Rock Cycle – An Introduction
Rock Cycle Diagram is the one that explains the processes through which the types of rocks namely – Igneous, Sedimentary and Metamorphic undergo transformation in Geologic times. Rocks of any type can be converted into any other type, or into another rock of the same type. Whenever the state of equilibrium of a particular rock type is disturbed, the rock undergoes a transformation e.g. when an Igneous rock like Basalt or Obsidian is subjected to weathering, it may break up into fragments. Due to Compressional forces it may get transformed into a Sedimentary rock like Sand Stone. A rock cycle diagram explains how the three rock types are related to each other, and how the forces of nature compel them to change from one type to another over time.
But before we learn details of the rock cycle diagram, it is important to have a detailed understanding of the nature of the three types of rocks namely – Igneous, Sedimentary and metamorphic.
Three Types Of Rocks
In general , any mass of mineral matter whether consolidated or not, which forms part of the earth’s crust or lithosphere is regarded as a rock. These minerals are compounds of more than one elements . All the rocks are categorized by their mineral content, chemical composition and the modes of formation. The important mineral groups are Silicates, Carbonates, Sulphides, etc.
- About 90 % of rock forming minerals are silicates ( compounds containing silicon, oxygen or more metals ).
- The important rock forming silicates mineral groups are feldspar, Quartz and Ferromagnesium.
- Feldspar is the most abundant rock forming silicate mineral ( making up 54 % of the minerals in the earth crust).
- Limestone’s and marbles are made up of calcite, an important mineral of carbonate group.
- Although most rocks are made of minerals, some substances of organic origin such as peat and guano are accepted as rocks.
Rocks are classified on the basis of their mode of formation into three broad categories: Igneous, Metamorphic & Sedementary.
Igneous rocks are formed by the cooling, solidification and crystallization of molten earth materials, known as magma and lava. These rocks are also called as primary rocks or parent rocks because these were originated first during the formation of crust through the process of cooling of the earth surface. They do not have distinct beds or strata like the sedimentary rocks. These are granular and crystalline rocks. The size of the crystals vary from one rock to another. Igneous rocks are generally hard and water percolates through them with difficulty along the joints. Since water does not percolate easily , these rocks are less affected by chemical weathering. These rocks are prone to mechanical weathering due to their granular structure. These rocks are non-fossiliferous. Most of the igneous rocks consist of silicate minerals.
On the basis of chemical composition, igneous rocks are divided into:
- Acidic igneous rocks having more silica. They are light rocks relatively e.g.—Graniotes.
- Basic igneous rocks have lower amount of silica. They are dark-coloured due to pre-dominance of Ferro-magnesium, e.g. –Gabbro, Basalt etc.
On the basis of mode of occurrence, igneous rocks are classified into two major groups: Intrusive and Extrusive
Intrusive Igneous rocks
When the rising magma is cooled and solidified below the surface of the earth , they are known as intrusive igneous rocks. These are further sub-divided into :
- Plutonic Igneous rocks: They result from the cooling of magma very deep inside the earth. Due to very slow cooling at that great depth, large grains are developed, e.g.-Granite.
- Hypabyssal Igneous rocks: They are formed when magma cools & solidify just beneath the earth surface. They take different shapes and forms depending upon the hollow places in which they solidify.
Different forms of Hypabyssal Igneous Rocks
- Batholith: These are large intrusive mass of igneous rocks , usually granite , formed by the deep-seated intrusion of magma on a large scale. They are known to be the largest kind of intrusive bodies and are usually dome shaped with very steep walls. They are known to be present in the core of most of the mountains.
- Lacolith: These are of mushroom shape having convex upper surface and a relatively flat lower one. The ascending magma forces the upper layer of the sedimentary rocks to take the form of a convex arch or a dome.
- Lopoliths: They represent inter-stratal bowl-like bodies formed by the solidification of magma in a concaves hallow basin and the sagging of rocks under the weight of the intruded magma.
- Phacoliths: These lens-shaped bodies are formed due to injection along the anticlines and synclines in the folded strata.
- Sills: They are bed-like intrusive bodies formed by the solidification of magma parallel to the bedding planes of the sedimentary rocks.
- Dykes: These wall-like formations of solidified magma are found mostly perpendicular to the beds of sedimentary rocks.
Sills, Lacoliths , Lopoliths and Phacoliths are concordant intrusive bodies while Batholiths and Dykes are discordant intrusive bodies.
Extrusive igneous rocks
These igneous rocks are formed by the cooling and solidification of molten lava on the earth’s surface. Basalt is the most important example of extrusive igneous rocks, others being Gabbro and Obsidian. These are generally fine grained or glassy because of quick rate of cooling of lava. The extrusive igneous rocks are divided into two sub-groups :
–Explosive type: Volcanic materials of violent volcanic eruptions include ‘bombs’ (big fragments of rocks) , Lapilli ( pea-sized fragments) and volcanic dusts and ashes.
–Quiet type: in this, lava appear on the surface through cracks and fissures and their continuous flow form extensive lava plateaus e.g. Deccan plateau, Columbia plateau.(USA)
Rocks formed from material derived from Pre-existing rocks and from organic sources by the process of denudation are known as sedimentary rocks. In other words, rocks formed due to aggregation and compaction of sediments are called sedimentary rocks.
