Rock system

Rock system

Igneous Rocks:

Igneous rocks form out of magma and lava from the interior of the earth, they are also known as primary rocks or Basic rock.Igneous rocks are created when magma cools and solidifies.

• When magma ascends and cools, transforming into a solid state, it forms igneous rock.This cooling and solidification can occur either within the Earth’s crust or on its surface.
• They do not occur in layer nor do they contain fossils, igneous rocks are much less affected by chemical weathering than physical and mechanical weathering.
• 90% of the crust is made up of igneous rock.

Characteristics

1. These rocks solidify from molten magma, making them impermeable to water.
2. Unlike sedimentary rocks, they do not typically form in distinct layers or strata.
3. Igneous rocks are usually non-fossiliferous.
4. They are generally characterized by a granular and crystalline texture.These are solidified from a molten magma and water cannot percolate through them.
5. They usually do not occur in distinct beds or strata like sedimentary rocks.
6. Igneous rocks are generally not fossiliferous.
7. Igneous rocks are generally granular and crystalline.
8. They are less affected by chemical weathering as the water does not percolate in them easily.
9. These rocks are generally weathered by mechanical weathering.

Igneous rocks are classified based on texture.The texture is determined by the size and arrangement of grains or the physical properties of the materials.

• When molten material cools slowly at significant depths, the mineral grains can grow very large.
• Rapid cooling at the surface produces small, smooth grains.
• Cooling under moderate conditions leads to grains of intermediate size in igneous rocks.
• Granite, gabbros, pegmatite, basalt, volcanic breccias and tuff are the examples of igneous rocks.

(i) Intrusive Igneous Rocks (Plutonic Igneous Rocks):

Intrusive igneous rocks form when magma cools and solidifies beneath a planet’s crust. Encased by pre-existing rock, known as country rock, the magma cools slowly, resulting in a coarse-grained texture.

• The mineral grains in these rocks are usually visible to the naked eye.
• Common intrusive formations include batholiths, stocks, laccoliths, sills, and dikes.

(ii)Extrusive Igneous Rocks (Volcanic rocks):

Extrusive igneous rocks form at the Earth’s surface due to the partial melting of rocks in the mantle and crust. These rocks cool and solidify more rapidly than intrusive igneous rocks, resulting in a fine-grained or small-crystal texture.

• The molten rock, containing suspended crystals and gas bubbles, is referred to as magma. Magma rises because it is less dense than the surrounding rock. Once it reaches the surface, either under water or in air, it is known as lava.
• Due to their fine-grained nature, it is more challenging to differentiate between various types of extrusive igneous rocks compared to distinguishing between types of intrusive igneous rocks.
• Basalt is common volcanic rock and form lava sheet, lava plateau e.g. Antrim in Northern Ireland, Deccan plateau in India, the Black soil or Regur is formed by the weathering of these rock.
• Acid igneous rocks containing a high silica content, greater than 63% SiO₂ (examples granite and rhyolite)
• Intermediate igneous rocks containing between 52 – 63% SiO₂ (example andesite and dacite)
• Basic igneous rocks have low silica 45 – 52% and typically high iron, Aluminum, magnesium content (example gabbro and basalt).
• Ultrabasic igneous rocks with less than 45% silica. (Examples picrite and komatiite)

Sedimentary Rocks:

Rocks on the Earth’s surface are subjected to denudational forces, breaking into fragments of various sizes. These fragments are transported by exogenous agents and deposited. Through the process of compaction, these deposits become rocks, a process known as lithification.

• In many sedimentary rocks, the original characteristics of the deposited layers are preserved even after lithification. This results in the formation of multiple layers of different thicknesses, as seen in rocks like sandstone and shale.

Sedimentary rock are layered or stratified rock are found over 75% area of the crust but contribute only 5% in the formation of the crust.

• This rock is used in the construction of roads, houses, tunnels, canals or other constructions. Sedimentary rocks are significant sources of natural resources, including coal, fossil fuels, groundwater, and mineral ores.

Important Characteristics

• 1. It contains strata or layers.
  2. The layers are rarely horizontal and generally tilted due to lateral compressive and tensile forces.
  3. It is formed of sediments derived from the older rocks, plants and animal remains.
  4. It covers 75 per cent of the surface area of the globe.
  5. Most of the sedimentary rocks are permeable and porous.
  6. They are distinguished by joints of varying sizes, which are typically perpendicular to the bedding planes.
  7. Riverine sedimentary rocks form polygonal-shaped cracks when exposed to sunlight.
  8. The most ideal locations for their formation are shallow sea floors adjacent to continents.
  9. The connecting plane between two consecutive beds or layers of sedimentary rocks is called ‘bedding plane’.
• Sedimentary rocks are classified into three categories:
  • Mechanically formed, E.g., Sandstone, limestone, conglomerate, loess, shale etc.
  • Organically formed, E.g., Chalk, limestone, geyserite, coal etc.
  • Chemically formed, E.g., Limestone, chert, potash, halite etc.

Important Points

• Coal: Coal is the most plentiful sedimentary rock rich in organic material.
• Oil  and  natural  gas:  Major natural gas varieties include methane, ethane, propane, and butane.
• These natural gases are commonly, though not invariably, intimately associated with the various liquid hydrocarbonsmainly liquid paraffins, napthenes, and aromatics that collectively constitute oil.

Metamorphic Rocks:

Metamorphic rocks are formed due to changes in pressure, volume, and temperature. Metamorphism takes place when rocks are pushed to deeper levels by tectonic forces, when molten magma rises through the crust and interacts with crustal rocks, or when underlying rocks experience significant pressure from overlying layers.

