Geologic Processes that Change the Earth's Structure

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.

The Structure of Earth

Principle surface features of the Earth –

(a) Continents

(b) Ocean basins

· 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

Composition of earth

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!!!

Composition of earth

The Earth is divided into several layers which have distinct chemical and seismic properties (depths in km):

• 0 - 40 Crust
• 40 - 400 Upper mantle
• 400 - 650 Transition region
• 650 - 2700 Lower mantle
• 2700 - 2890 D'' layer
• 2890 - 5150 Outer core
• 5150 - 6378 Inner core

Composition of earth

The internal structure of Earth

Model for Structure of Earth