(10)  Solid Earth. The student knows that plate tectonics is the global mechanism for major geologic processes and that heat transfer, governed by the principles of thermodynamics, is the driving force. The student is expected to:

(a)  investigate how new conceptual interpretations of data and innovative geophysical technologies led to the current theory of plate tectonics;

(b)  describe how heat and rock composition affect density within Earth's interior and how density

influences the development and motion of Earth's tectonic plates;

There are many forms of energy in our Universe, only some of which we have learned about so far. The principles of thermodynamics govern how and why these energy forms are transferred.

Law 1: Conservation of Energy.

The total amount of energy in the universe is constant. This means that all of the energy has to end up somewhere, either in the original form or in a different form. Tell one example that illustrates the first law that we’ve learned about so far.

Law 2: Entropy

Disorder in the universe always increases. As the disorder in the universe increases, the energy is transformed into less usable forms. Thus, the efficiency of any process will always be less than 100%. Tell one example that illustrates the second law that we’ve learned about so far.

Law 3: Law of Absolute ZeroAll molecular movement stops at a temperature we call absolute zero, or 0 Kelvin (-273oC). Since temperature is a measure of molecular movement, there can be no temperature lower than absolute zero. Since we don’t really focus on this 3rd Law, you really won’t find an example illustrating this that we’ve learned…however, you should still be familiar with it!

This law states that if object A is in thermal equilibrium with object B, and object B is in thermal equilibrium with object C, then object C is also in thermal equilibrium with object A.

This law allows us to build thermometers. For example, the length of a mercury column (object B) may be used as a measure to compare the temperatures of the two other objects.

Zeroth Law:

What we have learned before about convection currents within the mantle, should help you to make a connection with the science of how the Earth’s tectonic plates move about the surface.

You can see the hot plumes here, formed at the mantle-core boundary.

These plumes not only form the Earth’s “hot spots”, but they are responsible for helping to form the convection currents within the asthenosphere which drive the plates.

• The current scientific theory that explains the large-scale movements of the lithospheric plates.

developed during the early 1900s, and later accepted by the majority of the scientific community when the concepts of seafloor spreading were developed in the late 1950s and early 1960sLet’s start at the beginning…with Alfred Wegener

It’s not easy being a scientific genius, as Wegener found out early on! He was met with:

“Utter, damned rot!” “If we are to believe the continental drift hypothesis, we must forget everything we have learned in the last 70 years and start all over again,” Anyone who “values his reputation for scientific sanity” would never dare support such a theory”

As a Meteorologist, • he pioneered the use of balloons to track air circulation• became the first to use kites and tethered balloons to study the polar atmosphere as well.

Wegener began as a meteorologist, and studied climate patterns on Earth, largely in Greenland and polar regions. It is here he observed the movement of icebergs and glaciers.

• Early 20th century geologists viewed continents as fixed features that could rise and fall, but not move around. • Slow shrinking of the Earth was considered to be the cause of mountain building.• Connections of former land bridges and seaways could explain all stratigraphy and distributions of fossils. This was essentially the Atlantis myth of appearing and disappearing continents.

Wegener noted several inconsistencies.

• Close geographic fit of South America and Africa--like a torn newspaper.

• Narrow mountain belts restricted to continental margins.

• Isostasy of crust at two levels--oceanic and continental.

• Distribution of certain fossils, such as Mesosaurus.

• Distribution of ancient climatic indicators does not make sense.

• In 1912 the meteorologist Alfred Wegener described what he called continental drift, which spurred a debate that would end up fifty years later in the theory of plate tectonics.

• Wegener thought that the present continents once formed a single land mass (which we now call Pangea) that drifted apart, thus releasing the continents from the Earth's mantle and likening them to "icebergs" of low density granite floating on a sea of denser basalt.

The biggest problem for Wegener was that he had no convincing mechanism for how the continents might move. Wegener thought that the continents were moving through the Earth's crust, like icebreakers plowing through ice sheets, and that centrifugal (caused by Earth’s rotation) and tidal forces were responsible for moving the continents.

During the 1940s, scientists began proposing that convection currents might have driven the plate movements, and that spreading of the continents may have occurred below the sea within the oceanic crust.

• One of the first pieces of geophysical evidence that was used to support the movement of lithospheric plates came from paleomagnetism. This is based on the fact that rocks of different ages show a

variable magnetic field direction. The magnetic north and south poles reverse through time, and, especially important in paleotectonic studies, the relative position of the magnetic north pole varies through time.

Initially, this phenomenon was explained by what was called "polar wander" . It was assumed that the north pole location had been shifting through time, and that the plates and continents were “fixed”. An alternative explanation, though, was that the continents had moved relative to the north pole. This, of course, turned out to be true, and the “Theory of Plate Tectonics” was born.

We already know that the Earth’s interior is made up of progressively more and more dense material, as you travel towards the core. It also gets hotter, and hotter. Both of these facts influence the motion of Earth’s tectonic plates.

