About Rocks - Concise Geography

Do you get confused between Granite, Sandstone, Limestone and Marble?

Do you feel that all rocks are the same except their color?

Do you think that coal is not a rock?

Many of us get bogged down by the nuances of these rocky materials. There is no incentive for remembering all this (except for geologists) but it's nice to have things clear in your mind if it doesn't take much time. Which is why I am writing this once and for all.

What are rocks and how they are made?

Rocks are the solid constituents of earth's surface containing minerals in various proportions. They can be formed from a number of processes. Some rocks come ready made from under the surface of earth and are brought out through volcanoes and others are formed on the spot during eruptions. Some also get their shape and nature from the processes occurring on the surface of earth. The place of origin, nature of elements and processes determine the final product.

What separates various rocks? Some attributes which can help differentiate one piece of rock from another are necessary to recognize rocks. These are:

1. Chemical and Mineral Compositions: They are the rock DNA. They play a large part in determining their place of origin. For example, rocks from deep below the surface contain a larger proportion of higher metals as compared to others.
2. Texture and Structure: Texture refers to the size, shape and orientation of the grains. In lay terms, texture is the feeling you get when you rub your fingers over a piece of rock. On large scale, texture determines the structure and in turn physical properties like permeability, brittleness, etc.
3. Mode of Occurrence: The process which led to the formation of a rock also determines the final look and feel of the rock. For example, lava which cools down faster results in rocks which are more crystalline than the one which cools down gradually.

The big question: How do I differentiate Marble, Limestone, Sandstone and Granite?

To make it simple, rocks have only three main classifications, namely Igneous, Sedimentary and Metamorphic. The complex part is that each one has sub-categories.

1. Igneous Rocks: Igneous rocks are formed by the cooling and solidification of Magma (layer of molten mass of minerals below earth's crust) from beneath the Earth's surface. They reach the surface of the Earth through volcanic fissures. The process is called Crystallization because most of the Igneous rocks are crystalline in nature. They are generally crystalline. Igneous rocks have three sub-classifications:

• Acidic & Basic:

Those rocks which contain high proportion of Silica are called Acidic and those high in basic Oxides like Iron, Magnesium, Aluminium, etc. are called Basic.

• Plutonic & Volcanic:

The molten rocks which solidify before coming to the surface are called Plutonic rocks. In contrast to this, Volcanic rocks are the ones which solidify after molten lava reaches the Earth's surface.

• Intrusive - Extrusive:

Intrusive and Extrusive are nothing but the other names for Plutonic and Volcanic, respectively.

2. Sedimentary Rocks: These types of rocks are formed by the accumulation of sediments (broken down pieces) over a long period of time usually by the action of water and wind. They are also called stratified rocks because they form in layers. They often contain various types of fossils (remains of organic matter). They are sub-classified on the basis of their Origin Mechanism.

• Mechanically Formed Sediments: These rocks are formed by the cementing of material derived from other rocks. They are generally used for building materials as Sandstone, Clay, Sand and Gravel. Quartz is also formed by this process.
• Organically formed Sediments: They are formed by the remains of living organisms such as shell fishes and corals. The Great Barrier Reef of Australia is a great example of it. Limestone, Coal and Chalk are also examples of these type of rocks. They are sub-classified as:

Calcareous: Formed by the remains of living organisms. Some examples are Limestone and Chalk.

Carbonaceous: Formed by the remains of Vegetative matter -swamps and forests. Its examples are Peat and Lignite.

• Chemically Formed Sediments: These types of rocks are chemicals which have precipitated from solutions of some form. Gypsum is one such example formed by the evaporation of salt lakes which have a high level of salinity. Similarly, chemical rocks like Potashes, Rock Salts and Nitrates are formed.

Metamorphic (changed) rocks are the ones which get created when Sedimentary or Igneous rocks are subject to great heat and pressure for a prolonged period. In brief, they are altered Sedimentary or Igneous Rocks. By the process of metamorphism clay turns into slate, limestone into marble, sandstone into quartzite and coal into graphite.

It is easy to define Metamorphic rocks. But its definition opens a Pandora's Box of convertibility of rocks. This process of conversion of one type of rocks into another is a continuous one and is called Rock Cycle.

Rock Cycle is a way to depict the changes in rocks from one form to another in a recurring sequence. It was first suggested by James Hutton, the founder of Modern Geology. Let us examine the Rock Cycle. The diagram above contains 5 crucial agents.

Weathering and Erosion: This refers to the action of wind and water. It leads to the segmentations and layering of other types of rocks.

Deposition and Diagenesis: After weathering and erosion the rocks are remain buried. They undergo chemical, mechanical and biological change called diagenesis to form sedimentary rocks.

Pressure and Heat: This refers to the conditions of underground rocks which undergo high pressure and temperatures and are turned into metamorphic rocks.

Conversion of Rocks back to Magma: This happens around seismic zones. When two tectonic plates strike each other, chances are that one of them will slide underneath. This layer then turns into magma and gets recycled on some other part of the earth.

Crystallization: This has already been discussed in Igneous Rocks section.

Igneous rocks -

Granite - is light colored with large inter-locking crystals. It can't be scratched easily.

Gabbro - is dark colored with large inter-locking crystals. It can't be scratched easily.

Basalt - is dark colored with small crystals. It can't be scratched easily.

Sedimentary rocks -

Limestone - is made from shells, corals and calcite. It can be scratched with nail.

Sandstone - is made from sand grains. It can't be scratched easily.

Shale - is made from mud and fossils. It can be scratched with nail and easily broken.

Metamorphic rocks -

Marble - is converted from limestone. It can be scratched with nail

Slate - is converted from shale. It can be scratched with nail and easily broken.

Quartzite - is converted from sandstone. It can't be scratched easily.