Lecture 3: Plate Tectonics: Hypothesis to Theory
Reading: Chapter 2, pages 30-41 Topics:
· The scientific method
· Evidence for continental drift
· Evidence for seafloor spreading
· Evidence for subduction
· Theory of Plate Tectonics
Next lecture: Plate Tectonics: Mechanisms and Margins
3.1. Introduction
The internal structure of the Earth is a result of two processes: accretion and differentiation. Planetary accretion left the earth in a near molten state as kinetic energy was converted to heat during collision. Once molten the earth segregated into layers of different densities and chemical composition. This left the earth with a light crust, an intermediate mantle, and a dense core. Continued additions of heat released by radioactive decay of unstable elements in the earth have kept the Earth’s interior hot.
The distribution of pressure and temperature inside the earth (which together control a material’s melting point) has left the earth with layers of different physical properties. Layers below the melting point are rigid and strong (i.e., the lithosphere), while layers close to the melting point are plastic and weak (i.e., the asthenosphere). The rigid lithosphere (crust and uppermost mantle) is broken into pieces (plates) that float on the softer, deeper mantle asthenosphere.
Today we will discuss the evidence that lithospheric plates move. This is called plate tectonics.
3.2. Plate Tectonics and the Scientific Method
The development of plate tectonic theory is an excellent example of the scientific method in action. The scientific method is the way researchers work collectively over time to develop an accurate, reliable, and unbiased explanation of the world around us. This is accomplished by repeated observation, testing, and modification.
The scientific method consists of the following steps:
1. Observe: gather information about some phenomenon
2. Hypothesize: attempt to explain the observations
3. Predict: use the hypothesis to make testable predictions
4. Test: gather additional evidence from observations or experiments that may support or refute the hypothesis
5. Modify: modify the hypothesis in light of new evidence.
Repeat steps 3-5 until a hypothesis has been developed that is consistent with all available evidence and conceivable tests.
Once a hypothesis has been accepted by most researchers in a field, it is considered a scientific theory. Hypotheses gradually gain acceptance over many years through repeated testing and modification to become theories. A theory is a hypothesis that has withstood repeated scrutiny over time. However, it is still possible that a new observation will be made or a new experiment devised that produces evidence contradictory to theory. For example, future advances in technology might allow experiments that are inconceivable or impossible today; the results of which may or may not be consistent with established theory. If they are not consistent, a new round of modification and testing is required.
Be aware that the meaning of theory in everyday conversation is vastly different from the meaning of theory in science. We often use the word “theory” to refer to a possible explanation that may be nothing more than an educated guess. But to scientists, a theory is an explanation backed up by and consistent with a vast body of evidence. For example, the theory of evolution is a scientific explanation backed by evidence such as the succession of organisms in the fossil record and genetic relationships among modern organisms and includes defined and observable mechanisms of genetic variation and heredity. Despite the overwhelming evidence supporting evolution (essentially as Darwin originally envisioned it), whether or not evolution should even be taught in science classes is still debated by public school boards.
3.2.1 Discovery of Plate Tectonics
The theory of plate tectonics states that Earth’s outermost layer (crust) is broken into a number of large and small plates. The plates move relative to one another because they rest on top of hotter, more mobile material. The theory of plate tectonics is the foundation for much of our understanding of how the earth works, yet the processes of plate tectonics generally operate too slowly for direct observation. So how do we know that lithospheric plates move? We will discuss three independent lines of evidence:
1. Evidence for Continental Drift
2. Evidence for Seafloor Spreading
3. Evidence for Subduction
Plate tectonics ties these observations together and explains many related geologic phenomena, such as mid-ocean ridges, deep-sea trenches, volcanoes, and earthquakes.
3.3. Continental Drift
Fossil sea shells high above sea level and vertical land movements following earthquakes provided early scientists convincing evidence that the continents can move vertically. But could the continents move laterally as well?
Alfred Wegener first proposed the hypothesis of continental drift in 1915. According to continental drift the modern continents were once joined as parts of preexisting larger supercontinents. Wegener’s hypothesis attempted to explain the jig-saw fit between continents on either side of the Atlantic Ocean. Wegner stated that the continents were once joined as one land mass he called Pangaea which broke apart and the fragments (today’s continents) slowly drifted to their present positions.
This was not a new idea, but Wegner was the first to systematically describe the evidence that the continents were once joined. In addition to the fit of the coastlines, Wegener also noted the distribution of fossils across the southern continents. The same fossils of land animals (early reptiles) and plants (such as ferns) are found on continents that today are separated by vast oceans. This distribution could be explained if the continents were joined at the time these plants and animals lived.
