Week 7
Plate Tectonics
Click on the link below to hear a mini-lecture audio segment.
Plate tectonics is arguably the most important scientific revolution of the 20th century. It is so compelling because it provides and explanation for practically all of the things we observe on the surface of the earth, and those processes that we observe originating below the surface of the Earth. the earth is covered with 14 major plates that move around the surface of the Earth and have done so for over 500 million years.
Plate Boundaries Fit
Click on the link below to hear a mini-lecture audio segment.
Plate tectonics was first proposed in the 1800's by Antonio Snyder-Pellegrini based on the fact that the continents of the Earth fit together like pieces of a jigsaw puzzle. Based on the fit, he proposed that the continents were once together, forming a single large landmass.
Other Plate Tectonics Evidence
Alfred Wegener in the 1920's first proposed the concept of "Continental Drift" based on evidence he found on the margins of continents.
If you look at the continental boundaries there is much overlap of fossil assemblages, structures, and rock types up to the time in geologic history when the plate split apart.
If you look at glacial deposits from the past, before the split, there is also overlap.
Now, see if you can solve for the fit of a hypothetical groups of plates that were once part of a megacontinent, but have since broken off into four distinct plates.
Plate Movement History
Mechanisms of Plate Movement
Plate Boundaries
Click on the link below to hear a mini-lecture audio segment.
Convergent Boundaries
Convergent boundaries are plate boundaries in which one plate (Oceanic) is pushed down under another (Continental) plate. Oceanic crust is destroyed at this type of boundary. There are three types of convergent boundaries.
Ocean-continent
Ocean-ocean arc
Continental collision
Things Found at Convergent Boundaries:
Metamorphism
Volcanism
Reverse Faulted Mountains
Sedimentary Wedges
Divergent Boundaries
Divergent boundaries occur when plate are rifted apart and begin to move apart, creating large expanses of oceanic crust. Crust is created in this type of boundaries.
Things Found at Divergent Boundaries:
Volcanism
Normal Faulted Basins
Sedimentary Deposits of Arkosic Sandstone
Oblique-Slip Boundaries
Oblique Slip boundaries are boundaries in which one plate slide past another with neither loss or creation of crust.
Things Found at Oblique-slip Boundaries:
Dynamic Metamorphism
Faulting
Minor Igneous Structures (occasionally)
Summary of Plate Boundaries
A number of processes occur at the various types of plate boundaries. These processes result in many of the features we observe on the surface of the earth near these boundaries. In the exercises below, you may test your knowledge of the processes.
Sea Floor Spreading
Click on the link below to hear a mini-lecture audio segment.
The ocean floors provide a great deal of evidence about the mechanisms of plate movement and also provide some of the most compelling evidence in support of plate tectonics. For reasons that are not completely clear, the magnetic field of the earth periodically reverses itself. When the field is normal, the magnetic pole is roughly coincident with geographic north. When the field is reversed, the magnetic pole is south. The magnetic field at the time is preserved in any igneous rock that crystallizes at the time, because the the magnetically susceptible minerals point in the direction of the magnetic pole at the time of crystallization. This results is stripes of normal and reverse polarized stripes of oceanic crust that represent the magnetic field at the time that the particular part of the crust was created. This striping is shown below.
Wednesday, November 12, 2008
♥Pangaea♥
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| Each cell, under a specific grade level contains 3 lesson plans and student worksheets per week. Multimedia activities, web links, and dictionaries can also be found. Lesson plans increase in difficulty through the grades. Each grade level builds knowledge in a logical sequence. Printable version and workbooks can be downloaded by clicking here. Printable version of the below Scope and Sequence click here. | |
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Plate Tectonics Cycle at a GlanceIn the Plate Tectonic Cycle, students learn about the Earth's dynamics as it spins on its axis, revolving around the Sun. The Earth is restless inside, as it tries to cool its interior. Material inside the Earth become viscous and flow in certain areas. Movement within the Earth's interior is reflected on the outside crust. Convection currents inside the mantle (area between the crust and the outer core) create 2 types of crustal movements. When convections currents come together, convergent plate boundaries (earthquakes) are formed on the Earth's crust. When the convection currents pull the crust apart in two different directions divergent plate boundaries (volcanoes and earthquakes) are formed. A consequence of the Earth's surface moving faster along the equator than at the poles creates tension which in part forms transform boundaries.
In the Classroom
Hands-on activities teach students how scientists investigate the Earth through earthquakes and volcanoes. They learn to challenge and think about different theories. Learning about how to cope with the disasters caused by plate tectonics is also emphasized.
