The Dynamic-Earth And Consequence

THEORIES

THEORY OF ISOSTASY

Denudation has been going on the continents where tons and tons of materials are removed from mountains and hills and get deposited in ocean; but the hill of mountains are not reduced to the sea level.

WHY THIS IS SO?

The phenomena can be explained by the theory of Isostasy.

Origin of the term is Greek; Made of 2 words meaning ‘Equal standing’ is a state of equilibrium or balance in the earth’s crust with equal mass underline equal surface area. Therefore there is a state of equilibrium.

The theory suggest that the continents and their major features are maintained in a sort of equilibrium or are moving towards that equilibrium.

ISOSTATIC EQUILIBRIUM

Isostatic equilibrium can be disturbed

How? Processes on the surface of the continents.

Denudation

Weathering/mass wasting, transportation, and erosion.e.g    removal of material which lower the surface.

Deposition -Building up process (rising of the land)

Accumulation of ice masses and melting of ice masses.

For example

When denudation removes the material from mountains there is pressure released, uplift occurs where the material is removed and there is no Isostatic balance.

If the material is deposited on the sea bed there is compression which result in sinking (vertical movements) these vertical movements causes horizontal movements of simatic material from there is compression has taken place to where there was pressure release

In such process a state of equilibrium is restored.

EFFECT OF DISTURBANCE AT SURFACE AND READJUSTMENT OF ISOSTATIC
EQUILIBRIUM

EFFECTS OF DISTURBANCE OF ISOSTATIC EQUILIBRIUM

1. Earth quakes- Earth movements (gradual)

2. Subsidence +uplift (submerged coast /raised beaches)

3. Faulting -Result to rift valleys or block mountains, lift blocks

4. Fold- Fold Mountains

5. Volcanic eruptions forming different volcanic feature (volcano)

6. Displacement of lithosphere leading to plate tectonic

Evidences of Isostatic movements

1. The depression of the crust in the northern part of America and Europe was due to the weight of ice sheets of vast thickness during the ice age.

2. After the melting these ice sheets, the crust has been rising.

3. Slowly for example, there are numerous former beaches that occur around the coast of Scandinavia. They now lie between 8m-30m above the present day beaches. These old beaches have been raised because of the uplift of the land.

4. The continental shelf around Antarctica is covered with water to a depth of about 750m compared with 180m around other continents.

5. The presence of Rias and Estuaries between the coast lands of Gambia and Sierra Leone.

6. The submergence of forests on the shores of Britain.

IMPORTANCE OF THE THEORY

1. It provides the knowledge on the dynamic state of the earth’s crust that is the earth’s crust is not static but it is always dynamic as it tends to balance itself after some disturbances with the influence of the gravitational force.

2. The analogy that the crust floats on the mantle, just like the iceberg floats on and the ocean or sea water, is so crucial in the understanding of the theories of plate tectonics and continental drift.

3. The theory also helps in deciphering (understanding) how different landforms were formed.

4. It also gives the basic for predicting the future of the crustal state at any particular place on the earth’s surface.

5. It can also help human being in taking some precautions depending on the nature of the phenomena that might have been observed in the course of time, like the occurrence of ice sheets and melting of ice.

THEORY OF CONTINENTAL DRIFT

There are 7 continents Origin of these continents

Profounder

1. Francis Bacon (1620) Expanded earth

2. F .B Taylor

3. Alfred Wegener 1912.

According to his theory ,about 280 million years, the present day continents were united in a single block called Pangaea and surrounded by ocean called Panthalassa. He believed that Pangaea was located near the South pole. Later Pangaea split into two (2)Super-continental i.e Gondwanaland(south Pole) and Laurasia(along the equator in the northern hemisphere).

These two Super-Continents were separated by a narrow water body i.e Tethys sea.He assert that

Laurasia split to form present day N.America, Asia, Europe, and numerous landmasses found in northern Hemisphere-Greenland,Iceland and United Kingdom.

Gondwanaland split to give present day Africa, S.America, Australia and Indian sub- continent, Antarctica and other islands in the southern Hemisphere.

Drift

Since that time the continents have been drifting apart to occupy their present positions. And the drifting is a very slow one about 2cm per year. The drifting is still in progress.

