Our Changing Earth Class 7 Notes Geography Chapter 3
Our Changing Earth Class 7 Notes Geography Chapter 3
Overview
Lithospheric plates, also known as tectonic plates, are large rigid pieces of the Earth's lithosphere that fit together like a puzzle to form the Earth's surface. The theory of plate tectonics explains how the lithospheric plates move and interact with each other. There are seven major plates: the African Plate, Antarctic Plate, Eurasian Plate, North American Plate, South American Plate, Australian Plate, and Pacific Plate. Additionally, there are numerous smaller plates, such as the Arabian Plate, Caribbean Plate, Indian Plate, and Nazca Plate.
Endogenic forces and exogenic forces are two types of geological forces that shape and modify the Earth's surface. Here's an explanation of each:
Endogenic Forces: Endogenic forces are internal forces that originate from within the Earth. They are responsible for the formation of landforms by creating movements and deformations in the Earth's crust. Endogenic forces can be categorized into two main types:
a. Tectonic Forces: Tectonic forces result from the movement and interaction of lithospheric plates. These forces include:
Folding: When rock layers bend and buckle due to compression forces, forming fold mountains or anticlines and synclines.
Faulting: When there is a break or fracture in the Earth's crust, resulting in the displacement of rock layers along a fault line.
Earthquakes: Sudden release of energy along fault lines, causing ground shaking and tremors.
Volcanism: The eruption of molten magma onto the Earth's surface, leading to the formation of volcanoes and volcanic landforms.
b. Volcanic Forces: These forces specifically pertain to volcanic activity and include the movement of magma and the eruption of lava, gases, and pyroclastic materials.
Exogenic Forces: Exogenic forces are external forces that act upon the Earth's surface. They are primarily driven by atmospheric processes, such as weathering, erosion, and deposition. Exogenic forces include:
a. Weathering: The breakdown and alteration of rocks and minerals at or near the Earth's surface due to exposure to weather conditions, such as temperature changes, water, wind, and biological activity.
b. Erosion: The removal and transportation of weathered materials by agents such as water (rivers and streams), wind, glaciers, and waves.
c. Deposition: The settling and accumulation of eroded materials in new locations, resulting in the formation of sedimentary deposits and landforms like deltas, beaches, and alluvial plains.
d. Mass Movements: The downslope movement of rocks, soil, and debris under the influence of gravity, including processes like landslides, mudflows, and creep.
Major Landforms
The landscape is being continuously worn away by two processes – weathering
and erosion.
Weathering is the process of breaking down and altering rocks and minerals at or near the Earth's surface due to exposure to various weather conditions and environmental factors. It can be classified into three main types: mechanical, chemical, and biological. Mechanical weathering involves the physical breakdown of rocks into smaller fragments without changing their chemical composition. Chemical weathering involves the chemical alteration and decomposition of rocks through various chemical reactions. Biological weathering refers to the weathering processes that are influenced by the activities of living organisms.
Erosion is the process of removing and transporting weathered materials from one location to another. Erosion is caused by natural agents such as water, wind, ice (glaciers), and gravity. Water erosion occurs when flowing water dislodges and carries away soil and sediment, while wind erosion involves the lifting and transportation of small particles. Glacial erosion occurs when glaciers move and scrape the land, picking up rocks and soil, while gravity erosion refers to the downward movement of soil, rocks, and debris under the influence of gravity. This can include processes such as landslides, rockfalls, and slumping.
Work of a River
Rivers play a significant role in shaping and modifying the Earth's surface through various processes. Erosion: Rivers have the power to erode the land through the force of flowing water. Hydraulic action: The force of water itself can erode and dislodge rock and sediment. Abrasion: Rivers can carry and transport sediment, such as rocks and pebbles, which act as tools to physically wear away the riverbed and banks. Corrosion (or solution): The chemical action of the water can dissolve certain types of rocks and minerals.
Transportation: Rivers transport eroded materials, including rocks, sediment, and dissolved substances, downstream. Bedload: The heavier materials, such as pebbles, gravel, and larger rocks, are transported along the riverbed through rolling, sliding, or bouncing. Rivers can form a variety of landforms through erosion and deposition, such as river valleys, waterfalls and rapids, meanders, oxbow lakes, river deltas, floodplains, alluvial fans, and meanders. These landforms can be formed over time, such as V-shaped valleys, waterfalls and rapids, meanders, oxbow lakes, river deltas, floodplains, alluvial fans, and meanders. Deposition occurs when the velocity of the river decreases, and the deposited materials settle on the riverbed or accumulate in other areas, forming landforms known as depositional features. Over time, rivers can create a variety of landforms through erosion and deposition, such as river valleys, waterfalls and rapids, meanders, oxbow lakes, river deltas, floodplains, alluvial fans, and meanders.
Work of Sea Waves
Sea caves: Sea waves continually contact the rocks, causing fractures to emerge, and they erode and deposit coastal landforms. Over time, the fissures enlarge and widen. Rocks on the surface develop hollow-like caves as a result. They're known as sea caves.
Sea arches are created when these cave-like openings are so enormous that only the cave roof is left.
Stacks: When erosion destroys the roof, only walls remain; these wall-like structures are referred to as stacks.
Sea cliffs are steep, rocky coastlines that rise nearly vertically above the ocean.
Work of Ice
Glaciers: Referred to as "rivers of ice," glaciers erode the earth by removing rocks and soil to reveal the solid rock beneath. There, glaciers cut quite substantial hollows. The mountains become magnificent lakes as the ice melts and is filled with water.
Glacial moraines are formed when material carried by a glacier, such as large and small rocks, sand, and silt, is deposited.
Work of wind
Wind: In the desert, wind is a significant contributor to erosion and deposition.
Rocks that resemble mushrooms can be found in deserts and are known as "mushroom rocks."
Sand dunes: Sand is lifted and moved from one location to another by the wind. Sand falls and is deposited in low, hilllike structures after the wind ceases. We refer to these as sand dunes.
Loess: When sand particles are small and light, they can be transported across great distances by the wind. Loess is the term used when such sand is spread out across a vast area.
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