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Basic Theories/Interior of Earth/Rocks

Basic Concepts Interior of the Earth Classification of Rocks

Geomorphology Basic Theories

Geomorphology is a systematic and organized description and analysis of the surface landforms, processes and landscape evolution of the Earth.

The basic concepts related to geomorphology are:

A. Continental Drift
• The theory of Continental Drift is an attempt to explain the present arrangement of continents and ocean basins.
• Alfred Wegner,is regarded as the father of the concept of Continental Drift.
• Wegner hypothesized that the super continent (Pangaea) broke up to form: (i) Laurasia (N. America, Greenland and all Eurasia north of the Indian Subcontinent) and (ii) the Gondwanaland (South America, Africa, Madagascar, India, South West Asia, Australia and Antarctica).
• The Pangaea was surrounded by a huge sea Panthalassa (Pan = all, thalassa = oceans) or the Pacific Ocean.
• He opined that continents are made of SiAl and Ocean basins of SiMa.
• According to Wegener continents drifted in two directions:
a) Towards the Equator (due to : gravitational attraction exerted by the earth’s equatorial bulge). As a result of this drift, the Himalayas and Alps etc, came into existence. The Peninsular Indian and Africa were separated from the Antarctica and Australia.
b) The Second drift was towards the west (due to tidal force of the Moon and the Sun). North and South America got separated from Europe and Africa and the Andes and Rockies Mountains came into existence.

Evidence in support of Continental Drift
a) Similarities are found in the opposing coasts of the Atlantic Ocean. Jig saw of the opposing coasts of Atlantic Ocean.
b) Fossils of glossopteris (a fern – like plant) have been found in rocks of the same age from S. America, S. Africa, India and Australia.
c) Folded mountain ranges at Cape of Good Hope and rocks of Buenos Aires (Argentina) resemble.
d) Major lava plateaus of South America, W. Africa, Madagascar, India and Antarctica have the same geological structures.
e) Evidence from Glaciations as about 300 million years back (Paleozoic Era) glaciers covered the Southern Hemisphere including South America, Africa, India, Australia.
f) Great coal deposits in the tundra and cold climatic regions of Antarctica, Siberia, North Europe.
g) According to Wegener, Poles have changed their position, e.g. during the Carboniferous Period, the South Pole was near Natal and the North Pole was in the Pacific Ocean.

Criticism
• Wegener was criticized for failing to explain the forces that would permit continents to plough through the ocean of rocks.
• He was declared as a crank who carefully selected only those data which supported his hypothesis, ignoring contrary evidences.
• Wegener died on an expedition in Greenland in 1930. His theory was already eclipsed.

B. Sea-floor Spreading
• The hypothesis of ‘Sea-Floor Spreading’ which proves the theory of Continental drift was put forward jointly by Harry H. Hess and Robert Deitz in 1960.
• According to Hess, the submarine mountain ranges (Mid-Ocean Ridges) were direct results of upwelling flows of magma from the Asthenosphere.
• When magma rises from below, the earth’s crust is fractured. The magma spills out and cools to form new sea floor, building the ridges and spreading laterally.
• The following three important facts were established by Hess:
(i) The crust below the ocean floor was found to be only 6 to 7 km thick, whereas below the continental surface it was 30 to 40 km.
(ii) Mid-Oceanic Ridges were present in all the oceans.
(iii) The ocean floors, nowhere, were found to be older than 135 million years, though the oceans are 3.9 billion (3900 million) years old.
• Ocean floor is constantly regenerated at the mid-ocean ridges and subjected to continents lateral spreading until it is destroyed in the trenches.

Following are the Evidence for Sea-Flooring Spreading:
• Occurrence of earthquakes along the crust of Mid-Ocean Ridges.
• The dearth of sediments at the crest of Mid-Ocean Ridges and active volcanic islands like Iceland and Canary Islands.
• Thickness of the sedimentary deposits increases away from the Mid-Oceanic Ridges.

• There is reversal, in the main magnetic field of the earth known as ‘magnetic di-pole’ (magnetic field).
• The normal and reverse magnetic anomalies are found in alternate manner on either side of the Mid-Ocean Ridges.

Following Inferences can be down from the evidence:
(a) There is continuous spreading of sea floor.
(b) The basaltic crust (igneous) is continuously formed along the Mid-Oceanic Ridges.
(c) The newly formed basaltic layer is divided into two equal halves and thus displaced away from the Mid-Oceanic Ridge.
(d) Alternate stripes of positive and negative magnetic anomalies are found on either side of the Mid-Oceanic ridges.

C. Plate Tectonics
• It assumes that the lithosphere is broken into a series of separate plates which move in response to convection in the upper mantle (Asthenosphere).

Major Plates
1. African Plate, 2. American Plate, 3.Antarctica Plate, 4.Australian Plate, 5.Eurasian Plate, 6.Pacific Plate.

Minor Plates
1. Arabian Plate, 2. Bismark Plate, 3. Caribbean Plate, 4. Carolina Plate, 5. Coos Plate, 6. Nazca or East Pacific Plate, 7. Juan De Fuca Plate, 8. Philippines Plate, 9. Scotia Plate.