Evidence to support Wegeners theory of Continental Drift

1. Structural evidence ( Jig saw fit)

If the continents were to be brought together it will form one single landmass called Pangaea. Hence proves that all the continents come from one land mass. For example S. America could fit into Africa, N. America into Europe, Antarctica, Australia, India and Malagarasy formed in a single landmass with S. America.

2. Geological evidence

Similar rock types in the coastal margin of the continents for example if you take the rocks of west Africa coastal margin and those of Eastern coast Brazil coastal margin the rocks will be the same (similar ) These rocks where one before the split. (Similar type, age, structure, formation)

3. Biological evidence

The study of earlier life in sedimentary rocks. That reveals life similar fossils of different time where found in different places. Similar animals, plants and e.g. .These fossils are found in all continents hence prove that they all come from one landmass

4. Geomorphologic evidence

Structure of mountains ( fold mountains) e.g. The Alps and Atlas have similar features and where also formed under similar conditions, type of rocks the mountain case, structure, alignment and were formed when Africa moved north wards ending up colliding with the European continent. This gives evidence that the drifting movement took place.

5. Pale-climatic evidence:

The discovery of ancient is in the Congo basin where the climate is warm is used as evidence that the Africa continent drifted from parts which were cold to the current warm parts. For example, Africa has been shifting north wards from the south likewise coal deposit found beneath Antarctic ice caps and Greenland show that they were deposit when the continents had not drifted led to

those places when the climate coast warm. This is because the organisms that led to the formation of those deposits cannot east in areas where there is very low temp.

6. Paleo magnetism (palae-magnetic evidence).

This is the most conclusive proof of the continental drift which was done through pale magnetic dating. When the rocks cooled they were magnetized in the same direction (magnetic North) but pale magnetic dating shows that rocks older than 200,000 years ago from different parts of the earth, have shifted their relative positions magnetic fields show new paths representing relative migration of the earth’s materials

This is evidence that the continents have drifted or have been drifting.

Ocean floor spreading.

Weakness of Wegener / critiques of the theory

1. He did not explain how the movement has occurred / the continental drift processes.

2. He was not a geologist but meteorologist hence he was criticized to have involved in the field where he was probably less informed.

3. Not all continents are able to fit exactly as argued in the jig-saw fit theory, but the theory does not account for this aspect.

4. Other scientists argue that the plant remains might have been spread by aspects like wind, blowing from one continent to another.

Wegner failed to explain the development of glacier in the hot arid Australia.

PLATE TECTONIC THEORY (Unifying theory)

It takes the combination of theory of theory of isostacy, continental drift and ocean floor spreading.

According to this theory, the earth has an outer shell(lithosphere) made up of several rigid piece called tectonic plates.

Geometrical part – The crust is made up of segment called plates, which are of various sizes, Large and small plates. pacific, N. American, Nazca, S. American, Africa, Indian, Antarctica plates

Moments- Movement of the plates. The plates are in motion. Either they are diverging or converging cause of movement – Convectional currents from the interior of the earth’s especially aesthnosphere.

The movements can cause plates to collide or slide past one another. The rate of motion is very slow of the plates relative to one another 1cm to 2cm per year.

TYPES OF PLATE BOUNDARIES:

Divergent boundary

Is a boundary which is created when the plates are moving away from each other. This normally occurs in the ocean, where there are mid-oceanic ridges.

Example

Mid Atlantic ridges

Island Arcs ( volcanoes)

Rift. valley Graben

Convergent boundary

Is boundary where by the plates are moving towards one another

Neutral / trans current boundary-
This occurs when plates are sliding past one another. Normally occur on the Transform faults. (No uplift or submerge of the land (crust)

MECHANISMS OF THE PLATE MOVEMENTS AND THE PLATE BOUNDARIES:

Plates are either continental or oceanic. Oceanic plates are simatic (denser).

Continental Plates are Sialic (light).

CONVERGENT PLATE BOUNDARY

Continental and continental – Both will go uplifted and result to folds and faults

Oceanic and continental -The denser will down lift and lighter will uplift and may result to volcanic eruption and subduction result to trenches for the denser plate.

