The Earth consists of three broad layers:
Crust: The outermost solid layer which is made up of rocks and minerals, having an average thickness of 35 km (continental crust) and 7 km (oceanic crust).
Mantle: The middle layer, extending up to 2,900 km, consisting of semi-solid rock that is subjected to slow convection, powering plate tectonics.
Core: The innermost part, split into the outer core (liquid, iron and nickel composition, causes Earth’s magnetic field) and the inner core (solid because of intense pressure, mostly iron and nickel alloy).
The layered nature of the Earth is read through seismic research, as various waves move at different velocities through the layers.
The Earth’s Crust
The crust is the outermost and thinnest layer of the Earth, covering the surface on which life is found.
The crust is mostly made up of silicate minerals and is subdivided into:
1.Continental crust: More thick (30-70 km), less dense, made mostly of granite (aluminum and silica rich, “sial”).
2.Oceanic crust: Less thick (5-10 km), denser, made mostly of basalt (magnesium and silica rich, “sima”).
The Mohoroviić Discontinuity (Moho) is the contact between the crust and the mantle. The crust is dynamic in nature, constantly changing because of tectonic processes, weathering, and erosion.
Composition of Earth’s Crust
The Earth’s crust consists mainly of oxygen (46.6%) and silicon (27.7%), which form silicate minerals. Aluminum, iron, calcium, sodium, potassium, and magnesium are the other significant elements. The most abundant minerals in the crust are feldspar, quartz, mica, and amphibole.
The crust holds a range of rocks, divided into:
1.Igneous rocks (created from cooling lava/magma).
2.Sedimentary rocks (created from deposition of sediments).
3.Metamorphic rocks (created from alteration of current rocks under pressure and heat).
These rocks hold the key to Earth’s economic and geological resources like coal, petroleum, and metals.
Types of Rocks
Rocks are solid aggregates of minerals that naturally occur and fall into three types:
1.Igneous Rocks: Produced by the cooling and solidification of magma or lava. Examples: Granite, basalt, obsidian, pumice. They are hard, crystalline, and don’t have fossils.
2.Sedimentary Rocks: Produced by the deposition, compaction, and cementation of sediments over millions of years. Examples: Limestone, sandstone, shale, conglomerate. They usually have fossils.
3.Metamorphic Rocks: These are produced when igneous or sedimentary rocks are exposed to heat and pressure over millions of years. Examples: Marble (from limestone), slate (from shale), schist (from basalt).
Every type of rock has a central role to play in the rock cycle, as rocks undergo continuous change on a geological time scale.
The Rock Cycle
The rock cycle is the steady sequence of alteration among igneous, sedimentary, and metamorphic rocks due to Earth’s internal heat and external forces.
The most important processes are:
- Cooling and solidification of magma creates igneous rocks.
- Weathering and erosion disintegrate rocks to form sediments.
- Sediments compact and cement to create sedimentary rocks.
- Heat and pressure change rocks into metamorphic rocks.
- Melting of the rocks produces magma, and the cycle is complete.
This cycle is responsible for the dynamic state of the Earth’s crust and aids in the creation of natural resources.
Plate Tectonics
The lithosphere of the Earth is composed of a number of rigid plates that float on the semi-fluid asthenosphere of the upper mantle. Plate movement is caused by convection currents within the mantle.
The large plates are:
Pacific Plate (greatest), Eurasian Plate, Indo-Australian Plate, North American Plate, South American Plate, African Plate, and Antarctic Plate.
Movements of plates occur in three forms:
Divergent Boundaries: Plates drift apart and develop mid-ocean ridges (e.g., Mid-Atlantic Ridge).
Convergent Boundaries: Plates collide to create mountains (e.g., Himalayas) or subduction zones (e.g., Peru-Chile Trench).
Transform Boundaries: Plates slide horizontally over one another to produce earthquakes (e.g., San Andreas Fault).
Plate tectonics accounts for earthquakes, volcanoes, and the formation of mountains.
Giant Landforms
Landforms are categorized according to their mode of formation:
1.Mountains: High landforms with high slopes (e.g., Himalayas, Rockies).
2.Plateaus: Uplifted flat-topped surfaces (e.g., Deccan Plateau, Colorado Plateau).
3.Plains: Large flat or gently sloping surfaces developed by deposition (e.g., Indo-Gangetic Plains).
4.Valleys: Low surfaces between mountains or hills (e.g., Great Rift Valley).
Types of Mountains
Mountains are categorized according to their formation processes:
1. Fold Mountains- Developed due to compressional forces folding rock layers.
Have anticlines (upfolds) and synclines (downfolds).
Example: Himalayas (Asia), Alps (Europe), Andes (South America), Rockies (North America).
Young fold mountains: High, rugged, and actively rising (e.g., Himalayas).
Old fold mountains: Lower, eroded, and rounded (e.g., Aravallis in India).
2. Fault-Block Mountains (Horst Mountains)- Developed as a result of tensional forces that caused blocks to be uplifted along fault planes.
Surrounded by normal faults.
Example: Sierra Nevada (USA), Vosges (France), Salt Range (Pakistan).
3. Volcanic Mountains- Composed of lava, ash, and volcanic debris deposited by accumulation.
Examples:
Active: Mount Etna (Italy), Mount St. Helens (USA).
Dormant: Mount Fuji (Japan), Mount Kilimanjaro (Tanzania).
Extinct: Mount Popa (Myanmar).
4. Residual Mountains – Composed by erosion of original mountains over time.
Resistant rock masses survive.
Example: Aravalli Hills (India), Black Forest (Germany), Nilgiri Hills (India).
Types of Plateaus
Plateaus are large flat uplands developed because of tectonic forces, volcanic activity, or erosion.
They are of the following types:
1. Intermontane Plateaus
Surrounded by mountain ranges and created as a result of uplift.
Highest and largest plateaus in the world.
Example: Tibetan Plateau (highest, “Roof of the World”), Colorado Plateau (USA).
2. Piedmont Plateaus
Situated at the foot of mountains, created by erosion.
Example: Patagonian Plateau (Argentina), Malwa Plateau (India).
3. Volcanic Plateaus
Created by accumulation of lava flow over a period of time.
Example: Deccan Plateau (India), Columbia Plateau (USA), North Island Volcanic Plateau (New Zealand).
4. Dissected Plateaus
Developed by erosion of pre-existing plateaus, forming valleys and hills.
Example: Scottish Highlands (UK), Chotanagpur Plateau (India).
5. Continental Plateaus
Developed by continental uplift without volcanic activity.
Example: Canadian Shield (Canada), Brazilian Plateau (Brazil).
Types of Plains
Plains are extensive, relatively flat regions developed due to deposition of sediments by rivers, wind, and glaciers. They are categorized as:
1. Structural Plains
Developed by the uplift of horizontal rock layers with little disturbance.
Example: Russian Plain (Eastern Europe), Great Plains (USA-Canada).
2. Depositional Plains
Developed by deposition of sediments due to rivers, glaciers, or wind.
Subdivided into:
- Alluvial Plains: Developed by rivers (e.g., Indo-Gangetic Plain, Mississippi Plain).
Lacustrine Plains: Developed by deposition due to lakes (e.g., Kashmir Valley, USA Great Basin).
- Glacial Plains: Developed by glacial deposits (e.g., Northern European Plain).
- Loess Plains: Developed by deposits due to wind-blown silt (e.g., Loess Plateau in China).
3. Erosional Plains
Developed by landform erosion, lowering them to low, rolling surfaces.
Example: Peneplains (low-lying eroded lands such as Western Australia’s Yilgarn Craton).
