Slate stones can also last well, although they may split along their layers. Comparing stones of known age gives you a rough sense of how fast different rocks break down in that climate, and geologists have used gravestone studies in exactly this way. Town buildings offer more examples: granite kerbs, slate roofs and limestone walls each show the effects of weather, pollution and time on a rock from a different part of the cycle.

Because rocks are constantly recycled, very little of Earth's earliest crust survives. Geologists date rocks using radioactive elements that decay at known, steady rates. Zircon, a tough mineral that forms in many igneous rocks, is especially useful. When it crystallises, it takes in uranium but almost no lead, so nearly all the lead found inside later must have come from uranium decay. Measuring the ratio gives the crystal's age. Zircon is also hard and resistant to weathering, so a single grain can outlast the rock it formed in, travel with sand and be locked into a new sedimentary rock while still carrying its original birth date.

That is exactly what happened in the Jack Hills of Western Australia. Sedimentary rocks there contain zircon grains dated to about 4.4 billion years ago, only around 150 million years after Earth formed, making them the oldest known pieces of our planet. The rocks they first grew in have long since been weathered away. Some of these grains carry chemical hints that liquid water may already have existed at the surface, though how to interpret them is still discussed. By contrast, the oldest widely accepted intact rocks, such as gneiss in northern Canada, are around 4 billion years old. Almost everything older has been recycled.

A common picture shows Earth's crust floating on a layer of molten magma. In fact the mantle beneath is almost entirely solid rock, although over millions of years it can flow very slowly, like extremely stiff putty. Magma forms only in particular settings where conditions change. At mid-ocean ridges, hot mantle rises as plates pull apart, and the falling pressure lowers its melting point so that a small fraction melts. This decompression melting produces basalt that builds new ocean floor. At subduction zones, a sinking plate carries water down with it, and the released water lowers the melting point of the hot mantle above.

Subduction explains the chains of volcanoes in the Andes and Japan, while collisions between continents drive metamorphism on a huge scale. As India pushed into Asia, rocks were buried, heated and squeezed into new forms, while others were thrust high into the air. The summit of Mount Everest is made of limestone containing fossils of sea creatures, sediment laid down on an ancient seabed and lifted nearly nine kilometres. Plate motion also controls how long rocks survive. Ocean floor is constantly recycled at subduction zones, so little of it is older than about two hundred million years, while continents keep far older rocks.

The rock cycle describes many possible pathways rather than one circular conveyor belt. Rocks can melt, cool, weather, become buried or transform under heat and pressure depending on where they move in Earth's crust. Learn the processes first, and the three main rock types become connected parts of one changing system.