Earth, Our Environment - Class Notes
Chapter 6, Plate Tectonics
Overview
Earth, Our Environment - Class Notes
As soon as good maps were available (around 1600 AD), people noticed the jig-saw-like fit of continental margins. However, it wasn't until the early 1900's that scientists presented hypotheses that included the drift of continents. Perhaps the most comprehensive hypothesis was that of Wegener, a climatologist who read about observations of similar fossil plants and animals being discovered in South America and Africa, and was impressed by the similarity of coastlines between those two continents.
What the original drift ideas lacked was a mechanism for the continents to "plow" through or over the ocean floor. Our knowledge of the sea floor was limited, but we knew it was made of a relatively strong rock - basalt. After World War II, our knowledge of the sea floor exploded as we mapped the sea-floor geology. During the 1950's and 1960's, knowledge of sea-floor bathymetry and the development of paleomag-netism cleared the way for plate tectonics as we know it.
Plate tectonics united a wide-range of observations from geology, paleomagnetism, and seismology into a relatively simple model of how the upper part of Earth works.
As we discussed last chapter, when rocks cool in a region with a magnetic field, the rocks "record" the orientation of that ambient magnetic field. As scientists began to exploit this record of the magnetic field, they discovered that the rocks of different age, suggested that the "magnetic poles" had "wandered" throughout Earth's history. However, the wander of the magnetic poles was only apparent. It turned out that it wasn't the poles that had been wandering, but the continents.
Around the same time that paleomagnetists were figuring out what the ancient observations of apparent pole positions meant, another group of scientists were systematically studying the history of the magnetic field itself. What they discovered is one of the most important properties of Earth's magnetic field - it reverses polarity at irregular time intervals.
Very soon after, marine geoscientists began to measure the intensity of strength of the magnetic field over the sea floor. They discovered symmetrical patterns in the strength of the magnetic field. And the symmetry coincided with the ocean ridges - a long interconnected chain of high relief covering Earth's sea floor like threads on a baseball. The observation of symmetrical patterns on the sea floor was combined with the observations and hypothesis of geomagnetic field reversals to synthesize the sea-floor spreading hypothesis.
We assume that the major influences on climate that we observed today were also the likely controls throughout Earth's history. We observe today that the predominant influence on the major factors influencing Earth's global climate is the distribution of sunlight (as measured by light intensity). The intensity of sunlight hitting Earth is largest when the Sun is near vertical. Thus the hottest places on Earth are near the equator and the coldest regions are at the poles (local climate can be strongly influenced by topography, wind direction, and proximity to the ocean).
The rotation and differential heating of Earth drives the atmospheric circulation patterns and control the temperature distribution on Earth. The rainfall and temperature have a great impact on the populations of plants and animals - these populations are often (since about 500 million years ago) are recorded in the fossils.
By combining information found in the fossil record with paleomagnetic measurements, we can reconstruct the past latitudes of the continents. For example, if we see a region now a hot desert, that has evidence that it was once covered with glaciers, we must assume that either at that time Earth was much colder, or that region was much closer to the poles. Glaciation has influenced the "temperate zones" in times of a cooler Earth, but some regions could only have been glaciated by having drifted northward or southward. The climate and magnetic information do not provide information on the longitude of the continents - they are only "sensitive" to variations in a north-south direction.
However for the last two-hundred million years we have a fairly complete record of plate motions recorded in the ocean floors. By "reversing" those movements we can reconstruct the past positions of many pieces of the continents (and ocean floor). We also can map the geologic structures - rock types, source magma type and age across continents and discover areas that were once connected but subsequently rifted.
Wegener was able to speculate back to an age about 250 million years ago when the present-day continents were united into a super-continent, which he named Pangaea (for "all land"). Although he had no acceptable explanation for how the continents could move, his intuition was correct. Recent research added to the evidence for the existence of Pangaea. And the reconstruction of the plates unites some disparate observations of geology, paleoclimate observations, and paleontological (fossil) evidence.
Flood basalts are a particularly interesting set of geologic observations pertaining to plate tectonics. They provide evidence that continents were once joined and clues to the timing and mechanism for continental rifting.
A tectonic plate is some coherent piece of lithosphere that is bounded by a combination of spreading ridges, subduction zones, or transform boundaries.
