Igneous Rocks
Igneous rocks originate from magma or lava derived from the Earth’s interior and are therefore referred to as primary or basic rocks. They form when molten material cools and solidifies, either beneath the Earth’s crust or on its surface.
As magma rises upward and undergoes cooling, it transforms into solid igneous rock. Since these rocks do not form in layers and lack fossils, they are relatively resistant to chemical weathering but are more susceptible to physical and mechanical weathering. Nearly 90% of the Earth’s crust is composed of igneous rocks.
Key Characteristics
- Formed by the solidification of molten magma, making them generally impermeable to water.
- Do not occur in distinct layers or strata, unlike sedimentary rocks.
- Typically non-fossiliferous.
- Exhibit granular and crystalline textures due to interlocking mineral grains.
- Less affected by chemical weathering because water cannot easily penetrate them.
- Predominantly weathered by mechanical processes.
Igneous rocks are classified mainly on the basis of texture, which depends on the size, shape, and arrangement of mineral grains:
- Slow cooling at great depths → large mineral crystals.
- Rapid cooling at the surface → very fine-grained or glassy texture.
- Moderate cooling conditions → intermediate-sized grains.
Common examples include granite, gabbro, pegmatite, basalt, volcanic breccia, and tuff.
Types of Igneous Rocks
(i) Intrusive (Plutonic) Igneous Rocks
These rocks form when magma cools slowly beneath the Earth’s crust, surrounded by pre-existing country rock. Slow cooling allows large, coarse crystals to develop, which are often visible to the naked eye.
Typical intrusive forms include batholiths, stocks, laccoliths, sills, and dikes.
(ii) Extrusive (Volcanic) Igneous Rocks
Extrusive igneous rocks form on the Earth’s surface when molten material erupts from the mantle or crust. Rapid cooling produces fine-grained textures or very small crystals.
Molten rock beneath the surface is called magma. Once it emerges onto the surface—either on land or underwater—it is termed lava. Because of their fine-grained nature, distinguishing between extrusive rock types is often more difficult than with intrusive rocks.
Basalt is the most widespread volcanic rock and commonly forms extensive lava sheets and plateaus, such as the Deccan Plateau in India and the Antrim Plateau in Northern Ireland. The weathering of basalt gives rise to black (regur) soil.
Classification by Silica Content
- Acid igneous rocks: >63% silica (e.g., granite, rhyolite)
- Intermediate igneous rocks: 52–63% silica (e.g., andesite, dacite)
- Basic igneous rocks: 45–52% silica; rich in iron, aluminium, and magnesium (e.g., gabbro, basalt)
- Ultrabasic igneous rocks: <45% silica (e.g., picrite, komatiite)
Sedimentary Rocks
Sedimentary rocks are formed from fragments produced by the denudation of pre-existing rocks. These materials are transported by external agents such as water, wind, and ice, deposited in layers, and later compacted and cemented through lithification.
Many sedimentary rocks preserve their original depositional features even after lithification, resulting in well-defined layers or strata of varying thickness, as seen in sandstone and shale.
Although sedimentary rocks cover about 75% of the Earth’s surface, they account for only around 5% of the Earth’s crust by volume.
These rocks are widely used in construction activities such as roads, buildings, tunnels, and canals. They are also major sources of natural resources, including coal, petroleum, natural gas, groundwater, and mineral ores.
Key Characteristics
- Clearly stratified or layered structure.
- Layers are rarely horizontal and are often tilted due to tectonic forces.
- Composed of sediments derived from older rocks, plant remains, and animal matter.
- Highly porous and permeable, allowing water storage and movement.
- Characterised by joints that usually cut across bedding planes.
- Riverine sedimentary rocks often develop polygonal cracks upon drying.
- Typically formed in shallow marine environments near continental margins.
The surface separating two successive sedimentary layers is known as the bedding plane.
Classification of Sedimentary Rocks
- Mechanically formed: sandstone, conglomerate, shale, loess
- Organically formed: coal, chalk, limestone, geyserite
- Chemically formed: limestone, chert, halite, potash
Important Notes
- Coal is the most abundant organic sedimentary rock.
- Oil and natural gas (methane, ethane, propane, butane) are commonly associated with sedimentary basins and liquid hydrocarbons such as paraffins, naphthenes, and aromatics.
Metamorphic Rocks
Metamorphic rocks form when existing rocks undergo changes in temperature, pressure, and volume without melting. This process, known as metamorphism, occurs when rocks are buried deep within the crust, subjected to tectonic stress, or come into contact with rising magma.
Metamorphism leads to recrystallisation and reorganisation of minerals within the original rock.
Types of Metamorphism
- Dynamic metamorphism: Mechanical crushing and deformation without major chemical change.
- Thermal metamorphism: Chemical alteration and recrystallisation due to high temperature.
- Contact metamorphism: Occurs when rocks come into contact with hot magma or lava.
- Regional metamorphism: Large-scale metamorphism caused by tectonic forces, pressure, and heat.
During metamorphism, minerals may align into layers or lines, producing foliation or lineation. Alternating light and dark mineral bands form banding, characteristic of banded metamorphic rocks.
Metamorphic rocks are classified into:
- Foliated rocks
- Non-foliated rocks
Examples include gneiss, slate, schist, marble, quartzite, granite gneiss, and syenite.
Rock transformations include:
- Clay → Slate
- Limestone → Marble
- Sandstone → Quartzite
- Granite → Gneiss
- Shale → Schist
- Coal → Graphite
Distribution of Metamorphic Rocks
Most continents contain large Precambrian shield areas (e.g., Canadian, African, Brazilian, Australian shields), composed mainly of granitic and gneissic rocks. These regions have experienced multiple metamorphic events, including greenschist, amphibolite, and granulite facies metamorphism.
Major orogenic events such as the Caledonian and Hercynian (Variscan) orogenies played a key role in shaping metamorphic belts across Europe, North America, Central Asia, and Australia.
Rock Cycle
The rock cycle is a continuous and dynamic process through which rocks transform from one type to another over geological time. It involves melting, cooling, weathering, erosion, deposition, compaction, cementation, and metamorphism.
Stages of the Rock Cycle
- Igneous Rocks
Formed by cooling and solidification of magma or lava. They may later undergo metamorphism under high pressure and temperature. - Metamorphic Rocks
Formed when existing rocks are altered due to heat, pressure, or chemical changes. Under extreme conditions, they may melt and form magma again. - Sedimentary Rocks
Formed from weathered rock fragments that are transported, deposited, and lithified.
Key Processes
- Weathering and erosion break rocks into sediments.
- Transportation and deposition move and accumulate sediments.
- Lithification converts sediments into sedimentary rock.
- Subduction and melting transform rocks into magma.
- Solidification of magma forms new igneous rocks.
This cycle is driven by plate tectonics, volcanic activity, erosion, and mountain-building processes.
Recent Scientific Findings: Earth’s Inner Core Rotation
Recent studies analysing seismic waves from repeating earthquakes over the past 60 years reveal changes in the Earth’s inner core rotation.
Key Findings
- In the early 1970s, the inner core rotated slightly faster than the Earth’s mantle.
- By around 2009, its rotation slowed and matched the planet’s overall rotation.
- Currently, the inner core appears to be rotating more slowly than the Earth’s surface.
- The next shift in rotation speed is expected around the mid-2040s.
- The pattern suggests a cyclical change every 60–70 years.
Significance
- Encourages integrated Earth-system models linking core dynamics with surface processes.
- Inner core rotation may influence Earth’s overall rotation and magnetic field generation.
- Enhances understanding of long-term core evolution and deep Earth processes.