OBJECTIVES: | Students reconstruct the super continent Pangaea. |
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According to the theory of plate tectonics, the Earth's crust and upper mantle are broken into moving plates of "lithosphere." The Earth has two types of crust. Continental crust underlies much of the Earth’s land surface. The ocean floors are underlain by oceanic crust. These material have different compositions; the continental crust is like the igneous rock granite, and the oceanic crust is like basalt, another igneous rock.
Students and many adults often equate the geographic continents, i.e., land, with the plates. This is incorrect. The Earth’s various units of continental crust are actually embedded into plates. You may wish to explain this to your students by saying that the continental crust "ride on the back" of a plate. Moreover, continental and oceanic crust are often part of the same plate. For example, the North American plate has continental crust (essentially the land area of North America) at its core and is surrounded on most sides by oceanic crust.
As they move, plates interact at their edges or boundaries. There are three basic directions or types of boundary interactions. In some places, two plates move apart from each other; this is called a diverging plate boundary. Elsewhere two plate move together, which is called a converging plate boundary. Finally plates can also slide past each other horizontally. This is called a transform plate boundary. Volcanoes and earthquakes help define the boundaries between the plates. Volcanoes form mostly at converging and diverging plate boundaries, where much magma is generated. Earthquakes occur at all three types of boundaries. Because the plates are rigid, they tend to stick together, even though they are constantly moving. This builds up stress in the rocks at the plate boundary. When the strength of the rocks is exceeded, they move rapidly, "catching up" with the rest of the plates. We feel this release of energy as an earthquake.
One of the first observations used to suggest that the outer portion of the Earth is mobile is the fit of the continents, particularly the west coast of Africa against the east coast of South America. This observation predates plate tectonics. It was first noticed in the 18th century, and most recently proposed by a German scientist, Alfred Wegener in 1912. Wegener called his theory "continental drift", referring to the apparent movement of continents alone. However, "continental drift" is a only historical term. We now know it is not the continents that move, but the plates, in which the continents are embedded. South America and Africa were once together, but were split apart by the formation of a diverging plate boundary. This is also confirmed by matches between the rocks and fossils of the two continents. The two continents are still moving away from each other today.
A map of Pangaea shortly after it began to split. 160 million years ago
This exercise looks at the continents of North America, South America, Africa, Antarctic, and Australia, and how they have moved over the last 200 million years. At that time, these five continents were all part of a single large super continent, called Pangaea. Starting about 180 million years ago, Pangaea began to break up; new diverging plate boundaries formed within it. This eventually created the continents we see today. In this exercise, the students will reconstruct Pangaea. They will use the fit of the continental crust to put Pangaea back together.
PROCEDURE:
- Remind the students of the information they learned in the Pre Lab. Explain again that the plates are moving, due to convection and gravity. Explain that this movement causes stress within the plates, which generates earthquakes and volcanoes. You may want to show students a map of the plates.
- Review the composition of the plates with the class. Make sure the students understand that the continents make up the non-oceanic part of the crust. Discuss with them that the edges of the continents look as if they may have fit together at one time.
- Have the students label, color, cut out, and fit the continents together. The lines and numbers make this puzzle a little easier. You may want your students to work in pairs. Matching up the continents is not as easy as it looks.
- Once the students have placed the continents together have them move the pieces apart very slowly. They are to move the pieces until they reach their present positions.
- Ask students if they think this movement could have happened. Let them come up with stories about why it took place. Remind them of convection and the moving of the plates. This is a difficult concept to get across to the students.
Plate Tectonics - Plate Tectonics (2)
Evidence from Continents
Pre Lab -
Exploring How the Earth's Outermost Portion Moves Lab -
Exploring the Results of Movement on the Earth's Crust Exploring How Plates have Moved through Time
♥Continental Drift Theory♥
Continental Drift
(source: USGS at http://pubs.usgs.gov/publications/text/historical.html)
In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root "to build." Putting these two words together, we get the term plate tectonics, which refers to how the Earth's surface is built of plates. The theory of plate tectonics states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material. Before the advent of plate tectonics, however, some people already believed that the present-day continents were the fragmented pieces of preexisting larger landmasses ("supercontinents"). The diagrams below show the break-up of the supercontinent Pangaea (meaning "all lands" in Greek), which figured prominently in the theory of continental drift -- the forerunner to the theory of plate tectonics.
According to the continental drift theory, the supercontinent Pangaea began to break up about 225-200 million years ago, eventually fragmenting into the continents as we know them today. Plate tectonics is a relatively new scientific concept, introduced some 30 years ago, but it has revolutionized our understanding
of the dynamic planet upon which we live. The theory has unified the study of the Earth by drawing together many branches of the earth sciences, from paleontology (the study of fossils) to seismology (the study of earthquakes). It has provided explanations to questions that scientists had speculated upon for centuries -- such as why earthquakes and volcanic eruptions occur in very specific areas around the world, and how and why great mountain ranges like the Alps and Himalayas formed.
