Unit 3 Plate Tectonics

1 Introduction to
2 Tsunamis of
   the Past
3 Plate Tectonics
4 Tsunami Generation
5 Tsunami Propagation
6 Tsunami Inundation
7 Tsunami Aftermath
   and Response

3.1 What is a Scientific Theory?


Unit 3 introduces the concepts and vocabulary that describe plate tectonics, the processes that influence the movement and shaping of Earth’s surface. . In addition to covering basic Earth science, learners will gain a deeper understanding of how supported hypotheses contribute to a scientific theory or suggest further research.

Essential Question: What is the theory of plate tectonics?
Enduring Understanding: The movement, formation and interaction of continental and oceanic plates are part of Earth’s dynamic, ongoing and continuous geologic processes.

A Common Misunderstanding

What is a Scientific Theory?

A scientific theory is completely different than the word “theory” used in popular vernacular, which means nearly the opposite: a conjectural explanation, or to put it more simply, an idea.

Scientific theories are not guesses or random ideas. A scientific theory is a collection of well-supported hypotheses based on repeated testing that explains some aspect about the natural world.

Scientific theories give structure to explanations and interpret the natural world. However, all science is iterative and scientific theories are open to change to accommodate new data. The process is creative and messy: theories and facts are not rungs on a hierarchy of increasing certainty.

Below are several terms commonly used in science with explanations using the Universal Theory of Gravity as an example.
Fact Observations generally agreed to be correct. An example would be “a dropped object will fall to the ground.”
Hypothesis A testable potential answer to a question based on an observation.
Law Generalizes a body of knowledge but does not explain. An example would be the formula for Newton’s Law of Gravity F = mg (F= force, m= mass, g = acceleration due to gravity). This equation explains the behavior of a dropped object but does not explain why it falls. 
Observation A natural phenomenon noticed through the senses or through tools and technology.
Prediction What the investigator expects to occur in an experiment if his or her hypothesis is correct.
Scientific Theory A collection of well-supported hypotheses based on repeated testing that explains a fundamental question in science.


3.2 Earth's Surface Then and Now: Wegener's Theory

A Scientific Revolution

A scientific revolution occurred in the last century regarding the movement of continents and Earth’s tectonic processes: Continental Drift Theory, a precursor to today’s Theory of Plate Tectonics, stated that Earth’s continents are in motion, in contrast to previous conceptions of Earth’s surface as stationary and immobile.

Alfred Wegener was an interdisciplinary scientist trained in meteorology and climatology in the early 20th century. He is best known for the theory of continental drift. Now widely regarded as one of the most fundamental Earth science theories, it was initially greeted with ridicule and skepticism.

Wegener’s curiosity was piqued when he read a scientific paper that identified identical plant and animal fossils in South America and Africa, continents separated by the Atlantic Ocean. This began the start of his idea that separate landmasses were once connected and moved apart over time.

He gathered a large amount of supporting evidence, including observations of similar large scale geologic features that are currently separated, such as the Appalachian Mountains on the East Coast of North America and the Scottish Highlands. To support his idea, he also cited fossil evidence of climate that differs from climate today, pointing to fossilized tropical plants found in the Arctic.

Wegener claimed that all the continents were once a single land mass called Pangaea, which means “all the Earth” in Greek. While Wegener was not the first scientist to suggest the continents were once connected, he was the first to analyze and publish evidence from several different scientific fields to support his claim.

<Click Image to Enlarge

The Theory of Continental Drift Became the Theory of Plate Tectonics

Wegener did not know how to explain what causes plate movement.
He incorrectly proposed continental plates ploughed through the ocean floor. In the 1950’s and 1960’s new research found evidence of ocean floor spreading observed through magnetic reversal bands and oceanic plate subduction zones. Geologists were able to add these new discoveries to Wegener’s theory and develop the modern plate tectonic theory.


3.3 Geosphere

Earth as We Know It

Earth’s atmosphere, hydrosphere, biosphere and geosphere interact and produce the dynamic planet we live on. The geosphere refers to the solid Earth and extends from the surface to the center.

The geosphere is divided into layers based on their physical properties, or how the different materials behave. For example, the lithosphere is the rigid outer layer, and below it lies the malleable asthenosphere. Physical properties such as density, malleability (brittle or pliable), viscosity and melting point help differentiate one layer from another.

Another way to think about the layers is to differentiate between the chemical properties of the layers: the core made up of iron and nickel, the mantle made up of high-density rock, the crust made up of low-density rock, etc.

Earths Layers (click to enlarge)

Lithosphere: Solid and Brittle

The lithosphere includes the crust and uppermost mantle and is defined by rigid and brittle physical composition and relatively cool temperature compared to the lower mantle and core.