You are also aware that the continental crust (largely made up of granite) is less dense than the oceanic crust (largely made up of basalt).

When the new crust forms at mid-ocean ridges, this oceanic lithosphere is initially less dense than the underlying asthenosphere, but it becomes denser with age, as it conductively cools and thickens. The greater density of old lithosphere relative to the underlying asthenosphere allows it to sink into the deep mantle at subduction zones, providing most of the driving force for plate motions. (Ridge push, Slab pull)

Pillow Basalt

Although subduction is believed to be the strongest force driving plate motions, it cannot be the only force since there are plates such as the North American Plate which are moving, yet are nowhere being subducted.

Three Thoughts:

Mantle Dynamics

Gravity Dynamics

Rotation DynamicsWas Wegener right? Does the Earth’s rotation influence tectonic plate motion at all? There is new scientific evidence that centrifugal force, which causes Coriolis Effect, influences plate direction.

Ridge push, Slab pull. Remember, gravity dictates these two geological forces. At the mid-ocean ridge, gravity helps to “push” the plates apart. At convergent zones, gravity “pulls” one plate beneath the other. Slab pull is stronger than ridge push, by far.

Large scale convection currents in the upper mantle which are transmitted through the asthenosphere as the main driving force of the tectonic plates This theory was launched by Arthur Holmes

Sometimes called ridge push, because it happens near oceanic ridges, Slab-pull is one of the proposed mechanisms for plate motion in plate tectonics.

Because mid-ocean ridges lie at a higher elevation than the rest of the ocean floor, gravity causes the ridge to push on the lithosphere that lies farther from the ridge

Slab-push is the force caused by movement of the cold dense lithosphere into the asthenosphere at destructive (or convergent) boundaries.

The oceanic basalt crust is denser than the continental crust, and so it subducts beneath it, and is “sucked” under by gravitational forces.

In 1971, geophysicist W. Morgan proposed the hypothesis of mantle plumes.

In this hypothesis, convection in the mantle transports heat from the core to the Earth's surface in thermal columns, known as “plumes”.

Mantle plumes carry heat upward in narrow, rising columns, driven by heat exchange (thermodynamics) across the core-mantle boundary.

Core

Mantle

Lithosphere

Plume

Heat Exchange

These “hot spots” occur along plate boundaries, as well as mid-plate, such as with the Hawaiian Islands.

This cycle is nothing more than an “updated” version of the rock cycle, that includes plate tectonic theory. It was developed by J. Tuzo Wilson in the late 1950s and 60s. The problem with the traditional rock cycle is that it implies that rocks just cycle endlessly from one to the other, as you can see from this simplistic diagram. James Hutton in his Law of Uniformitarianism envisioned earth processes cyclically but never getting anywhere, never evolving.

The Wilson cycle, however, contains an evolutionary component: that is, it is not just cyclical, but it is cyclical with direction.

In the Wilson Cycle, there are 9 stages (or A-I). If you were to compare the Wilson Cycle Stage A continent with the Wilson Cycle Stage I continent, you would find stage I is a far more complex continent containing more felsic igneous rock because of all the rock evolution taking place in the Wilson cycle.

Stage A

Stage I

The Wilson Cycle simply depicts the rocks on planet Earth as having gone through a series of changes, and evolutions, and not a simple cycle, and describes the movement and formation of these rocks in terms of plate tectonics.

The student knows that plate tectonics is the global mechanism for major geologic processes and that heat transfer, governed by the principles of thermodynamics, is the driving force. The student is expected to:

• C) Explain how plate tectonics accounts for geologic processes and features, including sea-floor spreading, ocean ridges and rift valleys, subduction zones, earthquakes, volcanoes, mountain ranges, hot spots, and hydrothermal vents;

• E) distinguish the location, type, and relative motion of convergent, divergent, and transform plate boundaries using evidence from the distribution of earthquakes and volcanoes

How Plate Tectonics Accounts For:

Sea-Floor Spreading is the process in which the ocean floor is extended when two plates move apart.  As the plates move apart, the rocks break and form a crack between the plates. 

•Earthquakes occur along the plate boundary.  •Magma rises through the cracks and seeps out onto the ocean floor like a long, thin, undersea volcano.

How Plate Tectonics Accounts For:

As magma meets the water, it cools and solidifies, adding to the edges of the plates. 

•As magma piles up along the crack, a long chain of mountains forms gradually on the ocean floor.  This chain is called an oceanic ridge.  •When these boundaries form on the continents, they are called Rift Valleys.

The boundaries where the plates move apart are 'constructive' because new crust is being formed and added to the ocean floor. An example of an oceanic ridge is the Mid-Atlantic Ridge.  It is one part of a system of mid-oceanic ridges that stretches for 50,000 miles through the world's oceans. 

How Plate Tectonics Accounts For:

As we’ve already learned, the Earth's tectonic plates can move apart, collide, or slide past each other. The Mid-Ocean Ridge system - the Earth's underwater mountain range - arises where the plates are moving apart. As the plates part, the seafloor cracks. Cold seawater seeps down into these cracks, becomes super-heated by magma, and then bursts back out into the ocean, forming hydrothermal vents.

