Plate tectonics, earthquakes, and volcanism final(3)1

PLATE TECTONICS, EARTHQUAKES, AND

VOLCANISM

PLATE TECTONIC THEORY

Plate Tectonic Theory

• Based on a model of earth in which rigid lithosphere, consisting of oceanic crust as well as continental crust as well as about 100km of the underlying upper mantle, is made up of a number of pieces known as plates

• Lithospheric plates move over the hotter and weaker semi-plastic asthenosphere below. As plates move, they separate, mostly at mid-oceanic ridges, while colliding in other areas.

• When plates collide, one plate generally dives beneath the other, although in some cases the plates simply slide past one another.

CONTINENTAL DRIFT THEORY

Continental Drift Theory

• In 1915, Alfred Wegener published “The Origin of Continents and Ocean”

• Wegener suggested that a supercontinent called Pangaea (meaning all land), once existed

• About 200 million years ago, this supercontinent began breaking into smaller continents, which then “drifted” to their present positions

• Wegener and others collected substantial evidence to support these claims. The evidences includes:

• The fit of South America and Africa

• The geographic distribution of fossils

• Rock structures

• Ancient climates

Evidences of CDT

• Wegener first suspected that the continents night have been joined when he noticed the remarkable similarity between the coastlines on opposite sides of the South Atlantic

1. Continental Jigsaw Puzzle

• He learned that identical fossils or organisms were known from rocks in both South America and Africa

• He believed that there once existed a transoceanic bridges

2. Fossils Match Across the Seas

• If the continents were once together, the rocks found in a particular region on one continent should closely match in age and type those found in adjacent positions on the adjoining continent

3. Rock Types and Structures Match

• He was keenly interested in paleoclimatic (paleo – ancient) data to support continental drift

• He learned of ancient glacial deposits that indicated that near the end of Paleozoic Era (about 300 million years ago), ice sheets covered extensive areas of the Southern Hemisphere and India

• Layers of glacially transported sediments of the same age were found in Southern Africa and South America, as well as India and Australia

4. Ancient Climates

Rejection Of The TheoryAs late as 1953 – just five years before Carey introduced the theory of plate tectonics – the theory of continental drift was rejected by the physicist Scheiddiger on the following grounds.

• First, it had been shown that floating masses on a rotating geoid would collect at the equator, and stay there. This would explain one, but only one, mountain building episode between any pair of continents; it failed to account for earlier orogenic episodes.

• Second, masses floating freely in a fluid substratum, like icebergs in the ocean, should be in isostatic equilibrium (where the forces of gravity and buoyancy are in balance). Gravitational measurements were showing that many areas are not in isostatic equilibrium.

• Third, there was the problem of why some parts of the Earth's surface (crust) should have solidified while other parts were still fluid. Various attempts to explain this foundered on other difficulties.

Someday …

… will the continents come back together and form a

single landmass ?

SEAFLOOR SPREADING

THEORY

• Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.

• Seafloor spreading helps explain continental drift in the theory of plate tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to form new sea floor.

Seafloor Spreading Theory

• Older rocks will be found further away from the spreading zone while younger rocks will be found nearer to the spreading zone.

Age of oceanic crust; youngest (red) is along spreading centers.

PLATE TECTONICSPlate and

Plate Boundaries

Plate Boundaries• Lithospheric plates move a coherent units

relative to all other plates.

• Earth’s surface consist of great areas, or plates, that are geologically quiet, but are separated by narrow zones of seismic and volcanic activity where the plates separate, collide, or slide past one another.

• Plates are bounded by three distinct types of boundaries, which are differentiated by the type of movement they exhibit.

• Constructive Margins

• Where two plates move apart, resulting in up-welling of material from the mantle to create new seafloor

• Upwelling

• Rifting

Divergent Boundaries

• Destructive Margins

• Where two plates move together, resulting in oceanic lithosphere descending beneath an overriding plate, eventually to be reabsorbed to the mantle, or possibly in the collision of two continental blocks to create a mountain system

• Oceanic-Oceanic

• Oceanic-Continental

• Continental-Continental

Convergent Boundaries

• Conservative Margins

• Where plates grind past each other without the production or destruction of lithosphere.

Transform Plate Boundaries

Forces that Drive Plate

Motion

• The basic process responsible for plate motion

Thermal Convection

Slap Pull• The subduction of cold, dense slabs

of oceanic lithosphere is the main driving force of the plate motion

• As these slabs sinks into the asthenosphere, it “pull” the trailing plate along

• Results because old slabs of oceanic lithosphere are more dense than the underlying asthenosphere and hence “sink like a rock”

Ridge Push• Results from the elevated position of

the oceanic ridge, which causes slabs of lithosphere to slide down the flanks of ridge

• Ridge push appears to contribute far less to plate motions than does slab pull

Slab Suction• Arises from the drag of a subducting

slab on the adjacent mantle

• The result is an induced mantle circulation that pulls both the subducting and overriding plates toward the trench

What drives plate motion ?

1. Convective flow in the rocky 2900km thick mantle is the underlying driving force for plate movement

2. Mantle convection and plate tectonics are part of the same system. Subducting oceanic plates drive the cold downward-moving portion of convective flow where as shallow upwelling of hot rock along the oceanic ridge and buoyant mantle plumes are the upward-flowing arm of the convective mechanism.

3. The slow movements of Earth’s plates and mantle are ultimately driven by the unequal distribution of heat within Earth’s interior.

Measuring Plate Motion

VLBI• Very Long Baseline Interferometry

• Utilizes large radio telescopes to record signals from very distant quasars (quasi- stellar objects), quasars acts as stationary reference points.

• Useful in establishing large-scale plate motions

GPS• Global Positioning System

• Uses 21 satellites to accurately locate any individual who is equipped with a handheld receiver

• Useful in establishing small-scale crustal movements as those that occur along local faults in regions known to be tectonically active

The Theory of Plate Tectonic is supported

by:

1. Paleomagnetism, the direction and intensity of Earth’s magnetism in the geologic past.

2. The global distribution of earthquakes and their close association with plate boundaries.

3. The ages of sediments from the floors of deep ocean basins.

4. The existence of island groups that formed over hotspots and that provide a frames of reference for tracing the direction of plate motion.