Landforms of the Earth Chapter Notes | Science for Grade 7 PDF Download

Structure of the Earth

Crust (Lithosphere)

• Uppermost layer made of solid rocks.
• Thinnest layer, about 35 km thick on continents and 5 km on ocean floors.
• Continental crust contains silica and alumina, known as SIAL.
• Oceanic crust mainly consists of silica and magnesium, known as SIMA.

Mantle (Barysphere)

• Located below the crust, extends to a depth of 2,900 km.
• Divided into upper mantle and lower mantle.
• At 100–250 km depth, mantle is semi-molten, called asthenosphere.

Core

• Innermost layer with a radius of about 3,500 km.
• Made of nickel and iron, known as NIFE.
• Has very high temperature and pressure.
• Boundary between mantle and core is called Gutenberg Discontinuity.
• The discontinuity between the crust and the mantle is known as Mohorovicic Discontinuity. It forms the boundary between the crust and the mantle.

Isostasy

• State of gravitational balance between lithosphere and asthenosphere.
• Continental crust is less dense and extends deeper into asthenosphere than oceanic crust.
• Both continents and ocean basins float on semi-molten asthenosphere, maintaining balance.

Formation of Landforms

Plate Tectonics

• Lithosphere is divided into several plates floating on asthenosphere.
• Plates collide or push each other, causing landform formation.
• Orogenic movements form fold mountains.
• Sedimentary rocks between plates get compressed, leading to folding.
• Examples: Himalayas and Alps formed by plate collision.

Upliftment and Submergence

• Vertical movements (epeirogenic) cause land to rise or sink.
• Caused by faulting, forming block mountains, rift valleys, and basins.

Sudden Forces

• Volcanic activities and earthquakes are sudden forces.
• Known as constructive forces, they create relief features.
• Examples: Volcanic mountains, plateaus from volcanoes, lakes from earthquakes.

Exogenic Forces

• Occur on Earth's surface, caused by rivers, glaciers, and winds.
• Create landforms through erosion and deposition.
• Known as destructive forces due to weathering and erosion.

Various Types of Landforms

Mountains

• Formed by large-scale horizontal movements in Earth's crust.
• Caused by tectonic plates moving toward each other, folding Earth's layers.
• Upfolds are anticlines; downfolds are synclines.
• Examples: Himalayas (Asia), Andes (South America), Alps (Europe).
• Geosyncline: Sedimentary rock depression, like Tethys, where Himalayas formed.

Chief Characteristics of Fold Mountains

• Among the youngest mountains on Earth.
• Mostly made of sedimentary rocks from shallow ocean deposits.
• Longer in length but narrower in width.
• Associated with active volcanoes.
• Many islands, like West Indies, are tops of submerged fold mountains.

Block Mountains

• Formed due to faulting when tectonic plates move apart, causing cracks.
• Land between parallel faults rises, forming horsts.
  • Examples: Black Forest, Vosges (Rhineland).
• Also formed when crust sinks between faults, creating graben.
  • Example: East African Rift Valleys.

Formation of block mountains

Volcanic Mountains

• Formed by volcanic eruptions when magma erupts as lava.
• Lava accumulates and cools, building mountains.
• Examples: Mount Kilimanjaro (Africa), Mount Fuji (Japan).

Plateaus

• Elevated flat lands with a flat top called tableland.
• Four types: Intermontane, Piedmont, Volcanic, and Uplifted plateaus.

Intermontane Plateaus

• Surrounded by hills and mountains on all sides.
• Examples: Tibetan Plateau (world’s highest), Columbian Plateau.

Piedmont Plateaus

• Surrounded by mountains on one side and plains on the other.
• Also called residual plateaus.
• Examples: Patagonian Plateau, Appalachian Plateau.

Volcanic Plateaus

• Formed by lava flow from volcanic eruptions.
• Examples: Deccan Plateau, Ethiopian Plateau.

Uplifted Plateaus

• Formed when landmass center rises during volcanic activity with rounded sides.
• Examples: Chotanagpur Plateau, Ozark Plateau (USA).

Plains

• Large, flat lands with gentle undulating surfaces.
• Three types: structural, erosional, and depositional plains.

Structural Plains

• Formed by upliftment or submergence of land.
• Examples: Great Plains (USA), Coromandel Plains (India).

Erosional Plains

• Formed by erosion of hills, called peneplains.
• Also formed by wind and glacier erosion.
• Example: Niagara Plain (USA).

Depositional Plains

• Formed by deposition from rivers, winds, or marine activities.
• Examples: Great Indian Plains (Ganga), Mississippi Plains (USA).
• Wind-formed: Thar Desert, Sahara Desert.
• Marine deposition: East coast of India.

Valleys and Basins

• Valleys formed by tectonic or erosional forces.
• Example: Rift Valley (East Africa) by tectonic forces; Ganga-Brahmaputra Valley by erosion.
• Basins formed by river deposition.

Importance of Landforms

• Shape Earth's relief features and affect regional climates (e.g., Himalayas influence northern India’s climate).
• Snow-capped mountains are sources of perennial rivers.
• Mountain slopes have forests, supporting biodiversity.
• Plateaus provide various minerals.
• Volcanic plateaus have fertile black soil for farming.
• Plains with fertile soil support ancient civilizations and agriculture.
• Mountains, valleys have scenic spots attracting tourists.

Maps

Major plains in the world