Fossil evidence for continental drift from the southern continents includes reptiles, freshwater crocodiles, and ferns which are found on continents now widely separated by oceans.
Wegner was also aware that the ages and types of rocks along the African and South American coastlines, as well as the European and North American coastlines, were similar. When the
continents were pieced back together, the different rock types lined up nicely. This is illustrated by the distribution of 250 million-year old glacial deposits across the southern continents. The glacial deposits also indicate the direction the ice was moving, and suggest an ice cap centered in South Africa and radiating across the adjoining continents.
Grooves carved by glaciers 250-300 million years ago (arrows) provide evidence for continental drift. The left diagram assumes the continents were in their present day locations. The distribution of the glaciers can best be explained if the continents were once joined as part of Pangaea.
The supercontinent Pangaea (meaning “all lands” in Greek) started breaking up about 200 million years ago. When Pangaea began breaking apart the continents Gondwanaland and Laurasia formed. The Tethys Sea separated the two continents. By 65 million years ago the land masses looked like our modern-day continents.
3.3.1. Rejection of Continental Drift
Wegener’s hypothesis of continental drift did not gain wide acceptance at the time because of an unresolved problem: he could not explain how the continents move through the solid rock of the ocean floor. He thought continents moved through Earth’s crust by tidal forces. Tidal forces are much too weak to move continents! Wegener was wrong on one big point: continents do not plow through ocean floor. Instead both continents and ocean floor are rigid plates that “float” on the asthenosphere.
3.4 Seafloor Spreading
Why was continental drift revisited? Discoveries about Earth’s magnetism in the 1950s and mapping of the ocean floor made possible by technologies developed during World War II eventually provided a mechanism for continental drift called seafloor spreading. Plate tectonics was widely accepted by the late 1960’s.
Technology developed to track German U-boats allowed detailed mapping of the seafloor in peacetime.
3.4.1. Origin of Earth’s Magnetic Field
The molten iron in the Earth’ outer core combined with the Earth’s rotation generates a magnetic field. Molten iron in the outer core flows around the solid inner core because of Earth’s rotation. The flow generates an electrical current, in turn creating a magnetic field. Earth’s magnetic field resembles that of a simple bar magnetic with two poles of opposite polarity. Because the magnetic field is generated by Earth’s rotation, the magnetic poles are within a few degrees of the rotational (geographic) poles.
Earth’s magnetic field resembles a bar magnet with two poles of opposite polarity. The magnetic poles are inclined at
11.50 relative to the rotational (geographic) poles.
Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earth’s magnetic field is not completely understood, but is thought to be associated with electrical currents produced by rotation in the spinning liquid outer core made of iron and nickel. The liquid iron in the outer core is also in motion due to convection, changes in convective currents may account for the observed variability in the strength and polarity of Earth’s magnetic field (Earth’s rotation changes only very slowly over geologic time). Note that Venus does not have a measurable magnetic field despite having a similar liquid iron core because the rotation of Venus (243 Earth days) is too slow to generate an electrical field.
3.4.2. Magnetic Inclination
The magnetic field at any point on the Earth’s surface can be described in terms of its inclination. Inclination is the angle (dip) of the magnetic field lines with respect to the surface of the Earth and varies from 0o (horizontal) at the magnetic equator to 90o (vertical) at the magnetic poles.
Magnetic field inclination varies as a function of magnetic latitude across the Earth’s surface.
Iron-rich minerals align themselves parallel to the magnetic field as they crystallize from molten lava. At temperatures above 580°C the magnetic poles of individual atoms point in random directions. When a magnetic field is present and temperatures are below 580°C, the magnetic poles of individual atoms align with the magnetic field. Now the minerals are permanently magnetized. Therefore rocks containing iron minerals preserve a record of the ambient magnetic field at their time of formation (paleomagnetism) indicating both field polarity (direction to magnetic north) and inclination.
At high temperatures magnetic mineral grains are randomly oriented (left). During cooling the magnetic minerals align themselves with the ambient field (right). Once the temperature drops below the Curie temperature that orientation is locked in place and will not change even if the ambient field changes (so long as the rock is not re-heated).
3.4.3. Apparent Polar Wander
When the magnetic direction and inclination of rocks of different ages from the same continent are plotted on the map, they appear to indicate that the magnetic poles have wandered great distances over time. Furthermore, rocks from different continents appear to show the magnetic poles taking different paths at the same time. This phenomenon is known as apparent polar wander.
