Geologic Processes that Change the Earth's Structure
Geologic Forces: Earth has undergone great changes over million of years. Generally processes of gradation, tectonism and volcanism.
(a)Gradation
Degradation: Erosion results from wearing of rocks by water, air and ice.
Aggradation: Deposition results in accumulation of sediment and ultimate building up of rock strata.
(b) Tectonism
Plate tectonics - a dynamic process of the lithospheric plate which moves over a weak plastic layer in the upper mantle known as asthenosphere.
These plates interact with one another along their boundaries. Indicative of crustal instability, produce faulting (fracture and displacement), folding, subsidence and uplift of rock formation. Responsible for formation of mountain ranges.
Earth’s lithosphere is composed of seven large plates
with thickness ranging from 75 to 125 km.
Pacific Plate
Eurasian Plate
Antartic Plate
North America Plate
Indian Plate
South American Plate
Africian Plate
20 other small plates in between
(c) Volcanism
Volcano - a vent in the earth's crust through which molten rock materials within the earth, lavas, ashes, steam and gas are ejected and responsible for the formation of plutonic rocks, once solidified at great depth.
Majority of volcanoes are located along the margins of tectonic plates.
·Both are distinctly different in composition, density, rock type, structure and origin
(a)Continental Masses:
·Part of the earth - covers about 29% of the earth’s surface and has an average elevation of about 5 km above the floors of the ocean basins and about 1 km above sea level.
·It composed largely of rocks known as granite. The continents rise above the ocean basins as large platforms.
·The highest mountain on the continental surface is Mount Everest which is 29000 feet above sea level but the deepest part of the ocean is about 35000 feet below sea level at Pacific Ocean.
(b) Ocean Basins
The greatest part of the hydrosphere - covers about 70% of the earth's surface.
The ocean floors are also as irregular and posses many deep trenches and mountain ranges as the continental masses.
The rocks of the ocean are rather dense, dark basaltic rock.
A graph of the elevation of the continents and ocean basins
The crust is composed of two basic rock types granite and basalt. The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt. Basaltic rocks of the ocean plates are much denser and heavier than the granitic rock of the continental plates. Because of this the continents ride on the denser oceanic plates. The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere. The layer below the rigid lithosphere is a zone of asphalt-like consistency called the Asthenosphere. The asthenosphere is the part of the mantle that flows and moves the plates of the Earth.
The crust
•The mantle is approximately 2900 kilometers thick, making it Earth's largest layer. The mantle has a property called "plasticity" (where a solid has the ability to flow like a liquid). You might call the mantle "partially molten". Remember that the temperature of the mantle increases the deeper you go. This difference in temperature causes CONVECTION CURRENTS to form. This type of current forms when hot things rise and cooler things sink. These convection currents tumble throughout the mantle. They cause the Lithospheric plates floating on the mantle to move around. These currents cause our continents and oceans to change location slightly each year. The currents are the driving force for Plate Tectonics or Continental Drift, which we will discuss in more detail in a later section. The forces which drive continental drift seem to come from the mantle. The hot rock, which boils up at mid-ocean ridges, comes from the upper mantle. This rock spreads out forming new oceanic plates. When these meet the continents they plunge back down into the mantle, sometimes going down as far as the outer core.
•In addition there are hot spots, which start at the outer core and rise up through the mantle to form islands such as Hawaii or Iceland.
Convection Currents - Large convection systems in the mantle may carry along the plates of the lithosphere like a conveyor belt
Outer Core & Inner core
Inner core
The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The inner core begins about 4000 miles beneath the crust and is about 800 miles thick. The temperatures may reach 9000 degrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!!
The top of the asthenosphere is about 100 km below the surface. Above the asthenosphere, the material is solid, strong and rigid. This layer is called lithosphere. Contains the continental crust of the uppermost part of the mantle.
Asthenosphere (weak sphere)
A major zone within the upper mantle where temperature and pressure are just the right balance so that part of the material melts.The rocks lose much of their strength and become soft plastic and easily deformed. The thickness is about 200 km.
Mesosphere
The rock below the asthenosphere is stronger and more rigid than the asthenosphere because the high pressure at this depth offsets the effect of high temperature. The region between the asthenosphere and the core-mantle boundary is called the mesosphere.
Core
The core of the Earth marks a change in both physical properties and composition. It is composed mostly of iron and is therefore distinctly different from the silicate (rocky) material above. On the basis of physical properties, the core has two distinct parts - a solid inner core and liquid outer core. Heat loss from the core and the rotation of the Earth probably causes the liquid outer core to circulate and generate the Earth's magnetic field.
·Outer layer of the Earth, extending from solid surface down to the first major discontinuity in seismic wave velocity in the lithosphere. Thickness of crust varies from about 8 km under the oceans to about 35 km under the continents.
