Rocks

Our Earth is made mostly of rocks. The rocks are composed of mineral grains combined in different ways and having various properties.

  • Minerals are naturally occurring chemical compounds in which atoms are arranged in three-dimensional patterns.
  • The kind of elements and their arrangements lead to a particular appearance and certain properties for each mineral.
  • The same chemical elements when arranged in different patterns show different characteristics.
  • Classic examples are minerals made of the element carbon (C). Flat planes of carbon atoms form the mineral graphite, which is a gray, slippery, soft material. Carbon atoms arranged in a different pattern form the mineral diamond — the hardest natural substance known. Although they have the same chemical composition, their different internal crystal structures form very different materials.
  • A mineral is composed of two or more elements. But, sometimes single element minerals like sulphur, copper, silver, gold, graphite, etc are also found.
  • The basic source of all minerals is the hot magma in the interior of the earth.
  • When magma cools, crystals of the minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks.
  • The minerals which contain metals are called as metallic minerals (eg: Haematite) and the metallic minerals which are profitably mined are called as the ores.
  • The crust of the earth is made up of more than 2000 minerals, but out of these, only six are the most abundant and contribute the maximum.
  • These six most abundant minerals are feldspar, quartz, pyroxenes, amphiboles, mica and olivine.

Some Major Minerals:

Quartz:

  • It is one of the most important components of sand and granite.
  • It consists of silica and it is a hard mineral virtually insoluble in water.
  • It is usually white or colourless.
  • They are used in the manufacturing of radio, radar, etc.

Feldspar:

  • Silicon and oxygen are major elements of all types of feldspar.
  • Sodium, potassium, calcium, aluminium, etc are found in specific feldspar varieties.
  • Half of the earth’s crust is composed of feldspar (plagioclase (39%) and alkali feldspar (12%)).
  • It has light cream to salmon pink colour.
  • It is commonly used in ceramics and glass making.

Pyroxene:

  • The common elements in pyroxene are Calcium, aluminium, magnesium, iron and silicon.
  • About 10% of the earth’s crust is made up of pyroxene.
  • It is commonly found in meteorites.
  • Its colour is usually green or black.

Amphibole:

  • Aluminium, calcium, silicon, iron and magnesium are the major elements of amphiboles.
  • They form 7% of the earth’s crust.
  • It is green or black in colour and is used in asbestos industries commonly.
  • Hornblende is another form of amphiboles.

Mica:

  • It is made up of elements like potassium, aluminium, magnesium, iron, silicon, etc.
  • It forms 4% of the earth’s crust.
  • It is commonly found in igneous and metamorphic rocks.
  • Mica is widely used in electronic instruments.

Rocks are aggregates of minerals. A mineral is an inorganic, crystalline solid. This means that it has a regular, repeating series of molecules that ultimately determine its form. Quartz is an example of a very common mineral. Another is halite, commonly known as table salt. You can have very large quantities of a particular mineral, and even very large single crystals of minerals. Once you have more than one mineral together though, you’ve got a rock.

It is in the lithosphere that rocks are formed and reformed. And depending on the type of rock, the process through which they are created varies. In all, there are three types of rocks: igneous, sedimentary, and metamorphic. Each type of rock has a different origin.


How Are Igneous Rocks Formed?

Igneous rocks are formed when melted rock cools and solidifies. Melted rock may come in the form of magma, when it is found underneath the Earth’s surface. It can also come in the form of lava, when it is released unto the Earth’s surface during a volcanic eruption.

    • Some examples of igneous rocks are granite, scoria, pumice, and obsidian.Pumice, for instance, is formed when lava made up of melted rock, water, and trapped gas is ejected from a volcano during a violent eruption. As the ejected material undergoes very rapid cooling and depressurization, some of the trapped gas escape, leaving holes and gas bubbles on the solidified material.

How Are Sedimentary Rocks Formed?

Sedimentary rocks start forming when soil and other materials on the Earth’s surface are eroded and finally settle down, forming one layer of sediments. As time passes, more and more materials get eroded and settle on the older layers. Thus, layer upon layer is formed. The lower layers undergo intense pressure due to the weight of the upper layers, eventually evolving into rocks.

    • Some examples of sedimentary rocks are sandstone, limestone, shale, conglomerate, and gypsum. Sandstone, for instance, is a result of depositions of sand from beaches and rivers. One can find them mostly in deltas, since this is where the rivers flow into the ocean.

How Are Metamorphic Rocks Formed?

To metamorphose or simply to morph means ‘to change in form’. Metamorphic rocks are actually products of rocks that have undergone changes. Thus, a metamorphic rock may have originally been an igneous, sedimentary, or even another metamorphic rock. The changes occur when the original rocks are subjected to extreme heat and pressure beneath the Earth’s surface.They may also occur when the the original rocks are caught in the middle of two colliding tectonic boundaries.

    • Some examples of metamorphic rocks are marble, slate, schist and gneiss. Marble, for instance is the result of the metamorphism of limestone and dolostone. When limestone metamorphoses, its calcite grains grow and interlock with one another. As such, marble is denser and harder compared to limestone.

