Science Geography Class 7 Notes | CBSE Revision by YoLearn.ai

Our Dynamic Earth: Internal and External Forces

Our planet Earth is a constantly changing body, both from within and on its surface. These changes are driven by powerful forces that can be categorized into two main types: Endogenic (internal) and Exogenic (external) forces.

Endogenic Forces originate in the interior of the Earth. These forces are responsible for major geographical features and phenomena. Key endogenic forces include:

• Plate Tectonics: The Earth's lithosphere is broken into several large and small plates (lithospheric plates) that float over the molten magma in the asthenosphere. These plates are constantly moving, albeit very slowly, due to the convection currents within the mantle. The movement of these plates causes immense pressure, leading to phenomena like earthquakes, volcanoes, and the formation of mountains (e.g., fold mountains like the Himalayas).
• Earthquakes: Occur when lithospheric plates move, causing vibrations that travel through the Earth. The place in the crust where the movement starts is called the focus, and the place on the surface directly above the focus is the epicentre. The intensity of an earthquake is measured on the Richter scale.
• Volcanoes: Are vents in the Earth's crust through which molten material (magma), ashes, and gases erupt. The molten material that reaches the Earth's surface is called lava.

Exogenic Forces work on the surface of the Earth. These are primarily responsible for shaping the landscape through processes of degradation and aggradation. Key exogenic forces and processes include:

• Weathering: The breaking up of rocks on the Earth's surface due to exposure to elements like temperature changes, frost, wind, and plant roots. It's a static process.
• Erosion: The wearing away and transportation of rock and soil material by natural agents like water (rivers, glaciers), wind, and sea waves. Erosion constantly reshapes the land, creating features like V-shaped valleys, beaches, and deltas.
• Deposition: The process by which eroded material is laid down or dropped in new locations. This occurs when the carrying capacity of the agent (water, wind) decreases. Deposition leads to the formation of features like floodplains, levees, sand dunes, and moraines.

Understanding these internal and external forces is fundamental to comprehending how Earth's diverse landscapes are continuously formed and modified over geological timescales.

Key Geographical Terms & Definitions

Lithosphere

The solid outer layer of the Earth, including the crust and the uppermost part of the mantle, consisting of rocks and soil.

Hydrosphere

The domain of water, encompassing all forms of water on Earth: oceans, rivers, lakes, ice caps, groundwater, and water vapour in the atmosphere.

Atmosphere

The thin layer of air that surrounds the Earth, held by the planet's gravitational force. It protects us from harmful solar radiation and extreme temperatures.

Biosphere

The narrow zone of contact between land, water, and air where life exists. It includes all living organisms and their interactions.

Weathering

The breaking down of rocks on the Earth's surface into smaller fragments by physical, chemical, and biological processes.

Erosion

The wearing away of the Earth's surface by agents like water, wind, and ice, and the transportation of the eroded material.

Deposition

The process by which sediments and eroded materials are laid down or settled in new locations, typically when the transporting agent loses energy.

Epicentre

The point on the Earth's surface directly above the focus or origin of an earthquake.

Tsunami

A large ocean wave caused by sudden movements of the Earth's crust, often from underwater earthquakes, volcanic eruptions, or landslides.

Must Remember: Key Points for Revision

• Earth's interior has three main layers: Crust (outermost), Mantle (middle), and Core (innermost).
• The Earth's crust is made up of several large and small plates called lithospheric plates that are constantly moving.
• Endogenic forces (internal) cause earthquakes, volcanoes, and mountain building, while Exogenic forces (external) cause erosion and deposition.
• The three types of rocks are Igneous (formed from cooling magma/lava), Sedimentary (formed from compacted sediments), and Metamorphic (formed from existing rocks under heat and pressure).
• The atmosphere has five main layers: Troposphere, Stratosphere, Mesosphere, Thermosphere, and Exosphere.
• The water cycle describes the continuous movement of water on, above, and below the surface of the Earth.
• Factors affecting ocean currents include Earth's rotation, wind, temperature differences, and salinity.
• Major landforms created by rivers include waterfalls, floodplains, meanders, ox-bow lakes, and deltas.
• Glaciers are 'rivers of ice' that erode the landscape to form U-shaped valleys and deposit moraines.
• Wind is an active agent of erosion and deposition in deserts, forming sand dunes and mushroom rocks.

Weather vs. Climate: Key Differences

The Water Cycle (Hydrologic Cycle)

1. Evaporation — Water changes from liquid to gas (water vapour) due to heat from the sun, primarily from oceans, lakes, and rivers.
2. Transpiration — Plants release water vapour into the atmosphere through their leaves.
3. Condensation — Water vapour in the air cools and changes back into liquid water, forming clouds.
4. Precipitation — Water falls back to the Earth from clouds in the form of rain, snow, hail, or sleet.
5. Runoff & Collection — Precipitation flows over land as surface runoff, collecting in rivers, lakes, and oceans, or seeping into the ground to become groundwater.

Section 6

Diagram-based Questions: Many geography questions involve diagrams (e.g., Earth's layers, water cycle, types of landforms). Practice drawing and labeling these diagrams. For process questions (like the Water Cycle), ensure you describe each step clearly and sequentially. Use scientific terms correctly. Pay attention to keywords like 'describe', 'explain', 'differentiate', and 'identify' in the questions.

Quick Check: Test Your Knowledge

• Q: Name the three main layers of the Earth's interior. A: The three main layers are the Crust, Mantle, and Core.
• Q: What is the difference between weathering and erosion? A: Weathering is the breaking down of rocks in situ (in place), while erosion is the wearing away and transportation of weathered material by agents like wind or water.
• Q: List the five layers of the atmosphere in order from the Earth's surface upwards. A: Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.
• Q: What is an ox-bow lake and how is it formed? A: An ox-bow lake is a U-shaped lake formed when a wide meander of a river is cut off from the main river channel, typically during floods.

Frequently Asked Questions