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They are very tiny grains of different minerals, compressed together in chemical reaction to form a bigger mass. Rocks make up non-water part of the earth’s crust.

A mineral and a rock do not mean the same thing. Minerals are made of individual or a combination of elements. A mineral is composed of the same substance throughout. A rock is made of a combination of two or more different minerals. They may also contain organic compounds.

They come in infinite sizes, shapes, colors, weights, textures and strengths. Rocks constantly change from one type to another, and from place to place, in a cycle known as the rock cycle. This change does not occur in short periods—they take thousands to millions of years to change.

Rocks may be hard or soft. It depends on the way the tiny grains interlock with each other. Rocks with rough grains fitting perfectly into each other tend to leave no room for moisture. As a result, they are harder and non-porous in nature. Granite is a good example.

Rocks with rounded or fine grains often have spaces that hold moisture, and tend to be softer. They crumble quicker than hard rocks. Rocks that have lots of spaces holding water, or through which water can pass are called porous rocks. Sandstone is a great example.

Soils, sands and rocks.

Sand is the fine debris resulting from broken down rocks (weathered). Sand therefore carries the same or similar properties of the rocks that they came from. Soils are sands that contain air, water and organic materials, in proportions that support the growth of plants and organisms that live in them. Many plants and animals rely on soils for survival.

How do rocks form

Different rocks form from different processes. In the next few pages, we will see how the three main rock types (Igneous Rocks, Sedimentary Rocks and Metamorphic Rocks) form. Let us begin with igneous rocks…

Igneous Rocks

Igneous rocks form when magma cools off. Igneous means ‘firery’ (from heat or fire). Such rocks do not contain fossils or organic matter.

Below the earth’s crust, there is melted rock called magma. Magma is kept in molten form (liquid form) because of extreme temperatures (between 625 and 1200° Celsius) existing there.

Sometimes magma cools off very slowly under the earth’s crust and solidifies into rocks. Granite and gabbro are examples of igneous rocks formed in this way. They are very hard rocks. Slowly cooled magma produces bigger crystals than fast cooling magma. These are known as intrusive igneous rocks because they are formed from magma underground.

During a volcanic eruption, magma may also pour out to the surface of the crust. Magma that has come out to the surface is called lava. Lava cools off quicker on the surface than below. Rocks resulting from cooled-off lava include obsidian, lava rock, basalt or pumice. These are known as extrusive igneous rocks because they are formed outside of the crust.

There are about 700 different types of igneous rock. More examples of igneous rocks include tuff, diorite and andesite.

Here are some uses of igneous rocks: Granite is used for building and construction because of its strong properties. It is also used for head stones and kitchen counter tops. Pumice is lightweight and has lots of air spaces. It is used by cosmetic and cleaning industries to make things like excess skin removing stones. It is also used in some toothpaste products.

Sedimentary rocks

When mountains are formed, they are rugged in nature, but after many years, they are flattened and weathered away. Where do all the eroded particles go?

Tiny debris (also known as sediments) from eroded mountains and rock masses, together with the sand and other particles are often washed down the slopes of highlands into water bodies. Sediments are often rounded or smoothened by the abrasion it undergoes as it moved with water and other sediments. The sediments slowly settle under water in a process called sedimentation.

As the years go by, layers of sediments that have settled under the ocean or water bodies, harden under the weight of the topmost layers and water. This process is known as compaction. Sometimes, organic matter and fossils settle in them too and are compressed together with the sediments. Compaction leads to cementation, which is the gluing or cementing of pieces of rock together by salt compounds. When these harden, they form sedimentary rocks.

During the formation of sedimentary rocks, massive amounts of vegetative and animal residue may be trapped in the layers. They go on to form carbon pockets in these rocks, which humans extract later on in the form of crude oil. Note that the formation of coal takes millions of years.

Sedimentary rocks appear in layers called strata. The oldest rocks tend to be at the bottom with newer rocks above them. There are six main types of sedimentary rocks including conglomerate, sandstone, shale, limestone, gypsum and breccia. Each of these has its’ own properties and characteristics.
Metamorphic rocks Metamorphic rocks are the least common of all the three types of rocks. They are igneous and sedimentary rocks that have been transformed under extreme heat and pressure.

‘Metamorphic’ originates from the greek word ‘Meta’ which means ‘change’ and ‘morph’ which means ‘form’

Earth movements causes rock beds to shift and move. The movement causes other rock types to be squeezed, putting them under extreme pressure.

With time, extreme pressure and heat causes some chemical changes to occur, transforming the original rocks into metamorphic rocks.

Heat from the magma beneath the crust, geothermal heat and heat from friction along faults are the main sources of heat that cause the existing rocks to change form. Metamorphic activities occur in heats between 300° and 1470° Fahrenheit.

Even though they do not melt like magma, the extreme heat causes the rocks to change form. Two good examples include slate or gneiss, which is formed from shale, and marble, which is formed from limestone.

Physical Weathering

Physical factors such as freezing and thawing, temperature, rain, winds, waves, water pressure and others can cause rocks to break up into tiny pieces. Specific types of physical weathering occur in specific places.

Here are a few examples:

Weathering by temperature changes:

The sun’s energy can heat up rocks to very high temperatures. This causes rocks like granite to expand. As temperatures fall, the rocks cool down and contract. Continuous expansion and contraction causes pressure on the outer layers of the rock. Cracks develop as a result, and eventually, the outer layers of the rock wear off. This is also known as exfoliation.

