UNIT 2: RELIEF

1.      THE HISTORY OF HE EARTH

THE EARTH’S SURFACE

The Earth’s surface is made up of water and land:

Oceans are bodies of saltwater that cover 71 per cent of the Earth’s surface. There is submerged land beneath the oceans.

Continents are large areas of land that cover 29 per cent of the Earth’s surface.

We classify material that makes up the Earth’s surface in different zones:

- The lithosphere is the solid, rocky covering of the Earth’s surface.

- The hydrosphere is all the water on the Earth, including oceans, seas, lakes, rivers and underground water.

- The atmosphere is a layer of gases, including nitrogen and oxygen, that surrounds the Earth.

- The biosphere is the thin layer of atmosphere, earth and water where life exists.

THE SCIENCE OF GEOLOGY

The science of geology is the study of all the materials that make up the solid part of the Earth. Geologists examine rocks to find evidence of all the changes that have occurred. They also study fossils, which are the remains of living things preserved in the rocks. Both rocks and fossils are found in layers called strata. These are formed of sedimentary rocks, which gradually accumulate at the Earth’s surface. The oldest materials are found in the lowest strata.

GEOLOGICAL AGES

The Earth’s surface has changed very slowly. Its present-day relief was formed over many millions of years.

1.      Archaic Age. Water originally covered all the Earth. Land surfaced and then became submerged again repeatedly. The earliest plants grew in the sea. Fish probably appeared about 500 million years ago.

2.      Primary Age. About 200 million years ago all the Earth’s land masses were joined together in a single supercontinent called Pangea.

3.      Secondary Age. Pangea gradually broke up into the different continents that we know today. Dinosaurs lived for about 170 million years until they died out around 65 million years ago.

4.      Tertiary Age. The continents continued to form between 65 and 1.6 million years ago. New mountain chains appeared. There were mammals, including the very early ancestors of human beings. Hominids lived in Africa over 2 million years ago.

5. Quaternary Age. For most of the last two million years ice has covered much of the Northern Hemisphere. This has shaped the Earth’s surface. Modern human beings have only existed for about 200,000 years.

Unit 2: Relief

2.      THE EARTH’S STRUCTURE

The Earth has three layers:

- The core is the inner part of the Earth. It is divided into two parts:

‣ -The inner core is extremely hot. It is solid because of the great pressure.

‣ -The outer core is made up of extremely hot liquid rock.

- The mantle surrounds the core. Its temperature is so high that materials can melt and form a hot liquid called magma. This magma is sometimes pushed up to the Earth’s surface and can cause volcanic eruptions. When magma surfaces it is called lava.

- The crust is the solid, outer part of the Earth’s surface. We live on the Earth’s crust. We can distinguish two parts:

‣ The continental crust is under the continents and is 30 to 70 kilometres deep.

‣ The oceanic crust is under the oceans and is about 10 kilometres deep.

CONTINENTAL DRIFT AND TECTONIC PLATES

The single supercontinent that scientists call Pangea existed about 200 million years ago. Over time, it broke up into separate continents that gradually moved apart. This theory is called continental drift. According to the theory, modern continents are like pieces of a huge jigsaw puzzle that once fit together.

The Earth’s crust is made up of tectonic plates that move continually. There are seven or eight major plates and several minor ones. These plates slide against each other or move apart. The boundaries between the tectonic plates are very unstable. When pressure builds up under the Earth’s surface it causes earthquakes and volcanic eruptions along these boundaries.

3.      INTERNAL FORCES OF RELIEF

Relief is the shape of the Earth’s surface. It is caused by internal forces that originate deep inside the Earth. These forces may move, raise or sink the land on the Earth’s surface. They act through continual pressures and through earthquakes and volcanoes.

PRESSURES ON THE EARTH’S SURFACE

There are continual pressures on the Earth’s surface near the boundaries of tectonic plates. Folds and faults are created as a result of these pressures:

- Folds are deformations of the Earth’s surface where rock layers bend.An anticline is created when rock layers fold upwards, and a syncline is when they fold downwards.