·There are two kinds of earth crust classified according to two different kinds of rock they contained where each with its own general composition, thickness and density.
(a) Continent Crust:35 - 60 km thick
relatively low density
granitic rock
average density: 2.8 g/cm3
(b) Oceanic Crust :thickness rarely exceed 5 km
denser material
basaltic composition
average density: 2.9 g/cm3
2)Mantle
The next major compositional layer of the Earth which covers the core and this zone constitute 82% of its volume and 68% of mass of the Earth. The mantle is composed of iron and magnesium silicate rock, and it goes down to about 2900 km from surface of Earth. The earth's mantle is a heavy layer of rock which lies under the earth's crust. In places where the crust is broken, part of the upper mantle can melt and form a substance called magma, or molten rock. Magma can be forced through the earths crust. It flows out of volcanoes as lava.
Average density: 4.5 g/cm3
3)Outer core
·It is speculated that the thickness is about 2250 km and it is made of molten iron and nickel. The outer core is so hot that the metals in it are all in the liquid state.
·Average density: 10.7 g/cm3
4)Inner core
·The thickness' is about 1300 km and probably consists of mostly iron and nickel. The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place as a solid. The temperatures may reach 9000 degrees F. and the pressures are 45,000,000 pounds per square inch. This is 3,000,000 times the air pressure on you at sea level!!!
>>The largest of four planets of inner group solar system i.e. Mercury, Venus, Earth and Mars and third closest to the sun.
•Shape - spherical
•Polar radius - 21 km shorter than equatorial radius
•Average radius - 6378 km (3965 miles)
•Surface Area - 510 x 106 km2 (29% is land)
•Overall Density - 5.5 g/cm3
•Mount Everest is 8.8 km above sea level
•Ocean floor is an average 3.7 km below sea level
•Average height above sea level is 7 km
Principal Division of Earth
>> Consist of three:-
1) Atmosphere – Troposphere and Stratosphere
2) Hydrosphere
3) Lithosphere
(1) Atmosphere:
Gaseous portion of the Earth extending upwards for hundreds of miles above sea level.
It is a mixture of 98% nitrogen, 21% oxygen, carbon dioxide, water vapour and minor amount of other gases. The atmosphere is divided into two parts which is Troposphere and Stratosphere.
Troposhere - the closest to the Earth ~ 13 km. It contains almost all of the water vapour, clouds and storms.
Stratosphere - the overlying layer ~ 55 km above the surface, contains the ozone layer. It acts as an insulating agent protecting us from the heat and ultra violet radiation of the sun and makes possible the evaporation and precipitation of moisture and is thus of crucial important to organisms on surface of Earth.
The atmosphere is an important geologic agent and is responsible for the processes of weathering which are continually at work on the Earth's surface.
(2) Hydrosphere:
Total mass of water or the surface of our planet which includes about 98% of water in the oceans and 2% in lakes, rivers as well as ground water which exist in the pores and crevices of the crustal rocks and soils.
71% of Earth covered by oceans to average depth of 4 km. Water is essential to man and of geologic important.
All of Earth's weather patterns, climate, rainfall and the extremely important carbon dioxide content of atmosphere are influenced by the seas and oceans.
Hydrosphere is in constant motion - evaporating through atmosphere, precipitating as rain and returning to Earth.
As water moves over the Earth's surface it erodes, transports and deposits weathered rock material, constantly modifying the Earth's landscape.
(3) Lithosphere:
Lithos means rock.
The solid portion of the Earth composed of crust and upper mantle.
It is a layer of rocks about 70 km thick, that rests upon soft weak material and is broken into about 12 major plates which is slowly moved by the flow of material in a layer that directly underlies the lithosphere called the asthenosphere.
There are three basic types of rock: Igneous, Sedimentary and Metamorphic.
Physical Features of the Earth
Major Concepts
Earth is segregated and concentrated into layers according to density.
The major internal layers based on physical properties are:
(a) lithosphere(b) asthenosphere
(c) mesosphere(d) core
Material within each of these units is in motion, making Earth a changing dynamic planet.
Continents and ocean basins are the principal surface features of Earth.
Major Structural Units of Earth
The constituents of Earth are separated and segregated into layers according to density.
The denser materials - concentrated near the center, the less dense - near the surface.
The internal layers are recognized on the basis of composition and physical propertieswhich are:
•Crust
•Mantle – earth mass, which compose of composed of iron (Fe), magnesium (Mg),
aluminum (Al), silicon (Si), and oxygen (O) silicate compounds. At over
1000 degrees C, the mantle is solid but can deform slowly in a plastic
manner.
•Core (inner & outer) - composed mostly of iron (Fe) and is so hot that
the outer core is molten, with about 10% sulphur (S).
The inner core is under such extreme pressure that
