Rocks

By: Umer Shakeel VII Champions

What are rocks?Rocks are mainly made up of minerals, sediment grains, fossils and lava/magma. The earths lithosphere or crust is made up of rocks.

Rocks can be changed from one to another. The main minerals found in rocks are iron and silica. Some rocks are rich in iron, while others are silica-rich. Rocks are believed to have been used by hominids since the Stone Age which was more than 2.5 million years ago.

Characteristics of Rocks

Rocks have many characteristics.Some are similar to each other while others are not. Porosity is a measure of the empty spaces between particles. Some rocks are smooth; some are not. Some have large crystals; some have small crystals while some have no crystals. They can be rough and jagged or they could be smooth and round. The grains can be well-sorted according to size, or they can be poorly-sorted. Rocks can be rough or they can be smooth. Some rocks, like obsidian, are glassy.

Classification of RocksRocks are classified into three major categories: 1. Sedimentary rocks2. Igneous rocks3. Metamorphic rocksAll of these three types of rocks have their own characteristics and different types of formation.Sedimentary RocksWhen weathering and erosion agents act upon rocks to break it down, the rock turns into tiny pieces, called sediments. When these are transported by rivers, they are deposited near the sea bed. If the sediments have been transported over a long distance, they will be well-arranged according to their size. Otherwise they will be poorly-sorted. When many layers have settled down, the layers at the bottom become more compact. The minerals present act as a sort of glue and makes the rock stronger. Over millions of years, this process continues to form a sedimentary rock e.g. siltstone, limestone, shale and coal etc. Igneous Rocks

When lava comes to the surface of the Earth as a result of volcanic eruption or earthquakes, it solidifies to form igneous rocks. There are crystals present in most igneous rocks. Slow cooling of magma or lava results in large crystals, while rapid cooling results in fine crystals. Sometimes, igneous rocks do not form crystals because it cooled very rapidly. Instead, it takes on a smooth and glassy appearance. For example: granite, gabbro, basalt and obsidian etc.Metamorphic RocksMetamorphism is a process in which rocks change from one form to another. This can occur as a result of high temperature or high pressure exerted on the rocks. The two most common examples of metamorphism in rocks are: 1. when magma heats the surroundings to change the rock.2. When the rock is buried underground and heat changes the rock.The two types of metamorphic rocks are foliated (layered) and non- foliated (without layers).

Weathering and Erosion

Weathering is the breakdown of rocks by physical or chemical methods. Erosion is the breakdown and transportation of the surface of rocks. Main weathering agents are: temperature, freeze-and-thaw and acid rain. When rocks expand and contract continuously, they begin to break up. When water in the rocks expands and contracts, it also weakens rocks. When rainwater combines with carbon dioxide in the atmosphere, it forms carbonic acid. This acid reacts with the minerals in rocks. Consequently, carbon dioxide is released and the rocks begin to break.