Chapter 3: Our Changing Earth

Earth Movements and Forces

The Earth's surface is not static; it is in a state of constant change. These changes are driven by the movement of the Lithospheric plates. The lithosphere, which is the Earth's solid crust, is not a single piece but is broken into a number of large and small, irregularly shaped plates. These plates carry both the continents and the ocean floor.

These plates move incredibly slowly, only a few millimeters each year. This movement is caused by the circulation of molten magma within the mantle. As the magma heats up, it rises, and as it cools, it sinks, creating circular convection currents that drag the plates along.

Endogenic and Exogenic Forces

The movements of the Earth are classified based on the forces that cause them:

• Endogenic Forces: The word Endo means 'inside' and genic means 'origin'. These are forces that originate and act in the interior of the Earth. They are responsible for large-scale changes on the surface.
  • Sudden Forces: These cause rapid and often destructive events like earthquakes and volcanoes.
  • Diastrophic Forces: These are slow movements that build major landforms like mountains over millions of years.
• Exogenic Forces: The word Exo means 'outside'. These are forces that work on the surface of the Earth. They are responsible for wearing down the landscape and creating new features through erosion and deposition. Examples include the actions of river water, wind, sea waves, and glaciers.

Volcanoes and Earthquakes: Sudden Forces

Sudden endogenic forces can cause mass destruction over the Earth's surface in a very short time. The two most prominent examples are volcanoes and earthquakes.

Volcanoes

A volcano is a vent or opening in the Earth's crust. Through this opening, molten rock (magma), gases, and ash from the Earth's interior erupt suddenly onto the surface. The molten rock that flows out is called lava. This material builds up around the vent, often forming a cone-shaped mountain.

Earthquakes

An earthquake is the shaking of the Earth's surface caused by the sudden movement of the Lithospheric plates. The point inside the Earth's crust where the movement originates is called the focus. The point on the surface directly above the focus is called the epicentre. The vibrations, or seismic waves, travel outwards from the focus in all directions. The greatest damage typically occurs at the epicentre, and the strength of the earthquake decreases as the distance from the epicentre increases.

Earthquakes are measured using a machine called a seismograph. The magnitude of the earthquake is recorded on the Richter scale. An earthquake of 2.0 is barely felt, while one over 5.0 can cause damage, and anything 7.0 or higher is considered a major earthquake.

There are three main types of seismic waves:

• P waves (Primary waves): These are the fastest waves and travel through solids, liquids, and gases. They are longitudinal, compressing and expanding the ground.
• S waves (Secondary waves): These are slower and can only travel through solids. They are transverse, moving the ground up and down or side to side.
• L waves (Surface waves): These are the slowest waves and travel along the Earth's surface. They cause the most destruction.

Earthquake Preparedness

Although earthquakes cannot be predicted with certainty, their impact can be minimized with proper preparation and awareness. Some traditional methods of prediction include observing animal behavior, like fish getting agitated or snakes coming to the surface.

During an earthquake, it is vital to know where to take shelter:

Safe Spots:

• Under a kitchen counter, sturdy table, or desk.
• Against an inside corner or wall of a building.

Stay Away From:

• Fireplaces and areas around chimneys.
• Windows, mirrors, and picture frames that can shatter.
• Heavy objects that can fall.

Spreading awareness among family and friends is key to facing any disaster confidently.

Major Landforms

The Earth's landscape is continuously being shaped and reshaped by the two primary exogenic processes: weathering and erosion.

• Weathering: This is the process of breaking down rocks on the Earth's surface into smaller pieces. It can be caused by temperature changes, frost, plants, and chemical reactions, but it does not involve the movement of material.
• Erosion: This is the wearing away and transportation of the landscape by natural agents like moving water, wind, and ice. The eroded material (sediment) is carried away and eventually deposited elsewhere, creating new landforms.

The following sections explore the landforms created by the main agents of erosion and deposition.

Work of a River

Flowing river water is a powerful agent of erosion and deposition, carving out and building up landforms along its course from the mountains to the sea.

• Waterfall: When a river flows over a layer of hard rock onto softer rock below, the softer rock erodes more quickly. This creates a steep drop, forming a waterfall. The highest waterfall in the world is Angel Falls in Venezuela.
• Meanders and Ox-bow Lakes: As a river enters a plain, it slows down and begins to twist and turn, forming large loops called meanders. Continuous erosion on the outer banks and deposition on the inner banks cause the meander loops to become more pronounced. Eventually, a loop may become so tight that the river cuts a new, straighter channel, abandoning the old loop. This cut-off loop forms a crescent-shaped lake called an ox-bow lake.
• Floodplains and Levees: During floods, a river overflows its banks and deposits fine soil and sediment over the adjacent areas. This process creates a flat, fertile area known as a floodplain. The raised banks of deposited sediment along the river's edge are called levees.
• Delta: As a river approaches the sea, its flow becomes very slow. It breaks up into a network of smaller streams called distributaries. The river deposits its load of sediment at the mouth of each distributary, building up a triangular, fan-shaped landform called a delta.

Work of Sea Waves

The constant action of sea waves erodes and deposits material along coastlines, creating distinctive coastal landforms.

• Sea Caves, Arches, and Stacks: Waves continuously strike rocky coasts, finding and widening cracks in the rock. Over time, this forms hollow, cave-like structures called sea caves. As erosion continues, these cavities may cut all the way through the rock, leaving only a roof and forming a sea arch. Eventually, the roof of the arch collapses, leaving only a pillar of rock standing in the sea. This is called a stack.
• Sea Cliff: A steep, rocky coast that rises almost vertically above the sea.
• Beaches: Formed when sea waves deposit sediments like sand and gravel along the shore.

Work of Ice

Glaciers, which are vast, slow-moving "rivers of ice," are powerful agents of erosion. They bulldoze soil and stone, carving deep hollows into the landscape. When the ice melts, these hollows can fill with water, forming beautiful mountain lakes. The material carried by the glacier, such as rocks, sand, and silt, gets deposited at its end or sides. These deposits are known as glacial moraines.

Work of Wind

In deserts, wind is the dominant agent of erosion and deposition.

• Mushroom Rocks: Wind erodes the lower part of a rock more than the upper part by carrying sand particles that act like an abrasive. This results in rocks with a narrow base and a wide top, resembling a mushroom.
• Sand Dunes: When the wind blows, it lifts and transports sand. When the wind stops or slows down, the sand is deposited in low, hill-like structures called sand dunes.
• Loess: When sand grains are very fine and light, the wind can carry them over very long distances. When this fine sand is deposited in large areas, it forms a fertile deposit called loess. Large deposits of loess are found in China.