PLATE TECTONICS
Plate Tectonics: how it evolved into a theory.
Wegener and Continental Drift
Evidences
Geographic
• matching continental outlines
Check out this great site! http://www.ucmp.berkeley.edu/geology/tectonics.htm
Also explore http://www.scotese.com/earth.htm
which contains links to
paleogeographic reconstructions for different periods of earth history
in the column to the right.
Geologic
• same geol. units across continents divided by oceans
• similar mountain chains across continents divided by oceans
Climatologic
• glacial striations in equatorial/tropical positions
• coal beds and evaporites
Paleontologic • same land and fresh water fossils in different continents
Wegener's reason: continental drift
Continents "plowed" through oceanic basins.
Rejected: No such evidence of plowing in the ocean basins.
THE NEW FINDINGS
Seismic studies:
Core
solid inner core
liquid outer core
The asthenosphere: a layer of low strength, partially molten, contained in the upper mantle. Plastic.
The lithosphere: a rigid layer on top of the asthenosphere. Rigid.
(Crust and uppermost mantle).
Oceanic crust: basaltic.
Continental crust: Granitic
Continental lithosphere thicker than oceanic lithosphere.
OCEANIC FINDINGS
=>Sea floor spreading confirmed
SEA FLOOR CREATED AT MOR, DESTROYED AT THE OCEANIC TRENCHES BY SUBDUCTION.
PLATE BOUNDARIES
Divergent Boundaries (know examples)
East Africa The Atlantic Basin
The Red Sea The Pacific Basin
__________ stress, ______________ faults, ______________ magmatism
Convergent Boundaries (know examples)
_________________ stress, _______________ faults.
Ocean. lithosphere - ocean. lithosphere:___________ magmatism
Ocean. lithosphere - cont. lithosphere:____________ magmatism
Cont. lithosphere - cont. lithosphere: ______________ magmatism
Transform Boundaries (know examples)
San Andreas Fault, Faults across MOR.
_________________ stress, _________________ faults, ______________magmatism
HOT SPOTS
Intense volcanism, anchored deeply in the mantle.
Not related to plate boundaries.
Hawaii's case.
Iceland's case.
Yellowstone's case.
PLATE MOVEMENT
Convection
Rate
THE TECTONIC ROCK CYCLE
CHAPTER 3
PLATE TECTONICS
The evolution of a theory. Even though the Theory of Plate tectonics is widely recognized today as the most important unifying theme in Geology, it took about four decades for this to happen. Its development shows a good example of the scientific method at work.
In 1912, the German meteorologist Alfred Wegener began to publish information suggesting that the continents had moved through geologic time. His evidences came from different kinds of observations:
a. Geographic: the perfect matching of the continents' outlines as a jigsaw puzzle;
b. Geologic: the correlation of geologic sequences across different continents, in particular the ones in the Southern Hemisphere (similar volcanic and sedimentary rocks of same age are found in South America, Antarctica, Africa, Australia; and also India, which is now in the Northern Hemisphere);
c. also geologic: mountains chains of the same age and structure that can be continued from one continent into another (like the Appalachians of North America and the Caledonides in Great Britain and Western Scandinavia;
d. Climatologic: features indicating extensive glaciation spreading out from Africa, found in rocks of equal age from Africa, South America, Antarctica, India and Australia. Notice that, with the exception of Antarctica, those are all continents which today are mainly occupying equatorial or subtropical positions.
e. Paleontologic: the same species of fossils, in particular, land plants, reptiles and fresh water amphibians appear simultaneously in the fossil record of Africa, South America, Antarctica, India and Australia. It is unlikely that the plants' seeds were carried by the wind across the oceans. It is even more difficult for land or fresh water animals to have swam those distances. How could those species have scattered through the vastness of today's oceans?
How could those similarities in the rocks of different continents
be explained?
Wegener proposed his Hypothesis of Continental Drift to the scientific
community in the 1920's. According to his ideas, the continents moved,
plowing through the oceanic basins; the continuity of distinct features
among different continents resulted from the fact that they had been together
in the past, forming a single landmass that broke up. The separated fragments
drifted over geologic time.
Scientists were quick to question the mechanism he proposed. If the continents plowed through the oceanic basins, like a bulldozer pushing around some dirt, why was there no sign of deformation or disturbance in the ocean floor?. Wegener's mechanism was not supported by the ocean evidence. His whole set of ideas was rejected and forgotten. Wegener died about a decade after, while attempting to cross the Greenland ice sheet. He never knew that the continents did, in fact, drift.
New information gathered with modern technology allowed the development of the Theory of Plate Tectonics. Continental drift became one of its multiple aspects.