These hydrothermal vent communities have evolved some of the most interesting communities of organisms on Earth…perhaps showing us the place where life on Earth originated.

How Plate Tectonics Accounts For:

When two oceanic plates collide, the younger of the two plates, because it is less dense, will ride over the edge of the older plate. Oceanic plates grow more dense as they cool and move further away from the Mid-Ocean Ridge

The Marianas Trench, where the enormous Pacific Plate is descending under the leading edge of the Eurasian Plate, is the deepest sea floor in the world.

The Challenger Deep at the trench’s southern end measures nearly 7 miles deep. If Mount Everest was set down in the Pacific at this place, there would still be well over a mile of water left above it.

The older, heavier plate plunges steeply through the athenosphere, and descends into the Earth, where it forms a trench that can be as much as 70 miles wide, more than a thousand miles long, and several miles deep.

Only three descents into the trench have ever been achieved. The first was the manned descent by Trieste in 1960. This was followed by the unmanned ROVs Kaiko in 1996 and Nereus in 2009. Convergent boundaries are referred to as “destructive”.

How Plate Tectonics Accounts For:

One need only look at a map of tectonic plates, and one of earthquake activity, to see the close association.

If we were to remove the clutter of the minimal earthquakes, and only show those equalling 7 or higher on the Richter scale, you can see that all major earthquakes have been centered along tectonic boundaries.

An earthquake, also known as a quake, tremor or temblor, is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquake intensity is measured most commonly in terms of the “Richter Scale”, where each increase in whole number, yields a10-fold increase in intensity. So, a magnitude 7 earthquake is 10 times stronger than a 6, and 100 times stronger than a 5…etc.

Japan 2011Haiti 2010

China 2008

How Plate Tectonics Accounts For:

A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, volcanic ash and gases to escape from below the surface

Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together.

Iceland Volcanoes

By contrast, volcanoes are usually not created along transform boundaries, but they may form where there is stretching and thinning of the Earth's crust in the interiors of plates, such as in the East African Rift, and the Rio Grande Rift in North America.

This type of volcanism falls under the umbrella of "Plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are hypothesized to arise from upwelling of magma from the core-mantle boundary, 3,000 km deep in the Earth.

Three Sisters, East of Albuquerque NM, the newest addition to the Rio Grande Rift Volcanic chain.

How Plate Tectonics Accounts For:

Although Hawaii is perhaps the best known hotspot, others are thought to exist beneath the oceans and continents. More than a hundred hotspots beneath the Earth's crust have been active during the past 10 million years. Most of these are located under plate interiors, but some occur near diverging plate boundaries. Some are concentrated near the mid-oceanic ridge system, such as beneath Iceland, the Azores, and the Galapagos Islands.

A few hotspots are thought to exist below the North American Plate. Perhaps the best known is the hotspot presumed to exist under the continental crust in the region of Yellowstone National Park in northwestern Wyoming. There are several calderas (large craters formed by the ground collapse accompanying explosive volcanism) that were produced by three gigantic eruptions during the past two million years, the most recent of which was 600,000 years ago.

As you can see from prior ash layers, the area affected by a volcanic eruption would be immense.

How Plate Tectonics Accounts For:

When two continents carried on converging plates ram into each other, they crumple and fold under the enormous pressure, creating great mountain ranges.

The highest mountain range in the world, the snow-capped Himalayas, is an example of a continent-to-continent collision. This immense mountain range began to form when two large landmasses, India and Eurasia, driven by tectonic plate movement, collided. Because both landmasses have about the same rock density, one plate could not be subducted under the other.

The pressure of the colliding plates could only be relieved by thrusting skyward.

The existence of linear mountain chains on the Earth makes the Earth unique in the solar system.

Although there is volcanism on Venus and Mars and on some of the larger moons, there is no evidence of linear mountain chains.

Linear mountains suggest the movement of a plate boundary and the existence of active plate tectonics

We’ve already shown evidence of convergent and divergent boundaries by looking at the earthquake/volcanic activity maps. What about transform boundaries?

Since transform boundaries are “sliding” past one another, they are obviously a place of great earth movement, and areas of high earthquake activity.Map of midoceanic ridges (red) and deep-sea trenches (blue).

Transform faults are shown in black, cutting the ridges.

Many earthquakes occur along transform plate boundaries.

Three types of transform faults:

• Ridge-ridge

• Ridge-trench

• Trench-trench

are by far the most abundant. The active displacement on these faults occurs only between the ridge segments, as shownabove. No movement occurs along the rest of the fracture zone.

are much less common. They form an important connection between diverging and converging plates. The longest transform faults are all of this kind.

are also rare. These faults appear to connect trenches together.

Transform faults can connect convergent and divergent plate boundaries in various combinations. In all cases, the trend (or movement) of a transform fault is parallel to the direction of relative motion between plates.