Oceanic and oceanic – Convergent boundary ( subduction result to trenches both will move down)

Plate Tectonic Theory states that “The lithosphere is made up of rigid segments called the

plates and the plates are in constant state of motion relative to one another”

CAUSES OF MOVEMENTS:

I) Convectional current – During mantle convection some materials rise due to the influence of radioactive heat generation and later moves laterally below the lithosphere. The lateral movements

drag the lithosphere leading to the plate tectonic movements. On cooling the materials sink down to the lower level of the mantle where they melt again due to the constant motion of the plates

II )Upwelling of magma in the lines of weakness for example in the mid-oceanic ridges where by the magma is pushed out of the surface through the weak lines of the crust, in order to form a

new crust. This may result to the cause of movements

Isostatic adjustment – May cause slight movement when trying to create balance.

Cooling and heating of the crustal rocks -Expansion and contraction of rocks. The heat in the interior of the earth (mantle) causes rocks inside to expand and when the heat reduces, the rocks

cool. This process causes the rocks to crack hence leads to the disturbance of the crust and causes movements.

EFFECTS OF PLATE MOVEMENT:

Changes on the plate boundaries which are lines of weakness and on these boundaries, major

Landforms of the earth’s surface are going to be formed.

Diverging plates

Oceanic

Mid oceanic ridges

Oceanic Islands

Rifts e.g. Red sea

Continental

-Volcanic mountain

– Block mountains

-Rift valley

Convergent plate boundary

Collision may lead to subduction and uplift.

Oceanic – oceanic trends (Marianna trenches, Japan trenches )

Oceanic and continental-Volcanic mountains on the coastal boundaries and also result into trenches.

( Continental )- Formation of Fold Mountains. Himalayas ( Indian and Russian plate formed)

Neutral / Trans current boundary – There is neither uplift nor seduction. There is lateral displacement of the plates. N. America, San Francisco – San Andrea’s faults displacement of features of about 1000km.

The plate tectonic areas are areas of instabilities which results to earth quake, volcanoes.

The theory of plate tectonic can help to explain almost all of the landforms on the surface of the earth.

Deep sea Trenches: A sea trench is a long deep valley along an ocean floor.they form along a convergent destructive term of depth.the Mariana in the western Pacific with a depth of more than 36000ft.

Mid Oceanic Ridge: It refer to a giant undersea mountain range made up mostly basalt.It may be more than 80000km long and 1500 to 2500km wide and it may rise to 2.3km above the ocean floor.The feature is associated with a divergent plate boundary.As plate diverge,magma rises repeated and eventually cools to form the mid oceanic ridge.

Example: East Pacific as Nazea and Pacific diverges North Atlantic as North America diverges the Eurasian.

III.Island Arcs:Sometimes basalt eruption along the ridge or near may build up volcanoes that protrude above sea level to become Oceanic Island.They may vary in size.Example:Iceland,Japan,Hawaiian is lands, Mauna Loa,Easter Islands near the East Pacific ridge,west Indies.

IV. Magmatic Arc:It refers to island arcs at sea and belts of igneous activity on the edges of continents such as batholiths in mountain belts. Example:Aleutian Island.cascade volcanoes of the pacific North West,along Andes.

Mountain Belts:At a convergent collision boundary,the sea floor is denser and will be subducted making the ocean thinner and narrower hence collision of the continents.Eventually the oceanic lithosphere breaks off leaving the continental crumple to form mountain ranges.

The thick sequences of sedimentary rocks that had built upon both continental margins are intensively.Example The Himalayas as India collided lided, Alps as African and Europe ,Atlas in Northern Africa.In addition to that at a convergent destructive boundary,young mountain like the Andes form due to the folding of the young sediments.

Rifting and associated features:At a passive divergent plate boundary ,the continental crust is stretched and thinned producing faulty landforms like the great East Africa Rift Valley.The faults

may be path associated volcanic landforms.This may explain the volcanic landforms in the rifted areas of East Africa.

Due to thermal expansion emanating from rising mantle plume,it causes uplift of landscape.

New Oceanic Crust:This is associated with a divergent plate margin where the would be gap is filled with the up welled magma to form a new oceanic crust.