The belief that continents have not always been fixed in their present positions was suspected long before the 20th century; this notion was first suggested as early as 1596 by the Dutch map maker Abraham Ortelius in his work Thesaurus Geographicus. Ortelius suggested that the Americas were "torn away from Europe and Africa . . . by earthquakes and floods" and went on to say: "The vestiges of the rupture reveal themselves, if someone brings forward a map of the world and considers carefully the coasts of the three [continents]." Ortelius' idea surfaced again in the 19th century. However, it was not until 1912 that the idea of moving continents was seriously considered as a full-blown scientific theory -- called Continental
Drift -- introduced in two articles published by a 32-year-old German meteorologist named Alfred Lothar Wegener. He contended that, around 200 million years ago, the supercontinent Pangaea began to split apart. Alexander Du Toit, Professor of Geology at Johannesburg University and one of Wegener's staunchest supporters, proposed that Pangaea first broke into two large continental landmasses, Laurasia in the northern hemisphere and Gondwanaland in the southern hemisphere. Laurasia and Gondwanaland then continued to break apart into the various smaller continents that exist today.
Wegener's theory was based in part on what appeared to him to be the remarkable fit of the South American and Africancontinents, first noted by Abraham Ortelius three centuries earlier. Wegener was also intrigued by the occurrences of unusualgeologic structures and of plant and animal fossils found on the matching coastlines of South America and Africa, which arenow widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have swum or have been transported across the vast oceans. To him, the presence of identical fossil species along the coastalparts of Africa and South America was the most compelling evidence that the two continents were once joined.
In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching fossil occurrences but also the evidence of dramatic climate changes on some continents. For example, the discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow. Other mismatches of geology and climate included distinctive fossil ferns (Glossopteris) discovered in now-polar regions, and the occurrence of glacial deposits in present-day arid Africa, such as the Vaal River valley of South Africa.
The theory of continental drift would become the spark that ignited a new way of viewing the Earth. But at the time Wegener introduced his theory, the scientific community firmly believed the continents and oceans to be permanent features on the Earth's surface. Not surprisingly, his proposal was not well received, even though it seemed to agree with the scientific information available at the time. A fatal weakness in Wegener's theory was that it could not satisfactorily answer the most fundamental question raised by his critics: What kind of forces could be strong enough to move such large masses of solid rock over such great distances?
Alfred Wegener proposed his theory of continental drift in the early 20th century. It offered an explanation of the existence of similar fossils and rocks on continents that are far apart from each other. But it took a long time for the idea to become accepted by other scientists.
Wegener said continental drift happens when tectonic plates - huge segments of the Earth’s crust and upper mantle, sometimes forming continents - move. They shift at a few centimetres per year, with earthquakes and volcanoes often occurring around their edges.
Wegener’s theory
Alfred Wegener proposed the theory of continental drift at the beginning of the 20th century. His idea was that the Earth's continents were once joined together, but gradually moved apart over millions of years.
Wegener’s evidence for continental drift was:
- The same types of fossilised animals and plants are found in South America and Africa.
- The shape of the east coast of South America fits the west coast of Africa, like pieces in a jigsaw puzzle.
- Matching rock formations and mountain chains are found in South America and Africa.
Before Wegener developed his theory, it was thought mountains formed because the Earth was cooling down, and in doing so contracted. This was believed to form wrinkles, or mountains, in the Earth's crust. If the idea was correct, however, mountains would be spread evenly over the Earth's surface. We know this is not the case.
Wegener suggested mountains are formed when the edge of a drifting continent collides with another, causing it to crumple and fold. For example, the Himalayas were formed when India came into contact with Asia.
Problems
Alfred Wegener (1880 - 1930)
Wegener’s theory of continental drift was rejected by many geologists. Some of the reasons for this were:
- The movement of continents could not be detected - because they only move by a few centimetres per year.
- no-one could provide a good explanation of how whole continents could move apart
- Wegener was not a geologist - he trained as an astronomer and meteorologist
- there were other, simpler, explanations for the same evidence
- it was felt his idea was too big for the evidence at hand
It was only in the 1960s, long after Wegener’s death in 1930, that the theory of continental drift was accepted by scientists.
Consequences of continental drift
Plate tectonics
The Earth’s crust and upper part of the mantle are broken into large pieces called tectonic plates, sometimes forming continents. These are moving constantly, at the rate of a few centimetres each year. Although this does not sound like a lot, over millions of years continents shift thousands of kilometres.
Seafloor spreading
New seafloor material is created when tectonic plates move apart. Molten rock rises between the plates, cools, and solidifies. This process is called seafloor spreading - a consequence of the solid mantle moving.