The apparent location of the magnetic pole follows vastly different tracks over time when reconstructed using rocks from different continents.
Wandering Pole or Drifting Continents?
The apparent polar wander curves for different continents appear to show a chaotic magnetic pole moving all over the place and even in different places at the same time! Although the magnetic pole does move, it remains within a few degrees of the geographic pole because the magnetic field is generated by the earth’s rotation. This is known as true polar wander. The apparent polar wander curves make more sense if the continents are moving in different directions with respect to the magnetic pole (and each other). This is another line of evidence for continental drift that Wegner was unaware of, but a mechanism for continental drift was still lacking.
3.4.4. Magnetic Reversals
For reasons that are not understood, Earth’s magnetic field occasionally reverses polarity. In other words, the magnetic field flips direction (a magnetic reversal). The polarity or direction of the magnetic field is recorded in rocks in same way that inclination is. Rocks indicating a field direction the same
as today’s (magnetic north pole near the geographic North Pole) are said to have normal polarity. Rocks indicating the opposite field (magnetic north pole near the geographic South Pole) have reversed polarity.
Earth’s magnetic field during periods of normal polarity (left) and reversed polarity (right).
The geologic record provides evidence of 171 field reversals in the last 71 million years. Time is divided into periods of predominately normal polarity or predominately reverse polarity. Periods are called magnetic chrons. Within each chron there can be subchrons (times of opposite polarity). Today we are in a period of normal polarity (the Brunhes chron). The period from 730,000 years ago to 2.45 million years ago was period of predominately reverse polarity called the Matuyama chron. Within the Matuyama chron are three short periods of normal polarity, these are subchrons. The importance of this discovery in understanding continental drift was not realized until the seafloor was mapped.
Polarity reversals dating back 20 million years.
3.4.5 Evidence from the Seafloor
When the seafloor was mapped in the 1950s, several discoveries were made that added to our understanding of continental drift and led to the development of the theory of plate tectonics:
1. The topography of the seafloor was found to be dominated by huge underwater mountain ranges called mid-ocean ridges running through the ocean basins like seams on a baseball.
2. Another curious feature of the ocean basins is the narrow but very deep trenches, most notably along the western margin of the Pacific Ocean.
On this map, the shallower water over mid-ocean ridges is shown in yellow green, the deeper parts of the ocean basins in blue, and the very deep trenches in purple. The shallowest parts pf the oceans, show in red, are actually the flooded, low- lying parts of the continents called continental shelves.
3. The age of rocks on the seafloor shows an orderly distribution. Rocks were youngest along the mid-ocean ridges and progressively older away from the mid-ocean ridges.
4. The thickness of seafloor sediments also supports this age distribution. Sediments are thinner toward the ridge because they have had less time to accumulate.
5. Although the oldest rocks on the continents (4 billion years old) are nearly as old as the earth itself, nowhere on the seafloor are rocks found to be older than about 200 million years.
In this map the youngest seafloor rocks are shown in red and the oldest in blue. Note the symmetry in ages on either side of the mid-ocean ridge system.
3.4.6. Paleomagnetism and Seafloor Spreading
When the magnetic field direction recorded in the rocks of the seafloor was mapped in detail, a striking pattern emerged. Bands of rocks recording alternating periods of normal and reversed polarity are symmetric on either side of the mid-ocean ridge.
This figure shows the magnetic orientation of rocks along a section of mid-ocean ridge south of Iceland. Rocks shown red are rocks that formed during the current period of normal polarity. Other colors show rocks formed during earlier periods of normal polarity. White areas indicate rocks formed during periods of reversed polarity. Note the symmetry with respect to the ridge axis.
The Atlantic Ocean did not exist 250 million years ago! The continents that now border it were joined into a single vast continent that Alfred Wegener named Pangaea. About 200 million years ago, new spreading centers split the huge continent. The Atlantic continues to widen today at 2-4 cm/yr. If the spreading rate is fast, a larger amount of young warm oceanic lithosphere is produced and the ridge will be wider. A slow-spreading ridge will be narrower. The Atlantic Ocean spreads slowly, growing wider at 2-4 cm/yr. The Pacific spreading center is fast by comparison: 6-20 cm/yr. Note the differences in widths of the mid-Atlantic and East Pacific ridges (due to the differences in their spreading rates) on the maps above.