• Metamorphism is a process by which already consolidated rocks undergo recrystallisation and re-organization of materials within original rocks.
• Dynamic Metamorphism: Involves the mechanical breakdown and reorganization of the original minerals in rocks through crushing and fracturing, without significant chemical changes.
• Thermal Metamorphism: Occurs when the rock’s materials undergo chemical alterations and recrystallization.
• Contact Metamorphism: Rocks come in contact with hot intruding magma, and lava and the rock material recrystallize under high temperature.
• Regional Metamorphism: Rocks experience recrystallization due to deformation from tectonic shearing, combined with high temperatures, pressure, or both.
• During metamorphism, minerals or grains in some rocks align in layers or lines. This alignment in metamorphic rocks is known as foliation or lineation.
• Sometimes minerals or materials of different groups are arranged into alternating thin to thick layer appearing in light and dark shades.This arrangement in metamorphic rocks is referred to as banding, and rocks with this feature are called banded rocks.
• The type of metamorphic rock is determined by the original rock that underwent metamorphism.
• Metamorphic rocks are categorized into two main types: foliated and non-foliated rocks.
• Examples of metamorphic rocks include gneiss, granite, syenite, slate, schist, marble, and quartzite.
• Clay can metamorphose into slate, limestone into marble, sandstone into quartzite, granite into gneiss, shale into schist, and coal into graphite.

Distribution of Metamorphic Rocks: The central and often dominant feature of most continents is their vast Precambrian-shield area; examples include the Canadian, Brazilian, African, and Australian Shield.

• These regions consist largely of granitic or granodioritic gneisses, with belts of sedimentary rocks located within, between, and extending over them.
• These rocks often undergo metamorphism in the greenschist, amphibolite, and granulite facies. The Caledonian orogeny (at the end of the Silurian Period) triggered tectonic metamorphic events along the eastern coast of North America, Greenland, the British Isles, Fennoscandia, Central Asia, and Australia.
• The Hercynian, or Variscan, orogeny occurred around 300 million years ago, impacting similar regions, including the Urals and the European Alps.

ROCK CYCLE

Rock Cycle Overview:

The rock cycle is a continuous process where rocks are transformed from one type to another over geological time scales. It involves various processes such as melting, cooling, weathering, erosion, deposition, compaction, and metamorphism.

1. Igneous Rocks:

• Formation: Igneous rocks are created through the cooling and solidification of magma or lava (molten rock beneath the Earth’s surface) or lava (molten rock at the surface).
• Transformation: Igneous rocks can undergo metamorphism if subjected to high pressure and temperature conditions deep within the Earth’s crust or during tectonic processes.

2. Metamorphic Rocks:

• Formation: Metamorphic rocks form from the alteration of existing rocks (igneous, sedimentary, or other metamorphic rocks) due to changes in temperature, pressure, or chemical environment without melting.
• Transformation: Metamorphic rocks can melt if subjected to extreme temperatures and pressures, becoming magma and potentially re-forming into igneous rocks.

3. Sedimentary Rocks:

• Formation: Sedimentary rocks form from the accumulation and lithification of sediments (fragments of other rocks, minerals, or organic materials).
• Transformation: Sedimentary rocks can be weathered into sediment (fragments) by physical and chemical processes. These sediments can then be transported and deposited to form new sedimentary rocks through compaction and cementation.

Processes in the Rock Cycle:

• Weathering and Erosion: Igneous, metamorphic, and sedimentary rocks are weathered and eroded by natural processes like wind, water, and ice, breaking them down into smaller particles.
• Transportation and Deposition: These particles are transported by rivers, glaciers, wind, or ocean currents and deposited as sediments.
• Lithification: Sediments undergo compaction (due to pressure from overlying layers) and cementation (binding of grains by minerals dissolved in water), forming sedimentary rocks.
• Subduction and Melting: Crustal rocks (igneous, metamorphic, and sedimentary) can be subducted into the Earth’s mantle at convergent plate boundaries. Increased temperature and pressure within the mantle can cause these rocks to melt, forming magma.
• Solidification: Magma that reaches the Earth’s surface cools and solidifies, forming new igneous rocks.

Continuous Cycle: The rock cycle is continuous and dynamic, with rocks constantly undergoing transformation from one type to another over millions of years. This cycle is driven by geological processes such as plate tectonics, volcanic activity, erosion, and mountain building.

Recent news

• Researchers analyzed seismic waves from repeating earthquakes recorded over the past six decades.
• They analyzed variations in the timing and movement of seismic signals to estimate the rotational speed of Earth’s inner core.
• The inner core is believed to rotate independently from the mantle and the rest of the planet, providing insights into its dynamics.

Findings:

• In the early 1970s, the inner core started rotating slightly faster than the rest of the Earth.
• By 2009, it had gradually slowed down and aligned with the planet’s overall rotation.
• Current Status: There is now a “negative trend,” indicating that the inner core is rotating slower than the surface of the Earth.
• Future Prediction: The next change in rotation speed is projected to occur around the mid-2040s.
• Cycle: The research indicates a periodic pattern in which the inner core alters its rotation speed roughly every 60-70 years.

Significance:

• Model Building: The study encourages researchers to develop and test models that consider Earth as an integrated dynamic system, where changes in the inner core rotation affect the entire planet.
• Effect on Earth’s Rotation: The inner core’s slowdown may have the potential to affect the Earth’s overall rotation.
• Core Evolution: Understanding these rotational dynamics provides insights into how Earth’s core evolves over time, influencing geological processes and magnetic field generation.