ISOSTACY THEORY – ADVANCED PHYSICAL GEOGRAPHY
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 studying gravitational pull of the 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
There are two principals’ kinds of movement in Isostacy disturbance and adjustment. These are: =
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
2. 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.
3. 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.
4. 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 presentday 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.
ANALOGY OF ISOSTACY THEORY
There are several things may be the occurrence of Isostacy theory in our local life, like as follow;
1. A floated iceberg
It always floats with a certain proportional of its mass below the surface of the water. The portion below the surface of water is compensation portion. If a layer of ice is somehow sliced off on the top of the iceberg, the remaining iceberg will rise. Similarly, the earth’s lithosphere floats in the asthenosphere as the iceberg floats on water.
2. A floated piece of wood
The series of wooden blocks of different heights floating in water, the taller one will remain tall and sink down more than a shorter one. The wooden blocks emerge by amounts which are proportional to their respective heights. The submerged pieces of wood are proportional to their respective emerged parts.
3. Loading and unloading ship
Floating ships show how Isostacy affects the behavior of the crust as it floats on the heavy mantle rock. For instance, when cargo is transferred from ship A to ship B, ship A rises and ship B sinks. Therefore, these processes of loading and unloading of the cargo is like a isostatic disturbance associated by erosional process as result of unloading and depositional process of sediments as result of loading.
EFFECTS OF ISOSTACY THEORY
The theory of Isostacy has the following effects;
i. Formation of faulting process and features like rift valley, block mountain, etc, its occur when subsidence of land cause faults, etc.
ii. Formation of Fold Mountain and other following features like Geosyncline, etc. intensive compressional force.
iii. Occurrence of earth quakes: Associated with faulting and down ward movement process.
iv. Formation of volcanic process and their features like volcanic mountain, and lava plateau,
v. Displacement of lithosphere leading to drifting movement and formation of new oceanic crust (formation of sea floor spreading).
vi. The theory is very much linked to the understanding of rock cycle concept.
QN: Examine Landforms (features) associated with Isostacy theory.
Landforms associated with Isostacy includes;
i) Rift valley due to faulting
ii) Block mountains due to faulting
iii) Volcanic mountains due to faulting
iv) Fold mountains due to folding (intensive compression force)
v) Formation of lava plateau due to volcanic eruptions
vi) Escarpments/fault scarps due to faulting
SIGNIFICANCE OF ISOSTACY THEORY
The following are the significance of Isostacy theory. These are: –
1. It gives an understanding on the earth’s crust dynamic state as it is influenced by internal and external forces.
2. It helps in understanding of plate tectonic and continental drift theory. Because the floating of lithospheric plates on lithosphere like floating of iceberg on the ocean water.
3. It helps to takes the precaution depending of a area especially in the occurrence of ice sheet and ice melting.
4. It provides the basis for predicting the future of the crustal state at any place on the surface of the earth.
5. It helps to understanding how different landforms were formed and mechanism involved in their formation.
WEAKNESS OF ISOSTACY THEORY:
It failed to explain the formation of inland and undersea mountain or presence of volcanic Island.
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