The new findings: The lithosphere, asthenosphere, inner core, outer core. Seismic studies (the study of the passage of earthquake waves through the Earth) allowed a better understanding of Earth's internal structure, from the point of view of its physical properties. We already know from Chapter 1 that the Earth is formed by three chemically differentiated layers: the core, the mantle and the crust. When analyzing the Earth considering its physical characteristics, a different division is apparent.
The core is divided into a solid inner core, surrounded by a liquid (molten) outer core. Most of the mantle is solid, except for a layer very close to the crust boundary: the asthenosphere. The asthenosphere is plastic and partially molten. On top of the asthenosphere there is a rigid layer of solid rocks called the lithosphere, formed by the uppermost mantle and the crust. Your text book shows a good diagram that will help you understand the relationships between the lithosphere and asthenosphere with the mantle and the crust. Make sure that you check it and also refer to the text book for the depths that the lithosphere and asthenosphere reach in the Earth. Note that the oceanic lithosphere is thinner and mainly basaltic in composition, and that the continental lithosphere is thicker, with an average granitic composition.
The topography of the ocean floor. The first records of the topography of the ocean floor were obtained in World War II, when ships were trying to detect enemy submarines with sonar. It was discovered that every ocean contains a submerged mountain ridge and that there were trenches reaching several kilometers of depth, usually adjacent to a continent. Harry Hess, a geologist serving on a ship in the Pacific during the war, observed that the height of submerged mountain ranges perpendicular to the mid-oceanic ridge (MOR) decreased with the distance from it. He coined the name Sea floor spreading to describe what he believed was happening. The mountain ranges of the sea floor were created at the MOR, and moved away from it. As they moved away, they were eroded by the waves and became more submerged. He had no way to prove this. Later other kind of studies were going to provide the means to do so.
Paleomagnetism. It is the property of iron rich minerals to become oriented parallel to the lines of force of the Earth's magnetic field. This happens shortly after an igneous rock is solidified, at a temperature known as the Curie point. It is similar to the fact that the needle in a compass orients itself parallel to lines of forces of the magnetic field of the Earth today. Paleomagnetism allows geologists to know the latitude at which a rock was formed and the position of magnetic North pole of at that time.
Polar wander curves. Paleomagnetic studies were carried out in
all the continents and in samples of the different oceans during the fifties
and sixties. Some contradictory data called the attention of scientists:
a. For samples extracted from the same continent which corresponded
to different ages, the location of the magnetic North pole was different
for each age, as if the pole had "wandered" through time. The name polar
wander curves was created to describe this fact.
b. For samples of the same age taken from different continents,
paleomagnetism indicated that the magnetic North pole was at different
positions at the same time, depending on the continent considered. It was
as if there were more than one magnetic North pole at any given time.
The logical explanation is that neither the poles wandered nor there was more than one magnetic North at the same time: the continents moved around the magnetic North pole through geologic time, recording in their paleomagnetism the continent's positions relative to a stationary magnetic pole. This was the kind of proof that Wegener's ideas of Continental Drift needed!.
Normal and reverse magnetism. Another set of data dealing with paleomagnetism puzzled geologists: One would expect to find in all analyzed samples the minerals oriented towards present day magnetic North, which is almost coincident with the geographic North. Samples from some specific ages began to appear where their iron rich minerals pointed to what today is the geographic South. In those times, the needle of a compass would have pointed to the South. It seems that the Earth's magnetic polarity has reversed several times in the past. The mechanism by which this happened is still unknown and a topic of intense research today. The fact to consider, though, is that all the samples of the same age always show the same polarity. Periods of time when the polarity of the magnetic field is like today's (compass' needle points to the geographic North) are called periods of normal magnetism, and periods when the polarity is opposite to today's are called periods of reverse magnetism (the compass needle would point to the South).
The proof for seafloor spreading. Having explained normal and reverse polarity, let us see how these findings influenced the thought about Plate Tectonics.
An important advance came with Vine and Matthews' study of sections of ocean floor traversing MORs. They found that the ocean floor had a characteristic striped pattern of normal and reversely magnetized rocks. The stripes are parallel to the MOR and symmetric with reference to it. This pattern surprised them, but even more surprised they became when the radiometric dating of samples corresponding to the different stripes of the sea floor, showed an equally symmetric distribution for the ages. Vine and Matthews went public with the proof for seafloor spreading in 1963. Their explanation for the symmetric patterns is as follows: basaltic magma rises from the asthenosphere and is extruded along central fissures at the MORs. The basalt crystallizes with the "printed" paleomagnetism of that time. Later, a new batch of basalt from the asthenosphere fractures the previous one, pushes it toowards the sides, and crystallizes in the center of the MOR. This process, repeated over geologic time, causes a striped pattern of paleomagnetism and radiometric ages in the basaltic oceanic lithosphere created at the MOR. The oceanic lithosphere grows outward from the MOR, its rocks becoming older as they move away from the MOR. The movement is possible without causing any strong deformation on the ocean floor because the rigid lithosphere, as a unity, moves away riding on a layer of partially molten plastic asthenosphere. This is the mechanism that Wegener failed to explain!.
