Lecture 5
PLATE TECTONICS AND RESTLESS EARTH
Plate Tectonics
Continental
drift is the idea that the continents fit together like a jigsaw puzzle.
Alfred Wegener
(German climatologist and geophysicist) published The Origin of Continents
and Oceans in 1915.
1.
1.
He called the supercontinent "Pangaea", which means, "all
land".
2.
2.
He suspected that the continents might have been joined when he noticed
the similarity between the coastlines on opposite sides of the Atlantic.
3.
3.
He provided the following evidence for the existence of Pangaea:
o
o
Fossils match across the seas.
o
o
Rocks and structures match across the seas. Mountain belts disappear at
on coastline only to reappear on another coastline across the sea.
o
o
Paleoclimates show glacial evidence where tropical environments now
exist, visa versa.
o
o
Wegener's continental drift idea could not explain how the continents
moved.
Plate
Tectonics: A modern version of continental drift
Plate tectonics indicates that there are about 20
solid rigid slabs on the Earth's crust called plates.
These plates move in continuous slow motion
relative to each other.
The main assumption of plate tectonics is that the
plates are rigid; therefore, the distance between two places on the same plate
does not change (e.g., the distance between New York and Denver, which are on
the same plate does not change, but the distance between New York and London,
which are on different plates does change).
1.
1.
Divergent plate boundaries are where plates move apart, resulting in
upwelling of material from the mantle to create new sea floor (sea floor
spreading).
2.
2.
Transform plate boundaries are where plates grind past each other without
creating or destroying lithosphere, such as the San Andreas Fault System.
3.
3.
Convergent plate boundaries are where plates move together causing one
of the slab of lithosphere to be consumed into the mantle as it descends
beneath an overriding plate. Some types of convergent boundaries are:
o
o
Oceanic-continental convergence, which is characterized by deep ocean
trenches and volcanic arcs.
o
o
Oceanic-oceanic convergence, which is characterized by island arcs.
o
o
Continental-continental convergence, which is responsible for most
mountain systems.
Testing the plate tectonics model.
1.
1.
Paleomagnetism evidence exists in rocks formed thousands of millions of
years ago. The location of the magnetic poles at the time of their formation is
recorded in the rock.
2.
2.
Polar wondering has shown that during the last 500 million years the
rocks have recorded that the location of the magnetic north pole has gradually
wondered from a spot near Hawaii, northward through eastern Siberia, and
finally to its present location.
3.
3.
Others believe that the pole did not wonder, instead the continents
drifted.
4.
4.
Magnetic reversals and seafloor spreading show that the Earth's magnetic
field periodically reverses polarity: 1) the north magnetic pole becomes the
south magnetic pole and vice versa, and 2) rocks solidifying during a reversal
will be magnetized with the polarity opposite that of rocks being formed today.
5.
5.
Earthquake
patterns show that the distribution of earthquakes corresponds with the
various plate boundaries.
6.
6.
Hot spot evidence, such as the Hawaiian island
chain, help to further support the plate tectonic model.
Driving Mechanism for Plate Tectonics:
o
o
Slab-push and slab-pull hypothesis, and
Hot
plumes (hot spots).
Restless Earth
Introduction
An earthquake can be defined as the vibration of
the earth caused by the sudden release of energy as a result of the displacement
of rocks along fractures or faults. The energy radiates in all directions from
the source (focus).
The waves produced are like those produced when a
stone is tossed into a pond.
The elastic
rebound process is the process in which rocks are deformed, rupture,
release energy, and spring back to their original position.
Seismology.
Seismology is the study of earthquakes. A
seismograph is an instrument that records and measures vibration produced by
earthquakes. The record made by the seismograph is called the seismogram.
The focus is the location within the crust where
rupture starts and energy is released. The are three types of focus:
1.
1.
Shallow focal point: less than 70 km;
2.
2.
Intermediate focal point: between 70-300 km;
3.
3.
Deep focal point: greater than 300 km.
The epicenter is the point on the Earth's surface
vertically above the focus.
Body waves travel through the interior of the
Earth and consists of primary (P) and secondary (S) waves.
o
o
Primary waves are: 1) fastest of all seismic waves, 2) travel through solid,
liquids, and gases, and 3) have a compressional or push-pull motion.
o
o
Secondary waves are: 1) slower than P-waves, 2) travel only through
solids, 3) travel perpendicular to the direction of travel, and 4) are also
known as shear waves, because they only travel through solids.
Surface waves travel around the outer layer of
the Earth.
This scale measure the earthquake magnitude,
which is the total amount of energy released by an earthquake.
1.
1.
This scale is an open ended scale with 1 being the start of the scale.
2.
2.
Each increase in magnitude indicates a ten fold increase in wave
amplitude (e.g., a magnitude of 6 is 10xs greater than that of 5).
Earthquake
destruction.
3.
3.
Man made structures are damaged due to vibration.
4.
4.
Tsunamis or seismic sea
waves inundate coastal areas.
5.
5.
Fires can not be controlled due to breakdowns in the infrastructure.
6.
6.
Landslides and ground subsidence affect slope and valley areas.
Geologic structures.
The Earth is dynamic and the forces applied to the
rocks are evident in the various structural features seen in the rocks.
During mountain building flat lying sedimentary
and volcanic rocks become bent in a series of wavelike folds.
o o
Anticlines
are formed by up folding or arching of rock layers.
o o
Synclines
are formed by down folding rock layers.
o o
It is common to see both anticline and synclines together.
Faults
are fractures in the crust along which displacement or movement has occurred.
Some of the properties and types of faults are summarized below.
·
·
A normal
fault is when the hanging wall moves down relative to the footwall
(dip-slip fault).
·
·
A reverse
fault is when the hanging wall moves up relative to the footwall (dip-slip
fault).
·
·
A thrust
fault is when rocks have been trusted upon each other (dip- slip fault).
·
·
A transform
fault is when displacement occurs along the strike of the fault
(strike-slip. fault).