our changing earth (grade 7)

OUR CHANGING EARTH

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STD VII

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CHANGES IN THE EARTH SURFACE

Mountains Volcano Mushroom Rock

Delta Sea Arches Sand Dunes

The changes on the Earth surface is due to two types of forces.

1. ENDOGENIC (Endogenous) FORCE

2. EXOGENIC (Exogenous) FORCE

ENDOGENIC (Endogenous) FORCES

It refers to the forces that occur beneath the surface of the Earth.

• Deep inside the earth’s core, heat is generated.• The heat is transferred upward to the mantle causing it to slowly circulate.• This movement of the lava applies force on the lithospheric plates above. These plates keep shifting and sliding at a very slow speed of a few mm in a year in different directions. The movement of these plates is called Tectonic Activity or Plate Tectonics.

ENDOGENIC (Endogenous) FORCE

ENDOGENIC (Endogenous) FORCE

Endogenic Force causes - Earthquake, Tsunami, Volcano and change in the earth’s landscape.

There are 20 lithospheric plates of different sizes. These are commonly known as Tectonic plates.

The earth's crust is cracked and the pieces of the Earth's crust formed by the cracks are called tectonic plates.The plates are continually moving (albeit very slowly, a few millimetres a year) and this movement helps to form volcanoes.

These tectonic plates can be seen in this map.

An earthquake is a sudden tremor or movement of the earth’s crust, which originates naturally at or below the

surface.

What causes earthquakes ? Sudden movement of tectonic plates.Earth surface is subjected to endogenic forces occurring from beneath the earth’s surface. This force makes the plates move at a very slow speed. However a sudden movement of tectonic plates triggers an earthquake. Seismometer or Seismograph records earth movements.

1. EARTHQUAKE

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Focus or HypocentreEARTHQUAKE – Focus & Epicentre

It is the point of origin of the earthquake. Sudden movement occur deep

below the crust or mantle causing tectonic plates to

move vigorously. Concentric shock waves

generated from this point determines the extent of

damage caused. (Since the Focus point is often deep below the surface, it is

difficult to map.)

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Epicentre• It is the point on the earth surface which is directly above the focus. • It is most commonly used to describe the point of concentration of an earthquake.The strength and magnitude of an earthquake is measured on the Richter Scale. An earthquake whose measurement is above 7 on the Richter Scale can cause major damages. Earthquakes are now measured using the Moment Magnitude Scale (or simply Magnitude scale). This measures the size of the seismic waves during the earthquake.

TsunamiWhen the epicentre of earthquake is in sea bed, it cause huge tidal waves which could have devastating effect on sea shores. Such phenomenon is known as Tsunami.

Tsunami

2. VOLCANO

A volcano is an opening, or rapture, in a

planet’s surface, which allows

hot, molten rock, ash and gases to

escape from below the surface.

Volcanoes are most commonly found along plate boundaries.

Mountain ranges & trenches –Slow movement of lithospheric plates causes change on the earth’s surface. Collision of plates

3. Change on the earth’s surface

creates mountain systems of earth like the Himalayas and huge trenches on the ocean floor.

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SUMMARY

ENDOGENIC (Endogenous) FORCE

1.It occurs beneath the surface of the Earth.

2.It causes natural devastation & physical change in the earth’s surface , like earthquake, volcanoes & formation of mountains or valleys.

EXOGENIC (Exogenous)FORCES

EXOGENIC (Exogenous) FORCES

It refers to the external phenomena that occur on or above the earth’s

surface.

The main elements of exogenic forces are –

AIR & WATER

Waterfall are often formed where a layer of harder rock overlays a layer of softer rock.

WATERFALL

Harder rock

Softer rock

As the river passes over the softer rock, it is able to erode it at a faster rate, forming a step in the river bed.

WATERFALL

Harder rock

Softer rock

The force of hydraulic action does two things:

Further erosion makes the plunge pool and notch bigger over time.

WATERFALL

Harder rock

Softer rock

As the notch grows, eventually there isn’t enough support under the harder rock and so it collapses into the plunge pool.

WATERFALL

Harder rock

Softer rock

WATERFALLThe processes of erosion continue, further eroding out the notch and plunge pool. Eventually the harder rock above will collapse again, meaning that the waterfall will retreat upstream over time.

As it retreats, it leaves behind a steep sided gorge.

Why do river curves

• Rivers cause change on the surface over which it flows.

RIVER

> Meanders – It is a snaking pattern of water stream formed by alternatively eroding sediments from the outside of a bend and depositing them on the inside.

> Ox-bow lake – An oxbow lake is a U-shaped body of water that forms when a wide meander from the main stem of a river is cut off, creating a free-standing body of water. This landform is so named for its distinctive curved shape, resembling the bow pin of an oxbow.

bow pin of an oxbow

Floodplain – A floodplain is a generally flat area of land next to a river or stream. It stretches from the banks of the river to the outer edges of the valley. Flood plains are natural flooding outlets for rivers.

Agriculture remains a critical industry for flood plains today. The silty sediment is rich in nutrients that support plant growth. (This farmer is harvesting on the fertile flood plain of the Omo River in Ethiopia)

As it takes more energy to carry large particles, these are deposited first and therefore build up along the banks of the river to form a natural embankment which are called levees. The levee will become higher every time the river floods.