There are three broad categories of rocks. They are Igneous, Sedimentary, and Metamorphic.

Classification of Rocks

 

Igneous Rocks

  • Igneous rocks start as molten material that cools and solidifies.
  • Granite, for instance was once a super heated liquid. When this liquid is beneath the surface of the earth, it’s called magma. Once it reaches the surface, perhaps as an eruption, it is called lava.
  • Lava cools very quickly, since the temperature of the surface (let’s say 32 degrees Fahrenheit, or zero degrees Celsius) is significantly cooler than the lava, which can be more than 1000 degrees F. This rapid cooling is called quenching, and is how glass-like rocks, like obsidian, form.
  • Bringing material as hot as lava to the surface is somewhat like sticking a red-hot iron into a pot of water. Igneous rocks can (and often do) cool beneath the surface of the earth, the molten material moving up from the mantle but never making it to the surface. Other times they extrude at the surface, either at mid-oceanic ridges or hotspots.

Sedimentary Rocks

  • Sedimentaryrocks are made up of grains that break off of other rocks through a process called weathering.
  • When rocks are exposed to rain, wind, temperature changes, roots, and some chemicals, they can be broken down into their basic components.
  • Physical weatheringbreaks off grains of rock, while chemical weathering breaks rocks down into more basic elemental components.
  • These grains can come from other sedimentary rocks, from igneous rocks, or from metamorphic rocks.
  • Grains are transported downstream, eventually settling in a basin, or low energy environment (for example a lake or an ocean).
  • Over time, as more sediment is deposited, layers of sediment are buried deep enough to be lithified, or turned to stone.
  • Because features of the environment are preserved in the sediments, one can tell something about where the rock was deposited.
  • For example, a rock with leaf fossils preserved must have been formed in a slow, stagnant environment – one in which the leaves would not be disturbed or moved away.
  • Sedimentary rocks are all about what remains.
  • Black shales, organic rich sediments from which we extract oil and gas, form in deep marine settings, where there is very little energy and there is very little oxygen to decompose the organic material that gets deposited.

Metamorphic Rocks

  • Metamorphicrocks are rocks that have been deformed.
  • They have been heated or squashed or buried (which often means they are both heated and squashed), which can cause minerals in the rocks to recrystallize.
  • Metamorphic rocks form in places where change is happening: they can form in mountain belts (collision zones) where rock is compressed or buried, along fault planes, along subduction zones, next to magma pockets as the neighboring rock is cooked, to name a few.
  • Marble is metamorphosed limestone (a chemical sedimentary rock). Slate was once used as the backing of chalkboards.
  • Any type of rock can be metamorphosed, including metamorphic rocks.
  • Each of the rock types is related to the others through different physical and chemical processes.
  • Mountains can expose igneous, sedimentary, and metamorphic rocks to weathering and erosion.
  • Grains of sediment (or boulders!) break off of these exposed rocks, and, due to gravity, move downhill either in a stream bed or by rolling. This travel causes further breakdown of the sediment, which eventually reaches the ocean. The sediment, once deposited in an ocean basin, gets buried by other sediment, and compacted into a sedimentary rock. If it’s buried deep enough, or if the basin becomes part of an orogen, it can be metamorphosed, becoming a metamorphic rock.
  • Eventually, the rock can either become exposed through sea level fall or mountain-building, where it starts the journey back to the ocean again, or it can be recycled back into the mantle at a subduction zone, perhaps one day returning to the surface as an igneous rock.
  • The following concept map shows the rock cycle in terms of earth processes. New material comes from mid-ocean ridges and volcanoes; it is weathered, deformed, compacted and cemented, until it is recycled back into the mantle at a subduction zone.

rock cycle

  • The early phase of soil formation starts by disintegrating the rock under the influence of climate.
      • Rainwater will dissolve rock elements, temperature fluctuations will cause cracks and fissures in the rocks.
      • Freezing and thawing of water captured in the rock will widen existing cracks and cavities.
      • Pioneer vegetation, at first lichens, will settle and their roots will further loosen the rock.
      • Moreover, decaying plant debris will produce organic acids, which further disintegrates the rock.
      • Organic matter will start to accumulate and be mixed with the mineral material provided by the rock.
  • Over time, rock minerals will be dissolved or transformed.
  • Elements released from the rock will precipitate and new minerals may be formed. For example, iron will be oxidized and precipitate as iron oxides or hydroxides, giving the soil reddish or yellowish-brownish colours.
  • Soil fauna will settle and mix (‘homogenize’) the soil. The soil will grow in depth through newly formed soil material at the bottom.
  • The soil matures.
  • Given sufficient time under stable environmental conditions, soils will reach a steady state, whereby soil build-up matches their breakdown.
  • Production of humus from decaying vegetation debris will equal its consumption by soil microbae, fauna and flora.
  • Transformation of rock minerals into soil minerals will keep pace with the removal of earlier formed soil minerals.
  • Slow surface wash of topsoil is matched by new formation of soil material from the bedrock.
  • The soil has aged.