- Faults are breaks in rock layers where the rock is too hard to bend. The hanging wall is land that is pushed up, while lower land is called the footwall.

Unit 2: Relief

EARTHQUAKES

Earthquakes are caused when the Earth’s crust is shaken. Earthquakes generally occur along the boundaries of tectonic plates, where the plates slide against each other. Energy is released in seismic waves from a focus, or hypocentre, inside the Earth. The epicentre is the point on the Earth’s surface that is directly above the focus. This is where the earthquake is at its strongest. Many earthquakes are light tremors, which are imperceptible to human beings. Only special instruments called seismographs can detect them. However, other earthquakes are extremely destructive. Earthquakes on the ocean floor cause tsunamis, which are huge tidal waves that cause great destruction in coastal areas.

VOLCANOES

A volcano is a crack in the Earth’s crust. Hot liquid rock, called magma, rises to the surface when it erupts. Rocks, ash and gases are ejected through its main channel, or vent, and material is deposited all around the crater. During the eruption, hot liquid lava flows out before it cools and becomes solid. Together with solid rocks, it accumulates around the crater and forms a cone. The cone grows over a long period of time, and may become a mountain. Lava flow and ash clouds from volcanic eruptions can cause great damage.

4.      EXTERNAL AGENTS OF RELIEF

Relief is also shaped by external agents on the Earth’s surface.

EXTERNAL AGENTS

Agents such as water, wind, changes of temperature and human action all modify relief.

- Water plays a very important role in erosion, transport and deposition.

- Wind. Aeolian erosion is when wind erodes and transports rocks and sand. This type of erosion often occurs in dry climates, such as deserts, where there is little vegetation to protect the soil.

EROSION CYCLE

The external agents modify relief by three processes:

- Erosion: materials, such as rocks and soil, are broken up and moved around by external agents.

- Transportation: these materials are then transported by seas, rivers, ice or wind.

- Deposition: eroded rocks, sand and mud are deposited in low areas where the sediments accumulate.

OTHER EXTERNAL AGENTS

- Ice causes glacial erosion in very cold regions and in mountainous areas. Glaciers are huge masses of ice. When water enters the cracks in a glacier it freezes and breaks off pieces of rock, and they are transported by the glacier. The ice from glaciers moves very slowly like a big river. When glaciers melt, they create lakes and U-shaped valleys.

- Sudden changes of temperature can break up rocks. This happens in deserts and some mountainous areas where it is much hotter during the day than at nighttime.

Unit 2: Relief

Water sometimes enters the cracks in rocks. It may break the rocks if it freezes and expands.

- Human action also modifies relief through activities such as agriculture, deforestation and mining. Roads and reservoirs are man-made features of the Earth’s relief.

5.      TYPES OF RELIEF

CONTINENTAL RELIEF

- Mountains are very high elevations of the Earth’s surface. They are sometimes grouped together in mountain ranges.

- Plateaus are high flat areas that are formed from eroded mountains.

- Plains are low flat areas. The sediments that have been deposited on plains often make them fertile for agriculture. Alluvial plains are formed by rivers. Coastal plains are near the sea.

- Basins are very low areas. Some basins are below sea level.

RELIEF ON THE OCENAN FLOOR

- The continental shelf is a great underwater plateau that surrounds a continent, and is usually less than 200 metres deep.

- The abyssal plains are huge underwater plains that are generally more than 4,000 metres deep. Low trenches on these plains are like cracks in the Earth’s surface. The deepest trench, in the Pacific Ocean, reaches a depth of 11,000 metres. There are also ocean ridges on the ocean floor, which are similar to mountain ridges.

- The continental slope is the sloping terrain that connects the continental shelf to the abyssal plains.

TOPOGRAPHIC PROFILE

This is a representation of all the different altitudes along a line drawn on a topographic map. It is like a view of the Earth’s surface from the side.

Unit 2: Relief