Subduction. If the oceanic lithosphere is created at MORs and the external diameter of the Earth does not increase, there must be a place where the lithosphere is being destroyed. In fact, that is what happens in the deep trenches of the oceans. The oceanic lithosphere sinks back into the asthenosphere in a process called subduction. The Theory of Plate Tectonics indicates that the outer, rigid layer of the Earth, the lithosphere, is fragmented in seven large plates and several smaller ones which ride like on a conveyor belt away from the mid-oceanic ridges, on top of the partially molten plastic asthenosphere.
Plate boundaries. The places where two plates come together or are separated are places that see a lot of action. Stresses build up and magma comes to the surface, producing two of the most awe inspiring geologic processes: earthquakes and volcanism, which will be discussed in next two lessons. Other events also occur, but they are not so obvious because they happen over a long period of time and, therefore, they are not perceived, like mountain building.
There are three basic types of plate boundaries: divergent boundaries, convergent boundaries and transform boundaries.
Divergent boundaries. These are places where two plates move away from each other, and where oceanic lithosphere, basaltic in composition, is created. Midocean ridges are examples of divergent boundaries. At midocean ridges, the predominant type of stress is tensional, resulting from the pressure from the basaltic magma from the asthenosphere oozing out along deep fissures it had previously cracked open.
Convergent boundaries. As you probably figured out, convergent
boundaries are places where two different plates meet. The predominant
kind of stress in this case is compressive. The nature of the lithosphere
at the leading edges of the plates determines the geologic processes to
expect at each particular kind of convergent boundary. There are three
possible situations:
a. Convergence of oceanic lithosphere in one plate with oceanic lithosphere
from the other plate. The cold oceanic lithosphere is denser than the asthenosphere.
The compression caused by convergence will force one of the plates down
into the asthenosphere: it will be subducted. As oceanic lithosphere is
brought down, its temperature increases and it might reach its melting
point. Magma will be generated, which, being hot is less dense than the
asthenosphere and therefore will ascend to the surface producing a volcanic
island arc in the middle of the ocean.
b. Convergence of a plate with oceanic lithosphere with another plate
with continental lithosphere. Compression will develop again in this boundary.
Of the two plates, the one to be subducted is the plate with oceanic lithosphere.
The continental lithosphere is far less dense than the asthenosphere and
will be buoyant on it, and therefore, will not undergo subduction. Again,
the oceanic lithosphere will go down and generate magma which will ascend,
this time through the continental crust. In its way up through the continental
crust, it will melt partially continental rocks and produce andesitic magma.
The result of this process will be a continental volcanic arc developed
on the continent above the subduction zone.
c. Convergence of continental lithosphere with continental lithosphere:
This case results in continent-continent collision. Because both plates
have continental lithosphere on their leading edges, none of them will
be brought down by subduction; their density is too low compared to the
asthenosphere's, and they will not sink. Large compressive stresses from
the collision of the continents will cause regional metamorphism, intense
folding of the lithosphere and considerable uplift, making the highest
mountain ranges of the time. Any melts formed will come from the continental
lithosphere and will be granitic in composition; they will not reach the
surface though, crystallizing in depth to form plutonic rocks. Volcanism
and subduction are absent in this type of boundary.
Note that the processes decribed above are in agreement with the fact that the rocks from the ocean floor are much younger than the rocks in the continents. There are no rocks in the oceanic lithosphere older than Jurassic, because older oceanic lithosphere has already been subducted. The continents though, mainly granitic in composition, remain always "afloat", and that is the reason why rocks with ages as old as 3.8 billion years can be found in the old cores of the continents, but not in the oceanic basins.
Transform boundaries. In transform boundaries lithosphere is neither created nor destroyed. Plates move past each other in opposite directions. Friction among them causes shear stress. Transform boundaries are characterized by transform faults: fractures in the lithosphere along which displacement occurs in the horizontal plane. These faults are usually found across mid-oceanic ridges, where they produce off-set of the submerged mountain chain. The area of San Francisco and Los Angeles owes its intense earthquake activity to the shear stress generated at the San Andreas Fault, a transform fault, that runs through them. One side of this fault, the Western portion of California which belongs to the Pacific Plate, is slowly moving towards the Northwest, whereas the other side with Eastern California belongs to the North American plate and is moving Southeast.