The Crust: Continental and Oceanic Plates

The crust is the relatively thin rocky outer layer covering the surface of Earth. Geologic features such as mountain ranges, ocean floor, islands and continents are all physical features of the crust. 

The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. The continental crust is more granitic in composition and less dense (2.7 g/cm3) than the oceanic crust (3.3 g/cm3) which is more basaltic in composition.  Continental crust is also much thicker (25-70 km) than oceanic crust (6-10 km).


The Asthenosphere: Why the Plates Move

The asthenosphere is the upper portion of Earth’s mantle characterized by weak, easily malleable rocks below the cooler and more brittle lithosphere. Material in the upper asthenosphere is at or near its melting point resulting in a weak structure that can be physically detached from the lithosphere. In effect the lithosphere moves independently on top of the asthenosphere. 

3.4 Plate Boundaries

Earth’s surface is comprised of 15 major dynamic plates. Studying the boundaries and plate interactions, whether plates are colliding, grinding past each other or moving away from each other, is key to understanding Earth’s age and material make-up of the crust and interior. Tsunami activity often occurs along oceanic convergent (colliding) boundaries subduction zone areas where one plate is subsumed under another. Studying convergent and transform (strike-slip) boundaries helps scientists predict and understand earthquakes, which if under or near water may produce tsunamis.

3.5 The Mechanism: Discovering How Plates Move

In the past fifty years, researchers developed the Theory of Plate Tectonics by identifying the movement, boundaries and interaction of Earth’s plates. Wegener’s Theory of Continental Drift was the basis for initial research. This new research found evidence of the mechanism for plate movement, described below.

Convection and Subduction

The most recent accepted hypothesis for what causes plate movement are convection currents, the unequal heating in Earth’s interior.  Much like bringing a pot of water to boil, unequal heating produces convection currents.

Convection currents propel the hot and less dense material in the mantle slowly upward, while the cooler and denser oceanic plates slowly sink into the asthenosphere. This cycle of seafloor spreading and oceanic plate subduction moves Earth’s tectonic plates.


Mapping the Ocean Floor

Geologic features humans are accustomed to observing on the surface, such as mountains, valleys and plateaus, also exist in the ocean. Mapping these features increases our understanding of plate formation and movement.    


Harry Hess, a geologist and Navy commander during World War II, made maps of the ocean floor. Through these and other investigations, he mapped out a ridge of mountains that circles Earth’s oceans. In 1960 he proposed that ocean floor spreading was the mechanism for how continental plates move and how new plate material is formed. Discoveries such as magnetic reversal lines and GPS tracking of plate movements have further supported seafloor spreading and the Theory of Plate Tectonics.

Age of the Seafloor

During the process of mapping the ocean floor with magnetometers, scientists discovered a regular pattern of “normal” and “reverse” polarity leading away from the Mid-Atlantic Ridge. Surprised, scientists realized that this striping effect indicated that new sea floor was produced at a regular rate as cooling magma is magnetized with the existing magnetic field. Known as magnetic reversal bands.

Scientists discovered that cooling magma contains a paleomagnetic signature. Earth’s magnetic field periodically reverses polarity. Magnetic fields are recorded in cooling magma and lava containing ferrous minerals.

In the early 1970’s the National Science Foundation funded a deep-sea drilling project the third phase consisted of drilling and taking ocean floor core samples between South America and Africa. When the core samples were dated they showed the youngest material to be near the Mid Atlantic Ridge and older material further from the ridge. This work produced a timeline that scientists were able to interpret to determine the age of the constantly renewing seafloor.


Sea Floor Spreading

As molten magma rises up from the mantle, it encounters cold seawater; the cooling magma expands, pushing the plates apart. This spreading process moves continents several centimeters per year and is identified as a Mid-ocean Ridge that circles Earth. 

The magma forms basalt, a dense rock, which as it cools records the current polarity. Periodically, the polarity of the planet reverses, and these reversals are recorded in the seafloor.

3.6 Review

Take the following practice quiz to review content covered in Unit 3.

  1. Which modern day theory was built upon on Wegener's Continental Drift Theory?

  1. What creates most earthquakes and volcanoes?
    The difference in age between tectonic plates.

  1. What was one piece of evidence Alfred Wegener cited to support his Pangaea hypothesis?

  1. What is "a collection of well-supported hypotheses based on repeated testing that explains some aspect about the natural world"?

  1. What evidence did scientists discover that indicated that new sea floor is produced?

  1. What causes Earth's tectonic plates to move?

  1. What happens to the younger, denser oceanic plates when they collide with older, less dense continental plates?

  1. What is a convergent plate boundary?

  1. What type of plate interaction formed the Himalayas?

  1. What kind of geologic activity occurs frequently along transform boundaries?