At the edges of tectonic plates
When tectonic plates meet, the Earth’s crust becomes unstable as they push against each other, or ride under or over one another. Earthquakes and volcanic eruptions occur at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges. It is difficult to predict exactly when an earthquake will happen, or how destructive it will be, even in places that are known for them.
These are some of the things public authorities can do to reduce the damage caused by geohazards such as earthquakes and volcanoes:
- assess how vulnerable local buildings are
- provide education and training for emergencies
- make risk assessments
- monitor natural hazards in the local area
- research the hazards themselves
Watch
You may wish to view this BBC News item from 2006 about the 100th anniversary of San Francisco’s great earthquake.
Seafloor spreading - higher
Magnetic field reversal
Seafloors spread by about 10cm per year. This spreading leaves a characteristic pattern of magnetism in the rocks on the ocean floor that can be "read" by scientists.
The Earth’s magnetic field has not always had the same North-South alignment. Every so often it reverses direction - for thousands or millions of years. Iron-rich minerals in molten magma line up in the magnetic field, and this alignment is preserved when the magma solidifies. Scientists can then 'read' the pattern of magnetic field reversal that forms.
The patterns on either side of a mid-ocean ridge are mirror images of each other. This provides evidence for seafloor spreading and plate tectonics in general.
Tectonic plates - higher
There are three types of boundary between tectonic plates. They can cause earthquakes and/or volcanoes:
Types of plate boundary and their effects
| cause earthquakes? | cause volcanoes? | what they do | |
|---|---|---|---|
| constructive plate boundary | yes | yes | make new crust |
| destructive plate boundary | yes | yes | make mountains |
| conservative plate boundary | yes | no | do not make or destroy land |
Constructive plate boundary
Destructive plate boundary
Now try a Test Bite
Tuesday, November 11, 2008
♥Litosphere♥
Lithosphere
The outer, rigid shell of the earth. It is composed of the entire crust and uppermost part of the mantle. The thickness of the lithosphere varies from around 5km to up to 100km where there is thick continental crust. The lithosphere is above the more ductile asthenosphere. The boundary between these two layer is the Mohorovicic discontinuity.
The lithosphere is fragmented into tectonic plates which move relative to one another. This movement of lithospheric plates is described as plate tectonics.
♥Ionosphere♥
The Ionosphere
This absorption of radiation is also responsible for the Ionosphere. Located within the thermosphere, the ionosphere is made of electrically charged gas particles (ionized). The ionosphere extends from 37 to 190 miles (60-300 km) above the earth's surface. It is divided into three regions or layers; the F-Region, E-Layer and D-layer. During the daytime the F-Layer splits into two layers and recombines at night.
The E-layer was discovered first. In 1901, Guglielmo Marconi transmitted a signal between Europe and North America and showed that it had to bounce off an electrically conducting layer about 62 miles (100 km) altitude. In 1927, Sir Edward Appleton named that conducting layer the (E)lectrical-Layer. Later, discovery of additional conducting layers were simply called the D-layer and F-Layer.
Since the ionosphere existance is due to radiation from the sun striking the atmosphere, it changes in density from daytime to nighttime. All three layers are more dense during the daytime. At night, all layers decrease in density with the D-Layer undergoing the greatest change. At night the D-Layer essentially disappears.
As seen around the 1900's, the ionosphere has the important quality of bouncing radio signals transmitted from the earth. Its existance is why places all over the world can be reached via radio.
As the radio signal is transmitted, some of the signal will escape the earth through the ionosphere (green arrow). The ground wave (purple arrow) is the direct signal we hear on a normal basis. This wave weakens quickly and is what one hears as a fading signal.
The remaining waves (red and blue arrows) are called "skywaves." These waves bounce off the ionosphere and can bounce for many 1000's of miles depending upon the atmospheric conditions.
♥Thermosphere♥
Thermosphere
From Wikipedia, the free encyclopedia
The thermosphere is the layer of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization.
The thermosphere, named from the Greek θερμός (thermos) for heat, begins about 90 km above the earth.[1] At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures are highly dependent on solar activity, and can rise to 1,500°C. Radiation causes the atmosphere particles in this layer to become electrically charged (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 2,000 km above the earth's surface, the atmosphere mixes into space.
The few particles of gas in this area can reach 2,500°C (4532°F) during the day. Even though the temperature is so high, one would not feel warm in the thermosphere, because it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat. A normal thermometer would read significantly below 0°C.
The upper region of this atmospheric layer is called the ionosphere.
The dynamics of the lower thermosphere (below about 120 km) are dominated by atmospheric tide, which is driven, in part, by the very significant diurnal heating. The atmospheric tide dissipates above this level since molecular concentrations do not support the coherent motion needed for fluid flow.
The International Space Station has a stable orbit within the upper part of the thermosphere, between 320 and 380 kilometers. The auroras also occur in the thermosphere.
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