Mercury, Venus, Earth, and Mars are called rocky planets because they are primarily made of rock। In general usage, the word “rock” is a more specific technical term than what we commonly understand as “stone।” This article aims to clarify that distinction। On Earth, rocks or “shilas” are of three types: igneous, sedimentary, and metamorphic। All rocks formed from solidified magma are igneous and serve as the fundamental basis for all rock types, since the other two types derive from the transformation of igneous rocks। Igneous rocks gradually change under the influence of agents like air and water to become sedimentary rocks। And sedimentary rocks turn into metamorphic rocks under high pressure and temperature beneath the Earth’s surface। The mystery of rocks is beautifully expressed in a poem by Pablo Neruda: “It is clear that rock has faced time, its fine substance carries the scent of age।”

Under a magnifying glass, the small fragments seen in a rock are the crystals of various minerals (khanij)। All rocks are composed of mineral crystals। Below are shown some of the major mineral classes on Earth, along with each one’s chemical formula and some properties। For example, all compounds like sulfates are placed in the sulfate class।

Among these, silicates are the most important because most of Earth’s rocks are made of silicates। Examples of four types of silicates are given। Pyroxene and olivine are heavy, forming basalt। Quartz and feldspar are lighter, forming granite। The Earth’s continental crust above sea level is primarily granite (60% feldspar), while the oceanic crust beneath the sea is mainly basalt। Granite is typically plutonic (intrusive), forming when magma cools slowly underground, which results in coarse, large crystals। Basalt is usually volcanic (extrusive), crystallizing rapidly when magma or lava cools on the surface, leading to fine crystals। However, exceptions exist; we’ve only stated the rule, and where there is a rule, exceptions naturally occur।

Basalt is more abundant than granite because basaltic magma has lower viscosity and can flow farther। Many rocky regions in the solar system are undoubtedly basaltic। The dark maria on the Moon, like Earth’s oceanic crust, are basaltic।

Sedimentary rocks form from igneous rocks through various environmental effects in several stages:

1. Mechanical weathering: Water can penetrate and break the rock। Quartz breaks down into sand, and feldspar breaks down into soil।
2. Sorting: Because of weathering, a piece of granite can turn into a mixture of soil and sand, but wind or water cannot carry soil and sand grains equally far due to their different sizes। This causes the grains of soil and sand to separate।
3. Lithification: Under the pressure of the overlying layers, soil compacts into shale, and sand compacts into sandstone।

On any planet with life, a large amount of sedimentary rock known as limestone, that is calcium carbonate (CaCO$_3$), is produced। It can also form through chemical processes। Below is shown the reaction for carbonate production, the raw material for limestone, by chemical means।

$$ CaSiO_3 + 2CO_2 + H_2O \rightarrow Ca^{2+} + SiO_2 + 2HCO_3^- $$

$$ Ca^{2+} + 2HCO_3^- \rightarrow CaCO_3 + CO_2 + H_2O $$

This calcium carbonate dissolves in water and forms limestone and chalk as sediment। However, most limestone on Earth comes from the shells of numerous small marine organisms। After these shells accumulate on the seafloor and undergo prolonged compression, they gradually become limestone। Limestone and chalk are very important because they are the largest reservoirs of carbon dioxide। They contain 20,000 times more CO$_2$ than all of Earth’s oil and coal reserves। Earth has much less CO$_2$ than Venus because of these reservoirs। When one plate subducts beneath another, the carbonate breaks down under pressure and CO$_2$ returns to the atmosphere; plants take up this CO$_2$, animals feed on the plants, and upon death, their carbon again deposits as limestone on the seafloor। This is the global carbon cycle।

Among metamorphic rocks, marble is a well-known example। Subsurface limestone transforms into marble under pressure and heat। Shale transforms into slate।