Weathering by water, wind and waves:

Winds, water and waves pound on rocks and wears them up. Prolonged action causes larger rocks with rugged surfaces to smoothen. During runoff, water carries sand and smaller debris and smashes them against larger rocks in their path. The resulting abrasion causes wearing of rocks.

Freeze and thaw:

Put a glass of water in a freezer and it will break up. Why? This is because water expands when it freezes. Take a look at the diagram below:

When water collects in rock pores and cracks and spaces they expand when they freeze, particularly in cold climates. The freezing widens and causes additional cracks. When the ice thaws, the water enters into new cracks again and causes further cracks as they freeze. Soon the rocks break apart.

Chemical Weathering

This is when weathering involves the reaction of some chemicals on rocks. Some rocks (such as limestone and chalk) are more prone to chemical weathering than others such as granite. This is because limestone contains minerals such as calcium carbonate, which readily reacts with rainwater. This chemical reaction produces new soluble substances that are easily washed away.

In the example below, see how the beddings of limestone beddings are exposed, because the joints are worn away by carbonic rain, forming clints and grykes, which are very commin limestone features.

Rain water contains an acid called carbonic acid. Rain gets acidic because carbon dioxide in the atmosphere dissolves in it. When acidic rainwater falls and stays on rocks, some minerals in the rocks may react chemically with it and cause the rock to weather.

Air pollution that results in more carbon dioxide and sulphur dioxide causes rain water to become even more acidic. When moisture in the atmosphere dissolves these gases, they form acid rain. When acid rains fall on rocks, the effects are even more than regular rainwater.

Chemical weathering is a key fact in the creation of caves and caverns. Carbonation has also resulted in sink holes, karst topography, stalactites and stalagmites.

Hydrolysis is another key reaction associated with chemical weathering. When water (H2O) separates into H+ and OH- ions, the elements can react with ions in the minerals and destroy their atomic compositions, usually forming new minerals. This is what happens when feldspar and hornblende come into contact with water. They form clay, a new mineral.

Chemical weathering can occur even under the top layer of the ground. Acid rain water can leach into deeper layers underground and come into contact with rocks for chemical reactions to occur.

Biological weathering

This process of weathering is very common and we see it around us. A good example is an animal that can burrow into a crack in a rock. There are many insects, rodents and bigger animals that live in holes in the ground or rocks. Constantly, they burrow and widen cracks and end up breaking rocks apart.

Algae, lichens, bacteria and mosses often grow on rock surfaces, especially in humid regions. They produce weak acids, which convert some of the minerals to clay. Algae growth can weaken many rock types and make it more open to weathering.

Humans are no exception. Walking and construction activities like road building, mining and the like involve people ripping rocks apart. They cause weathering too.

Also importantly, weeds and plant roots can get into cracks of rocks and grow from there. As the plant grows bigger the roots grow bigger and deeper, widening cracks and splitting them apart. Decaying roots also add organic acids to the joint, speeding up the weathering process.

Sometimes it is tricky to tell if a particular form of weathering is physical, chemical or biological. For example, if the pressure exerted by a root breaks up a piece of rock, what kind of weathering would it be? Pressure is physical weathering. At the same time, if chemicals released by roots enhance microbiological activity, then that can also cause chemical weathering. The same is true for biological weathering too, because weathering caused by plants and animals fall under that.

Erosion

Erosion is not the same as weathering. Weathering is the process where rock is broken down or dissolved into smaller pieces by physical, chemical or biological weathering process.

Erosion (or mass wasting) involves the movement of the weathered rock (now pebbles, sand or soil) from one place to the other by the action of wind, ice, water and gravity. Examples of mass wasting include rock falls, slumps, and debris flows.

Transport makes erosion complete, because it is the part that involves the movement of the eroded materials or sediments.

Transport by water:

Everyday, millions of tonnes of sediments are moved along rivers, coasts and deep oceans. Water transport occurs in four ways: Traction, saltation, fine particles and soluble salts.

Transport by wind:

Wind transport can result in stunning landscapes as sand is blown away from place to place. In the deserts, dunes are created. Wind can also create sand storms and destroy farms by blowing away the topsoil of large farms.

Faster flowing rivers can carry heavier rocks or drag them along the riverbed. As the rocks move, they are smoothened by the abrasive effect. Transport makes that possible. Winds can also carry tiny rocks or sand and move them to other places.

The Rock Cycle When rocks form, they do not stay the same forever. They also do not stay at one place forever. They move about. The rock cycle is the entire journey rocks make as they change. These take millions of years.

Let us start the cycle with molten magma in the earth core.

Molten magma may cool off and crystalize beneath the earth’s crust, forming intrusive igneous rocks. With time, pressure may cause uplift and rocks end up on the surface.

Molten magma may also flow to the surface by volcanic action, causing extrusive igneous rocks as they harden and crystalize.

On the surface, they undergo weathering, erosion and transport. Sediments are therefore carried to low lying places and into rivers and water bodies. The pilling up of sediments cause compaction and cementation. Sedimentary rocks form.

After a long period of pressure and heat from the overlying weight, the igneous and sedimentary rocks buried deep inside the crust change to metamorphic rocks, deep under the earth’s crust.

Some of the metamorphic rocks begin to melt as they get closer to the molten magma region. Some will also undergo uplift to the surface again, in places where volcanic activity is not common. If they melt, they get released back to the surface through volcanic activity, especially in places with high tectonic activity.