Different types of plate movements:
• Constructive Plate Margins or Divergent Plate Boundaries (Ocean Ridges)
The Plates diverge and move along mid-oceanic ridges and thus, new lithosphere is formed. The oceanic ridges stand high because their material is low in density.
The mid-oceanic ridges and rift valleys of East Africa are the constructive plates.
• Destructive Plate Margins or Convergent Plate Boundaries (Ocean Trenches)

At Ocean trenches, one plate slips under the margins of other at an angle of 45° (the Pacific and Nazca Plates). The volcanoes of the Andes and Pacific, and the earthquake of these regions are the results of destructive Plates (Mt. St. Helens).
• Conservative or Passive Plate margins
In this case the plates simply slip past each other. The crust is neither created nor destroyed. Example California’s San Andreas Fault.

Criticism
1. Plate tectonics theory is unable to explain why subduction is limited to the Pacific coast while spreading is found in all the ocean. The length of spreading (ocean ridges) is far greatest than the subduction zone. The rate of construction is more than the rate of destruction.
2. The Benioff Zone (Ring of Fire) is not present equally in all probable places. For example, the intermediate and deep focus earthquakes are absent in North America.
3. There are certain mountain ranges, such as the Eastern highlands of Australia, Drekenburg mountain of South Africa and Sierra-Dalmar of Brazil which can not be related to plate tectonics.
Despite all these omissions and commissions plate tectonic is a revolutionary and comprehensive theory which scientifically explains the present distribution and arrangement of the continents and ocean basin. It also provides as satisfactory explanation of the distribution of volcanoes and earthquakes. It has also confirmed the theory of Continental Drift.

D. Isostasy
• Isostasy is the state of balance.The gravitational adjustment of the earth’s crust is known as isostasy.
• It is based on the principle of buoyancy (Archimedes).
For example: A Ship sinks until it displaces a volume of water equal in weight of the ship and its cargo.Thus an empty ship displaces a smaller volume of water than the same ship when fully laoded.
Mt. Everest (8.84 km above sea level) and its adjacent peaks are not supported by the mechanical strength of materials within the earth because nothing on (or in) our world is that strong. The mountains upper surface of the continents floats high above sea level because the lithosphere of which it is part sinks into the asthenosphere until it has its own mass.
• The state of equilibrium, resembling floatation, in which segments of the earth crust stand at levels determined by their thickness and density.
• Isostatic equilibrium is attained by flow of material in the mantle.
• Isostasy takes place on the Earth wherever a large amount of weight is present. This weight might be due to a large mountain, ice from an ice age, or even from manmade structures, such as the weight from large manmade lakes.
• Isostasy also takes place when a large amount of weight is removed from an area, causing that portion of the Earth’s crust to rise, such as when ice caps melt.

 Interior of the Earth

Due to changing nature of its internal composition and huge size the direct observation of the earth interior is not possible.

There are three ways to observe the interior of the earth as used by Geophysicists and geologists:

a) Temperature: Temperature increases at the rate of 1°C per 32 metres of depth, reaching 4000°C at its core where no rocks or minerals can remain solid due to such high temperature.

b) Density of rocks:
• The density of the rocks increases going inside the earth.
• The outer shell consists of sedimentary rocks upto about 1.6 km with a density of 2.7g/cm3.
• Below sedimentary rocks lie crystalline rocks with a density of 3.0 to 3.5 g/cm3.
• The average density of the earth is 5.5 g/cm3.
• Density of the core is more than 11g/cm3 due to the presence of metallic minerals like nickel and ferrous.

c) Earthquake Waves (Seismology):
The energy generated by earthquake in the form of waves is the most reliable source to know the interior of the earth.

Seismic waves passing through the earth are refracted in a way that show distinct discontinuities within earth’s interior and provide the basis for the belief that earth has:
a) a solid inner core,
b) a liquid outer core,
c) a soft asthenosphere, and
d) a rigid lithosphere.

There are two types of Seismic Waves:


a) Body Waves travel or propagate through the body of the (interior) earth following ray paths refracted by the varying density and stiffness of the earth’s interior which in turn, vary according to temperature, composition, and phase.

• Primary Waves (P Waves): Analogous to sound waves, travels fastest through the solid materials but slows down while passing through liquid. They are observed on seismograph quickly as it travels nearly twice as fast as the S Waves.

• Secondary Waves (S Waves): Analogous to water ripples or light waves, travels through solid only, having high frequency, short wavelength which propagates in all direction from the focus.

b) Surface Waves (L Waves) travel or propagate along Earth’s surface are slower than body waves due to its low frequency, long duration and large amplitude. They are most violent and destructive covering the longest distance on the surface.

• Rayleigh Waves (Ground Roll): Generated by the interaction of P- and S-waves at the surface of the earth, Travel as ripples with motions that are similar to those of waves on water. It causes the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion. It emanates outward from the epicenter of an earthquake travelling about 10 times the speed of sound in air (0.340 km/s).


• Love waves (Q Waves):
It causes horizontal shifting of the Earth and circular shearing of the ground during an Earthquake. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity, and have the largest amplitude.