Plate boundaries that separate the plates are either divergent, convergent, or transform. And plate boundaries do not necessarily correspond to continental margins. For example, the coast of eastern North America is not a plate boundary - western half of the Atlantic sea floor is part of the North American plate. We call the margins between oceans and continents that are part of the same plate passive margins.
Plate move slowly, on the order of 0.1 to 10 cm/yr. The Atlantic ocean is growing by about 2 cm each year. We measure plate motions using
Plate boundaries are distinctive - different types of boundaries produce different types of earthquake and activity, volcanoes and volcanic activity, plate thickness, etc. Boundary types are not permanent, and plate boundaries as well as plates move, are created, and destroyed throughout Earth's history. One of the great successes of plate tectonic hypothesis is its consistent explanation for the distribution of earthquake and volcanic activity. Most (but not all) of this activity is associated with plate boundaries. And as we will see in chapters 8, 9, and 10, the type of volcano or character of seismic activity is distinctive for each plate boundary type.
For example:
The Pacific basin is mostly surrounded by subduction zones - this is why the Pacific rim is often called the "ring of fire".
A "hot spot" is a region on Earth's surface above a mantle plume. The best example is Hawaii, which is built of basaltic eruptions caused by compression melting of hot, upwelling mantle material. When compared with the plates, hot spots are relatively fixed, and as the plates move over them, a line of volcanoes often forms. This the case with the Hawaiian Islands. The chain of islands increases in age to the northwest. In fact, the pattern increases to just northwest of Midway Island, where a kink in the pattern occurs and the emperor seamount chain forms a northward lineation. We can use these "hot-spot trails" to calculate the absolute velocities of plates (often we discuss the relative motion between plates).
Using radioactive dating methods, we know the age of Midway Island is about 27.2 million years. The distance between the "Big Island" of Hawaii (which is still forming, i.e. age = 0) is about 2432 kilometers. From this information, we can calculate the velocity of the Pacific plate:
Other hot spot tracks in the southern Pacific Ocean support this velocity.
The topography on Earth is largely controlled by isostasy, the same principle that governs the floating of blocks in water. Several interesting trends can be observed on Earth. Foremost is the distinction between the continents and the ocean basins. Obviously, the oceans floors are lower than the continents. This is a consequence of the larger thickness of the continental crust. It is thick and light, and "floats" higher than the oceans.
Within the oceans, there is a systematic trend of increasing depth with distance from an spreading ridge. New ocean floor is formed at the spreading ridge. As that material moves away from the ridge it cools and grows more dense. The more dense it becomes, the more it sinks, due to isostasy.
The extreme elevations on Earth are also a consequence of plate tectonic activity. The high peaks of the Himalayas, Andes, etc. are a result of mountain building at convergent plate margins. The deepest parts of the oceans are trenches also associated with convergent margins and are particularly deep where one piece of the ocean floor is subducting beneath another, as near the Mariana Islands of the west Pacific.
As you know, the basic source of energy for tectonics is Earth's internal heat. We know that plate motions are related to mantle convection, but detailed maps of the flow direction in the mantle are not available. One force that acts on the plates is viscous drag on the plate bottoms which tends to resist plate movement. However, gravity pulls causes two forces that act on plates: ridge push and slab pull.
Ridge push is the force caused by the elevation of oceanic plates near spreading ridges. The elevated asthenosphere exerts an outward force all along the spreading ridge.
Slab pull is caused by the tug of dense, subducting oceanic lithosphere. This is believed to be the most important force since rapid plate movement correlates with the amount of subducted material attached to a plate.
A third force on plates is called slab suction and this is a force between plates. If a slab of material falls vertically, the subduction zone rolls back and the overriding plate is pulled along with it. This force can cause extension of the lithosphere behind the subduction zone.
We can trace the motions of continents back for about 500 million years, but the farther back we go, the more uncertain our estimates of plate movement. Accepting the principle of Uniformitarianism (or Universality) means that we assume that the laws of physics and chemistry operated the same way throughout Earth history. However, Earth has experienced some changes that may have meant different rates of such processes as weathering and volcanism.
For example, before the proliferation of plant life, the erosive process would have been much more effective at wearing down the continents. You might also hypothesize that changes in atmospheric chemistry may have affected this process as well. Also, the Earth is cooling, so it must have been hotter earlier, and this may have affected the rate of plate movement. Specific answers to these hypotheses are locked in rocks and are the targets of current geoscience research.