Collectively, these observations of the age distribution and magnetic symmetry of the sea floor led to the idea of seafloor spreading. Seafloor spreading is a result of rising magma at the mid-ocean ridges which cools to form new ocean crust. As new crust is formed the older crust is pushed away from the ridge. This explains both the age distribution of the seafloor and the symmetry with respect to the ridge.
Rising magma creating new crust at the mid-ocean ridge during a period of normal polarity while older crust is pushed away from the ridge (a). Following a period of reversed polarity, crust is once again being formed with normal polarity (b). The resulting pattern of rocks formed during alternating periods of normal and reversed polarity is symmetric about the mid-ocean ridge (c).
3.5. Subduction
Seafloor spreading also provides a mechanism for continental drift. The drifting continents do not plow through the seafloor, instead the seafloor acts as a giant conveyor belt in which the embedded continents are taken along for the ride. However, seafloor spreading creates a new problem: the
amount of new ocean crust created by seafloor spreading must be balanced by equal amount being destroyed elsewhere; otherwise earth’s crust would swell like a balloon! Where is ocean crust being destroyed?
The location of deep earthquakes near the deep-sea trenches provided the answer. Earthquakes are not randomly distributed globally. Adjacent to and inland from deep-sea trenches is one place where earthquakes are concentrated. Earthquakes are also concentrated along mid-ocean ridges.
Earthquakes are not randomly distributed globally. Adjacent to and inland from deep-sea trenches in one place where earthquakes are concentrated.
The origin of earthquakes becomes deeper along one side of ocean trenches with increasing distance from the trench. This marks the descent of old oceanic crust into the mantle as it is destroyed. This process is known as subduction and the places where it occurs are called subduction zones.
Top Left: Earthquakes get progressively deeper to the west of the Tonga trench. Remaining panels: Progressively deeper earthquakes away from the trench (arrow) mark the descent of subducted crust.
3.6. Plate Tectonics: Tying it all Together
The theory of plate tectonics ties the observations of continental drift, seafloor spreading, and subduction together while also explaining related geologic phenomena (such as volcanoes and earthquakes). The earth’s lithosphere is broken into about a dozen major pieces and numerous smaller fragments called plates. The lithospheric plates float on the denser but weaker asthenosphere and move in different directions and speeds relative to one another.
The major lithospheric plates
Where plates are moving away from each other, such as at mid-ocean ridges, new oceanic crust is being created by magma rising from the asthenosphere. Where plates collide with each other, denser oceanic crust is forced under lighter continental crust and is destroyed by subduction. As the subducted plate melts, magma is generated, resulting in a chain of volcanoes on the overriding plate. Through the processes of seafloor spreading and subduction, oceanic crust is continuously recycled. Continental crust is too buoyant to be subducted, so the oldest rocks are found on the continents because continental crust is not destroyed by subduction.
New oceanic crust is created by rising magma at mid ocean ridges. As the crust moves away from the ridge and ages, it eventually is forced back down into the mantle and destroyed by subduction.
The movement of the lithospheric plates is driven by convection currents in the mantle. This convection may overturn the entire mantle (as pictured below) or may be broken into different layers. Because each plate is a rigid body, two additional forces keep the plates in motion:
1. Ridge Push: as new crust is created at the ridge, it pushes the older crust aside.
2. Slab Pull: The weight of the subducted slab drags the rest of the plate into the trench, until it melts.
Mechanisms of plate motion: mantle convection, ridge push, and slab pull.
3.6.1. Satellite Observation of Plate Motions
Advancing technology continues to generate new evidence of plate motions. For example, Global Positioning System satellites are able to measure locations so accurately they are able to resolve the direction and velocity of plate motions that are no faster than about 8 cm per year (about the rate fingernails grow).
Plate motions as resolved by GPS. The arrows indicate direction, while length of the arrow indicates speed. Note that all locations within one plate are moving in the same direction and at the same speed. The Eurasian and North American plates are also rotating.
| Terms to Know
Scientific method |
Magnetic field |
True polar wander |
| Hypothesis Theory Continental drift Seafloor spreading Subduction Pangaea | Polarity Inclination Magnetic reversal Normal polarity Reversed polarity
Apparent polar wander |
Mid-ocean ridge Trench Subduction zone Plate Tectonics Plate |
| Questions for Review |
1. What is the scientific method? What is the difference between a hypothesis and a scientific theory?
2. Explain the geologic evidence for continental drift, sea floor spreading, and subduction. How does plate tectonics explain this evidence?
3. Why does Earth have a magnetic field? How do rocks provide a record of the magnetic field back through time? What evidence does this provide that supports continental drift and seafloor spreading?
4. What causes plates to move?