Shadow zone:
• Shadow Zone for P Waves: From a particular spot (labeled 0°) shadow zone for P-Waves exists between 103° and 143° from the focus of the earth quake. Evidently the core deflects the waves from a linear path where seismic waves travel relatively slowly.
• It is due to dense core which produces shadow zone in which no seismic waves passes and detected. It signifies that the core is liquid at a depth of 2900 km from the Earth surface in which S Waves does not pass through.
• ‘S’ Waves disappear at an angle of 120° from the epicenter.
• Since the outer core is fluid, and S-waves cannot travel through a fluid, the “S-wave shadow zone” is even larger, extending from about 100° to 180°.
• The Mantle (35 to 2900 km) is made of dense rigid rocks (magnesium and iron). As the velocity of seismic wave decreases from lower Mesosphere to upper Asthenosphere, signifies that the lower mantle is more liquid than upper mantle.
• The Crust made of Silica and Aluminum (solid) has a great impact of L Waves and S Waves, causing massive destruction on surface.

Classification of Rocks

• The earth’s crust is made up of various types of rocks differing from one another in texture, structure, colour, permeability, mode of occurrence and degree of resistance to denudation. Rocks also form the basis for soil, and determine to some extent the type of natural vegetation and land use.
• Generally speaking, all rocks may be classified under three headings according to their origin and appearance: Igneous, Sedimentary and Metamorphic.

Igneous Rocks: Crystalline in structure, formed by the cooling and solidification of molten rock (Magma) from beneath the earth’s crust. They do not occur in strata nor contain any fossils.

On the basis of mineral composition Igneous Rocks can be subdivided:
• When they contain high proportion of silica, are said to be ‘acid’ such as granite which is less dense and are lighter in colour.
Basic rocks contain a greater proportion of basic oxides of iron, aluminium or magnesium and thus denser and darker in colour.
• They are described on two axes:

1) Rocks that are quartz rich (felsic) and magnesium rich (mafic).
2) Fast cooling (small crystals) and slow cooling (large crystals).

On the basis of origin, Igneous Rocks are of two types: Plutonic Rocks and Volcanic Rocks.
• Plutonic Rocks (Intrusive Rocks): Formed at some depth in the earth’s crust, cooled and solidified slowly so that large and easily recognised crystals have been able to form. These are exposed at the surface by the process of denudation and erosion. Example: granite, diorite, gabbro etc.
• Volcanic Rocks (Extrusive Rocks): Formed on the surface of the earth when the molten lave is cooled slowly and solidifies after being poured on forming small crystals. Example: Basalt, Andesite, Rhyolite etc.

Sedimentary Rocks: Non-crystallized termed as stratified rocks found in layers, formed by the accumulation of sediments (brought by streams, glaciers, winds or even animals, over a long periods) usually under water.

The rocks may be coarse or fine grained containing fossils of animals, plants and other micro-organisms.

In accordance with their origin and composition sedimentary rocks are of: Mechanically Formed, Organically Formed and Chemically Formed.

a) Mechanically Formed Sedimentary Rocks:
• Formed by the accumulation of sand grains, often quartz fragments derived from granites which were cemented together on other rocks. Example: sandstone.
• A coarse type of sandstone is known as grit. The finer sedimentary materials form clay widely used for brick making, shale or mudstone. Sand and gravel may occur in uncemented form.
• When large rounded pebbles are firmly cemented to form a rock is called conglomerate or breccia when the fragments are angular.

b) Organically Formed Sedimentary Rocks:
• Formed from the remains of living organisms such as corals or shellfish when their fleshy parts are decomposed, leaving behind the hard shells. Example: Calcareous which include limestones and chalk.
• The carbonaceous rocks are also organically formed but from the vegetative matter such as swamp and forests. Example: Peat, Lignite or coal.

c) Chemically Formed Sedimentary Rocks:
• Such rocks are precipitated chemically from solutions of one kind or another. Rock salts are derived from strata which once formed the beds of seas or lakes.
• Gypsum or Calcium Sulphate is obtained from the evaporation of salt lakes, such as the Dead Sea. Similarly potash and nitrates are also formed.

Metamorphic Rocks: Under great heat and pressure all rocks including sedimentary and metamorphic can turn into metamorphic altering their original character and appearance.

By this process:
• Clay may be metamorphosed into slate, Limestone into marble, Sandstone into quartzite, Granite into gneiss, Shale into schist and coal into graphite.
Metamorphic rocks are classified into Foliated and Non-Foliated on the basis of their structure and dominant minerals.

a) Foliated Metamorphic Rocks: Parallel arrangement of mineral grains, formed when pressure squeezes the flat or elongate minerals within a rock to become aligned. These rocks develop a platy or sheet like structure that reflects the direction where pressure was applied. Example: Slate, Schist, Gneiss etc.

b) Non-Foliated Metamorphic Rocks: Rocks with only one mineral (such as limestone) or those that recrystallize in the absence of deforming stresses that do not develop strong foliation developing a granular texture. It does not have a platy or sheet like structure. Example: Limestone.

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