Rocks and the Rock Cycle

The solid Earth beneath our feet is made of rocks, and a rock is simply an aggregate of one or more minerals. There are three great families of rock, each formed in a different way. They are not fixed forever. Over vast time, one kind slowly turns into another through the rock cycle. (The science of rocks is called petrology.)

Igneous rocks form when molten magma (below ground) or lava (at the surface) cools and solidifies. Because all other rocks ultimately derive from them, they are called the primary rocks. Examples include granite and basalt.

Magma itself is not a pure liquid. It is a mixture of liquid melt, solid crystals and dissolved gases. The principal gases dissolved in magma are water vapour and carbon dioxide. Its temperature depends on its chemistry. Basaltic (mafic) magma is the hotter kind, at roughly 1200°C. Silicic (felsic) magma is cooler, at about 700–900°C.

They are classified in several ways:

• By texture: slow cooling deep down gives large crystals (coarse-grained, e.g. granite); fast cooling at the surface gives small crystals (fine-grained, e.g. basalt).
• By occurrence: intrusive/plutonic rocks cool deep inside (granite), while volcanic/extrusive rocks cool at the surface.
• By chemical composition: felsic/acidic rocks (high silica, light, e.g. granite) versus mafic/basic rocks (lower silica, dark and heavy, e.g. basalt).

When magma pushes into existing rocks and solidifies there, it takes characteristic shapes:

• Sill: a sheet of magma that solidifies horizontally between rock layers. It runs parallel to the layering, so it is called a concordant intrusion.
• Dike (dyke): a wall of magma that solidifies in a vertical crack, cutting across rock layers. Because it cuts the layering, it is a discordant intrusion.

Sedimentary rocks form from sediments (fragments worn from older rocks, or organic and chemical material). These sediments are transported, deposited in layers, and then hardened. The hardening process is called lithification.

All of this happens at the Earth's surface, and the driving force is the hydrological system. Water is the chief agent: it weathers older rocks, transports the loose sediments, and deposits them in layers in rivers, lakes and seas.

Key facts:

• they often show distinct layers (strata), and are the main rocks in which fossils are found,
• they cover about 95% of the Earth's surface but make up only ~5% of the crust by volume, and
• by formation they are mechanically formed (sandstone, shale), organically formed (coal, limestone from remains) or chemically formed (rock salt, gypsum).

India's most important sedimentary rocks belong to the Gondwana rock system. These are fluviatile deposits, meaning sediments laid down by rivers, which accumulated in ancient river basins. Buried plant matter in these basins turned into coal. As a result, the Gondwana system holds more than 90% of India's coal reserves, which is why it is regarded as the most important rock system of India.

Metamorphic rocks ("change of form") are existing rocks transformed by great heat and pressure, without melting. This process is called metamorphism. The original minerals recrystallise into new ones. Examples:

• marble from limestone,
• slate from shale, and
• gneiss from granite, schist from basalt.

Finally, all three families are linked in the rock cycle, a continuous process in which old rocks become new ones:

• igneous rocks weather into sediments → sedimentary rocks,
• heat and pressure turn rocks into metamorphic rocks, and
• deep melting returns rock to magma, which cools again into igneous rock.

So rock is never permanent; it is endlessly recycled.