- Sedimentary rocks contain different layers of sediments.
- Fossils are found in these rocks.
- About 75 % of the surface area of the globe is covered by the igneous and metamorphic rocks.
- Though sedimentary rocks cover largest area of the earth’s surface , they constitute only 5 % of the composition of the crust while 95 % of the crust is the composed of igneous and metamorphic rocks .
- Sedimentary rocks lack crystalline structure.
- Layers of sedimentary rocks are seldom found in original and horizontal manner. They are prone to folding and faulting due to Compressional and Tensional forces.
- Joints are also found in the sedimentary rocks.
- Most of the sedimentary rocks are permeable and porous but a few of them are also non-porous such as clay.
- Shale is the most abundant sedimentary rock.
Classification of sedimentary rocks:
- Mechanically formed or clastic rocks – Sandstone, Conglaromate , Shale etc.
2. Formed by the water action – Clay.
3. Formed by the wind action – Loess.
4. Formed by the glacial action – Boulder clay.
5. Organically formed sedimentary rocks – Limestone, Dolomites, Coals, Peats etc.
6. Chemically formed sedimentary rocks – Gypsum, Salt rock etc.
Metamorphic rocks are formed by the change in the texture, mineral composition and structure of the pre-existing rocks through temperature & pressure.
The pre-existing rocks may be igneous, sedimentary or even metamorphic rocks.
The fossils of the original sedimentary rocks are destroyed by the heat and pressure.
When already formed metamorphic rocks are again metamorphosed, they are known as Re-metamorphosed rocks.
Most of the metamorphic rocks result largely from the movement of tectonic plates and the processes related to it.
Some rocks after metamorphism become harder than their original form , e.g. Marble, Limestone, Diamond from carbon etc.
The Rock Cycle Diagram
Rocks are always in the process of transformation. Weathering erosion of igneous rocks lead to the formation of sedimentary rocks and both igneous & sedimentary rocks are transformed into metamorphosed rocks. Again metamorphosed rock can melt under high temperature to become igneous rocks & the cycle goes on.
Transformation to Igneous Rocks
When Sedimentary or Metamorphic rocks are pushed deep under the Earths surface, by the action of Plate boundary interactions that result in Subduction they may melt into magma. If the conditions no longer exist for the magma to stay in its liquid state, it cools and solidifies into an igneous rock. A rock that cools within the Earth is called Intrusive which is further divided into Plutonic and Hypabyssal. These rocks cool very slowly, producing a coarse-grained texture such as the rock Granite.
Similarly, when the magma is forced out due the volcanic activity, it cools rapidly by the action of atmospheric temperature and pressure and cool fast. The resultant formation is fine grained rocks such as Basalt. Sometimes there are no crystals and they form rocks with a glassy appearance such as Obsidian. These rocks are broadly categorized as Extrusive Igneous Rocks.
Transformation to Sedimentary Rocks
When Igneous or Metamorphic Rocks are subjected to mountain building activity, they get exposed and further get subjected to the processes of Weathering and Erosion. Eventually they undergo sedimentation. These sediments when compressed due to pressure from the mountains or otherwise get converted into rocks such as Breccia, Conglomerate, Sandstone, Siltstone, and shale . Some processes like Silification, Serpentinization, Uralitization, Epidotizations are instrumental in the creation of Sedimentary rocks.
Transformation to Metamorphic Rocks
When any type of rock is exposed to high temperature and pressure, there is a change its structure, composition and texture. These rocks are called Metamorphic rocks e.g Gneiss, Slate, Marble, Schist, and Quartzite etc. The pre-existing rock can be Igneous, Sedimentary or a Metamorphic rock itself. Local or regional metamorphism refers to the effects on large masses of rocks over a wide area, typically associated with mountain building activities. These rocks commonly exhibit distinct bands of differing mineralogy and colors, called Foliation. When a Metamorphic rock itself undergoes change by the action of high temperature and pressure, then it is called Re-Metamorphosed Rock.
Factors Influencing Rock Cycle
- Tectonic processes : Volcanic activity and uplift of land exposes rock that was underground to weathering and erosion.
- Weathering Rate : Climatic conditions such as precipitation and temperature alter the rate of weathering.
- Type of Vegetation : Roots can physically break up rocks and also change the environmental chemistry (e.g increase acidity), increasing the rate of chemical weathering. In turn, the kind of rock that is weathered determines quality of soils and levels of nutrients (especially Nitrogen and Phosphorus levels), and local biodiversity.
- Mining Activities : The extraction of fossil fuels and rocks and which in turn can destabilize soil topography, increase erosion, and decrease water quality by increasing sediment and pollutants in rivers and streams.
- Human Factors : Deforestation, Agricultural activities and water use can destabilize soil and increase the rate of erosion, eventually affecting the Rock Cycle Diagram.
- Action of wind, ice, gravity and water: These can alter the rock cycle significantly as atmospheric air circulation and ocean currents are constantly at work.
- Urbanization : Concretization increases water run off paving way for increase water runoff, increase edrosion and decreased soil quality in the surrounding areas.
- Extreme Natural Events : Flooding and turbulent oceanic waves can cause accelerated rates of erosion.
Thus, a closer look into the rock cycle diagram reveals that no rocks are everlasting. This is in tune with the non-permanence principle of nature.