TOPIC 2: THE DYNAMIC EARTH AND CONSEQUENCE - PHYSICAL GEOGRAPHY FORM 5 & 6
The following are the subtopics;
i. Isostacy
ii. Plate tectonic and drifting
iii. Materials of Earth's crust
iv. The impact of Earth quake
v. Vulcanism
vi. Denudation and deposition
THE EARTH
The Earth is the third planet in distance from the sun and the fifth largest; The Earth is the only planet that supports life. It is distance from the Sun is approximately 149,503,000 kilometers.
CHARACTERISTICS OF THE EARTH
The Earth has the following characteristics
1. It is spherical in shape or flattened sphere (oblate spheroid)
2. It is diameter from the North Pole through the South Pole is 12,713 kilometers.
3. It is diameter from the east-west is 12,757 kilometers (at the equator).
4. The circumference of the Earth is approximately 40,000 kilometers.
5. The Earth rotates from west to east.
THEORY OF EQUILIBRIUM AND EARTH GRAVITATIONAL FORCE
The earth has got a gravitational force which pulls thing towards the crust.
The gravitational influence very much the density as well as the weight of the materials such as materials and rocks in each zone of the earth interior.
The gravitational forces influence a balance restoration of the earth crust when there are forces leading to its destruction of such balance.
Therefore, detail study of earth gravitational force of equilibrium maintenance between its materials is called ISOSTACY.
ISOSTACY THEORY
The word Isostacy was derived from two (2) Greek words i.e "Iso" which means "equal" and "Stacy" which means "balance".
Therefore, in Geography, Isostacy is the theory describing the balance that exist between the weight of mantle rocks and crustal rocks.
This theory was developed in 1950's by various scholars who worked independently example George Everest, Dutton, and George Airy.
These scientists investigated by studied gravitational pull earth in various place of world.
This theory of Isostacy state that "the weight of crustal rocks and mantle rocks occurs in equilibrium similar between the balance of lithosphere and asthenosphere".
Mountains are therefore being similar structure projections penetrating deeply into the mantle
like a tooth in the gum. The less dense crust float on the denser mantle floating on the asthenosphere.
ISOSTATIC DISTURBANCE AND ISOSTATIC ADJUSTMENT
What is the Isostatic disturbance?
Isostatic disturbance refers to all the process in the balance between the crust and the mantle is destroyed by loading of materials by reducing the weight in balance.
For example, when there is long term of denudation on the crust, there is unloading process and there is deposition of sediments, glaciation or even lava accumulation there is loading process. Both loading and unloading processes disturb the Isostatic balance between the crust and mantle.
What is the Isostatic adjustment?
Isostatic adjustment refers to the process of adjusting, restoration or recovering of the equilibrium caused by either loading or unloading process.
The isostatic adjustment operates when is loading process example deposition along river delta, sinking movement occur which push the crust down to the asthenosphere.
In the asthenosphere horizontal movement takes place from loaded areas to the area where erosion (unloading) has occurred. Here uplifting takes place to restore the balance.
Isostatic adjustment occurs slowly over 1000 of years but maintain the balance between crust and mantle.
TYPES OF MOVEMENT IN ISOSTATIC ADJUSTMENT AND DISTURBANCE
1. Vertical movement:
It involves upward and downward movements of materials which enhance isostatic adjustment.
Its also known as the movement of weight. Its subdivided into two subtypes; Sinking movement and Uplifting movement
a. Sinking movement (down movement): It occur during loading process where weight is added on the crust, caused by deposition of sediment, lava accumulation due to volcanic eruption, accumulation of snow and ice resulted by glaciation, etc.
b. Uplifting movement (Up warping/upward movement): It occur due to decrease of weight of continental rock resulted by erosion and ice melting process on the earth surface. It is recovery movement in which the balance is restored.
2. Horizontal movement:
It also referred as the movement of heat accumulation. This take place in the asthenosphere where molten materials (lava) from place of loading where weight has been added to the place of unloading where weight has been reduced.
EVIDENCE OF ISOSTACY THEORY
The validity of Isostacy theory is demonstrated by the following evidences: -
1. Depression formation
The deposition (depression) of earth crust in the Northern part of America and Europe due to over weight of ice sheet during the ice age periods. Now these areas are uplifting followed of ice due to global rise of temperature. For example, in Northern and of the Gulf of Bothnia rises at the rate of about 1 meter per year.
2. Land uplift in Canada
Is another evidence in which Isostatic rebound has also occurred indifferent areas such as Eastern Canada and Southern Ontario where the land has risen as much as 100m during the last 6000 years, due to glacier melted because of the Isostatic adjustment.
3. Existence of mountain ranges
Despite the continuous process of erosion, several mountains have maintained their heights. Example Mount Kilimanjaro.
4. Existence of water bodies and depression areas
Despite the continuous deposition activity with large water bodies like ocean and lakes, still has not disappeared.
5. Presence of raised beaches in Scandinavia
This due to uplift of land resulted by isostatic adjustment. Scandinavia which now lies from 8m to 30m above the present- day beaches.
6. The depth of continental shelf
The presence deeper Continental shelf around the Antarctica continent than other continents, it covered by sea water to a depth of about 750m below the sea level due to isostatic disturbance resulted by overweight associated by ice sheet. This is contrary to 180m depth around the other continents.
7. Submergence of forest on the shore of British; etc.
TOPIC 2: THE DYNAMIC EARTH AND CONSEQUENCE - PHYSICAL GEOGRAPHY FORM 5 & 6