Mechanism of plate movement. The mechanism responsible from plate movement is a system of convection cells by which the Earth is continuously loosing heat to space. What heat? Heat that still remains from accretion times and heat that is generated by the decay of naturally radioactive elements like uranium, thorium and potassium. Rock materials in the convection cells follow a path similar to the one of water in a boiling pot. The bottom layer, closer to heat source, becomes hot and is melted. Hot melts are less dense than the cooler material above and start to ascend. They reach the surface at the Midoceanic ridges, where they solidify, cool and are pushed laterally by a new bath of uprising magma. The cooled rock at the surface looses its heat and becomes denser, dense material sinks back to the bottom of the convection cell along subduction zones. Then, the cycle starts again. Current research is focussed in finding out if convection affects only the asthenosphere or the whole mantle.
Hot spots. Even though it is not known how extensive convection in the mantle is, it seems to have deep regions where there is an anomalously high amount of heat, possibly caused by higher concentrations of radioactive elements. Those regions produce large amounts of magma, and are characterized at the surface by continuous basaltic volcanism. Such a place receives the name of hot spot. Hot spots have been stationary through vast spans of geologic time. They seem to be rooted in the mantle and do not move with the plates. The movement of lithospheric plates over hot spots have created long chains of old extinct volcanos which end in an active one, in the location of the plate currently over the hot spot. This is the case of the Hawaiian Islands, Yellowstone National Park, the Galapagos Islands and the island country of Iceland. This last case is slightly different. The Iceland's hot spot is located in the Atlantic Midocean Ridge, a plate boundary that does not move, therefore all the magma accumulates in a single spot, forming a huge volcanic island. Check the locations of Iceland and Hawaii in an atlas and observe where they are located in the map of your textbook that represents the boundaries of the plates.
Velocity of plate motion. The average rate of plate motion is about an inch a year, it is a rate similar to the speed at which the nails in you hand grow. In some places like the Pacific rise it is much faster, about six an a half inches per year.
How do you measure it? To calculate a velocity you need to have the distance covered by the moving object and the amount of time involved in covering that distance. Mid oceanic ridges and hot spots offer excellent situations to obtain this kind of information. Because new rock is formed at mid oceanic ridges or hot spots, if you sample rocks formed at those points, obtain their ages by radiometric dating and measure the distance from their present locations to the places of origin, you can calculate the velocity of the movement.
The tectonic rock cycle. In comparison to Mars, Mercury and the
moon which have lost all their heat a long time ago, the Earth is still
hot, and therefore, is a very dynamic system. The processes like convection
that started early in its history are still going on at a global scale.
Given that Plate Tectonics continuously reworks rocks between the stages
of creation, shifting and destruction of lithospheric plates, the rock
cycle can be explained as a consequence of Plate Tectonics.
EARTH RESOURCES AND ENVIRONMENTS
PHYSICAL PROPERTIES OF GEOLOGIC MATERIALS: REVIEW
1. Properties of geologic materials: Types of stresses.
2. Stress-Strain curve: elastic deformation, plastic deformation,
rupture, elastic limit, plastic limit. Ductile materials, plastic materials,
brittle materials. Fatigue.
3. How do heat and pressure affect the behavior of geologic materials?
PLATE TECTONICS: REVIEW TOPICS AND QUESTIONS
1. Sources of the heat that drive earth's internal systems.
2. What portions of the earth does Plate Tectonics affect?
3. Define Lithosphere and Asthenosphere: layers of the earth
according to the "physical behavior" of materials. Characteristics. Know
their thickness and relationships with the "chemical composition" layers
of the earth: core, mantle and crust.
4. Evidences for plate movement: examples. Scientists that researched
on Continental Drift and Seafloor Spreading.
5. Paleomagnetism. The magnetic field of the earth. Curie point.
Magnetic reversals. Polar wander curves. Know how to explain them.
6. Types of plate boundaries: convergent, divergent and transform.
Types of stress, faults and magmatism related to each type of boundary.
Examples from over the world and the US.
7. Mechanism of plate movement. Velocity.
8. Methods to measure velocity of plate movements: necessary
data, how it is collected and used.
9. Hot spots: What is a hot spot? Current and past hot spots
of the world.
10. Plate tectonics and Radioactive waste. (Read Box in your
textbook).
11. The rock cycle from the plate tectonics point of view.