> Levees - In times of flood a river may overflow its banks and spread over the flood plain. As it does so it loses energy and deposits its material across the flood plain.

Levees

DeltaDeltas are formed at the mouth of a river where the river flows into the ocean or other water body. When a river carrying sediments reaches a body of standing water, it reduces its velocity causing it to deposits the sediments at the mouth. Over time these sediments builds up to rise above the surface of water. The river then cuts across the land and forms different branches. These branches are called distributaries.

Bay of Bengal Delta

EXOGENIC (Exogenous) FORCES

Sea caves – continuous wave action of the sea erodes the rocky faces on the sea shore to form caves. Sea arches – over a period of time a sea cave convert into sea arch with increased erosion in the rock. Stack – Stacks are formed when part of a main landform is eroded by water action, like water crashing against the rock. Sea Beaches – Sea waves leave behind sediments in the form of fine sand at these places are called beaches.

Sea waves also create various changes in landforms.

GLACIER

• Glacier are accumulation of snow that are formed in areas where the amount of snow that falls exceeds the amount that melts. • Glacier form distinct landforms that looks like frozen river moving slowly over some land surface or down a valley.

Glacial Moraines

Moraine is sediment deposited by a glacier.

Wind flowing over a long period create different forms.

AIR (WIND)

Mushroom Rocks – Usually found in deserts areas. The wind blowing at high speed takes with it sand particles. The particles being heavier remain close to the surface and as wind passes along the rocks the sand particles erodes the portion of rock closer to the surface without affecting the upper portion. This gives the rocks a mushroom like shape.

Wind can create landforms when it deposits its sediments, especially in

deserts and along coasts.

Sand dunes & Loess are landscape features deposited by wind.

Sand Dunes –A dune is a mound of sand formed by the wind, usually along the beach or in a desert. Dunes form when wind blows sand into a sheltered area behind an obstacle. Dunes grow as grains of sand accumulate. Every dune has a windward side and a slipface.

Loess – It is a sedimentary deposit of mineral particles which are finer than sand but coarser than dust or clay, deposited by the wind. Loess is a type of silt which forms fertile topsoil in some parts of the world. Loess deposits are usually a few meters thick. The soil has few clay particles to hold it together.

EXOGENIC (Exogenous) FORCE

1.It occurs above the surface of the Earth.

2.It causes physical change in the earth’s surface , like Sea arches, mushroom rock, sea shore, sand dunes, etc.

ENDOGENIC (Endogenous) FORCE

1.It occurs beneath the surface of the Earth.

2.It causes natural devastation & physical change in the earth’s surface , like earthquake, volcanoes & formation of mountains or valleys.

Question & Answer

1. What cause Tsunami ?2. List two causes of earthquakes.3. What is volcano?4. What are exogenic forces?5. Where and where is ox-bow lake formed?6. How are sea arches formed?7. How are sand dunes formed?

Give Reason.1. Formation of Mushroom rocks in deserts.2. Earth surface is not a consistent state.

Answer the following in detail.3. How is delta formed? Draw a neat and labelled diagram of delta.4. Explain the depositional work of glacier.5. Explain the erosional and depositional work of wind.6. How are sea caves and arches formed?

1. What cause Tsunami ?A tsunami is a large ocean wave that is caused by sudden motion on the ocean floor. This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide.

2. List two causes of earthquakes.An Earthquake is a sudden tremor or movement of the earth's crust, which originates naturally at or below the surface.There are two main causes of earthquakes.1. They can be linked to explosive volcanic eruptions; they are in fact very common in areas of volcanic activity where they either proceed or accompany eruptions.2. They can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes world wide are of this type.

Question & Answer

3. What is volcano?4. What are exogenic forces?5. Where and where is ox-bow lake formed?

This is formed when a wide meander from the main stem of a river is cut off to create a lake. This is called ox-bow lake for its distinctive curved shape that results from this process.

6. How are sea arches formed?This is formed by the process of erosion

7. How are sand dunes formed?A dune is a mound of sand formed by the wind, usually along the beach or in a desert. Dunes form when wind blows sand into a sheltered area behind an obstacle. Dunes grow as sediments of sand accumulate.

Question & Answer

Answer the following in detail.1. How is delta formed? Draw a neat and labelled diagram of

delta.Deltas are formed at the mouth of a river where the river flows into the ocean or other water body. When a river carrying sediments reaches a body of standing water, it reduces its velocity causing it to deposits the sediments at the mouth. Over time these sediments builds up to rise above the surface of water. The river then cuts across the land and forms different branches. These branches are called distributaries.

2. Explain the depositional work of glacier.Like flowing water, flowing ice carry and deposits the rocks and sediments. Rock and sediments freeze and stick to the bottom of the glacier and get carried away by the flowing ice. Glaciers deposit their sediment when they melt.

Question & Answer

Home Work1. Explain the erosional and depositional work of wind.

2. Earth surface is not a consistent state. Explain

Give Reason.1. Formation of Mushroom rocks in deserts.

2. Agriculture remains a critical industry for flood plains today.

Home Work

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