"The striking similarity of the coastlines of Africa and Brazil must have been made by Satan."
-- Schuchert, 1928
As we now turn our attention to the internal processes in the Earth that drive catastrophic earthquake and volcanoes, let us take stock of the Earth that resulted from the topics covered in the first third of the class. It is a planet born in fire and storm. There are vast stores of heat energy within the interior, residual from the energy of accretion, core-formation, and radioactive decay processes that we have reviewed. The Earth's fluid envelopes, the oceans and atmosphere, were progressively exhaled by volcanic eruptions. The surface was again and again struck by large meteorites, but most of the scars of bombardment have been healed by cycling of the ocean floor or erosion of the continental surface. Life evolved on the planet, punctuated by rapid catastrophic environmental changes due to massive volcanism, impacts, and sea-level changes.
While the bombardment from space has slowed, and now involves rare chance events (that may or may not intersect and/or terminate human existence), the Earth rumbles along, producing a steady diet of catastrophic upheavals for those that dwell on its surface. Those associated with the atmosphere and oceans (storms, glacial ages, changes in atmospheric chemistry, changes in ocean nutrients) are largely due to variations in solar energy influx. Earthquakes and volcanoes arise from deep-seated processes that we will strive to understand. It is the cooling of the Earth that supplies the energy for these phenomena, and the pace of cooling is such that there has always been and will be for long into the future a steady supply of both earthquakes and volcanic eruptions.
We begin by considering the Earth stripped of its fluid layers. The globe is then most dramatically a bimodal surface, with 30% elevated continents and 70% deep oceans. These surfaces are themselves not level, with high chains of mountains in both the continents and the oceans, and very deep ocean trenches around much of the Pacific ocean margin. There is rough topography on both the ocean floor and on the continents, which must withstand the flattening tendencies of erosion. We will see that the roughness is itself a manifestation of deep dynamics.
The continents are the most accessible regions, and have been extensively studied by humans, both in the pursuit of economic resources and in the quest for understanding of how they evolved. The crust of the continents is comprised of relatively low density rocks (relative to oceanic crustal rocks), which are the result of extensive melting of early Earth mantle and segregation of the lighter components into the continental crust. This process began early in the Earth, and so the rocks of the continents, which are too buoyant to sink back into the mantle, preserve a 4 billion year history of the planet's surface. The light continental crust has an average thickness of 40 km, with a root extending downward much as the root of an iceberg compensates for the high elevation of ice above the sea level. The crust is embedded within the lithosphere, the stiff, coherently translating portion of the upper mantle. The lithosphere under oceans is 70-100 km thick, while under continents it is as much as 350 km thick. Thus, there is a deep 'keel' of stiff material under the continents, extending downward in a conical shape that has remained attached to the continental crust for great periods of time.
So, do continents really 'drift'? This notion is one that dates back to the work of Alfred Wegener, a meteorologist working in the 1920's. He was a very experienced traveler and naturalist, and he argued that continental drift has occurred based on several lines of evidence. The first was the remarkable symmetry of the eastern South American and western African coast lines. If one closes up the Atlantic, the puzzle pieces of the continents fit together amazingly well, and North America also contracts well back onto the northern coast of Africa and the western coast of Europe. Wegener also argued that connecting up the continents was required in order to explain continuity of geological structures across the boundaries of South America and Africa. Furthermore, fossil dinosaurs (and plants) of the same species are found in Brazil and southwestern Africa. These organisms could not swim great distances, so there was either a land bridge across the Atlantic (now sunk out of sight?) or the continents were close together. Wegener also noted that there is evidence for past differences in locations of all continents, such as the presence of areas with scars of past glaciation located near the equator today.
But, the idea of large lateral motions of the continents, or Continental Drift, met great resistance. In part, this stemmed from the conservatism that had gripped the early geologists, fostered by the ideas of Uniformitarianism and the legacy of Hutton and Lyell. Moving continents around smacked of catastrophism, and was dismissed as improbable. In part, the human experience was again a limiting perspective. While the notion of slow vertical motions was broadly accepted, as required to explain the exposure of marine fossils high in the Alps, this was viewed as a consequence of localized buckling and folding of the surface, with changes in sea level. The Earth still seemed very rigid overall, and unable to endure large horizontal motions of continents. There was also a focus on local observations, the result of the intensive localized geological studies that were mapping out the fossil record and the history of rock formation at each site around the world. There was not a global perspective, sitting back from the planet and viewing it as a dynamic system.
Wegener published maps with continental reconstructions dating back 100 million years or so, and argued forcefully that this had taken place, but geologists and physicists responded by saying that there is no mechanism to propel continents through a sea of ocean floor, and despite the logic of some of Wegener's arguments, he must be wrong. Sir Harold Jeffreys quibbled with the continental reconstructions, arguing that the fit is not really all that good, and there are 15 degree mismatches (this overlooked the then unknown extent of the submarine continental shelves of Africa and South America). Other notions were proposed, and gained some currency in the frustrating search for a reconciliation. One was the notion advanced by Warren Carey of an Expanding Earth, which in the past had a smaller radius, bringing the continents back into connection. This idea persisted from the 1930s to the 1950s, and even finds some supported today. However, no mechanism for expanding the Earth could be found; there simply is no source of energy to do this, and the idea languished.
In the late 1950s and early 1960s there began a revolution in the thought about the Earth, and the mobility of the surface. This is the time of the Plate Tectonics revolution, and the old ideas of a geologic history dominated by localized vertical motions has given way to a history of vast horizontal motions of the surface continents and even of the ocean floors. The observations that underlay this hypothesis are numerous, and some were around for many decades prior to the synthesis that took place in the mid to late 1960s. However, some new observations were essential to making the puzzle fit together (so to speak), and as with many scientific advances, it was the confluence of completely unexpected lines of study that made the breakthrough possible.
One of the essential lines of evidence was the global observation of earthquake locations. This was mapped out in primitive form as early as the mid-1850s (at least on the continental regions where there was history of earthquakes occurring in long belts of activity). Advances in the study of earthquakes, or the field of Seismology, had led to accurate global earthquake locations around the world by the 1930s. If one looks at relatively shallow events, in the upper 60 miles of the Earth, the distribution of events is non-uniform, with long chains or bands of earthquake activity lying out in the oceans, and around the margins of the Pacific ocean. The latter region is also ringed with volcanoes, leading to the "Ring of Fire" observation. Following the 1906 San Francisco earthquake disaster, it was quite well established that earthquakes involve sliding of rock across breaks in the ground or faults. Thus, the earthquake distribution clearly reflects where there is active deformation and breaking of rock around the Earth. So, what causes the long chains of earthquake activity and the concentration in regions near volcanoes? Another puzzle was that by the 1920s it was clearly established that some earthquakes occur deep in the mantle, down as deep as 700 km. This was particularly difficult to understand, since the pressures and temperatures at such depths are such that rock is expected to deform ductily, and there should be no abrupt breaking of rock or sliding on faults. The physicists fought for a long time, arguing that deep earthquakes (below 100 km) could not exist, and the seismologists must be locating them incorrectly, but improved models of the seismic wave properties of the Earth reaffirmed the observation. The deep events are also not uniformly distributed, but occur in limited regions, primarily around the Pacific margin. No deep events are found out under the central oceans. The question then became; how to explain the occurrence and location of deep earthquakes? The answer was not forthcoming for many decades after the basic observations were accepted.
The most critical line of observations that broke through the resistance to large-scale horizontal motions came from the unlikely area of studying the Earth's magnetic field. The magnetic field is produced by a complex system of convection in the outer core. The spinning rotation of the Earth and the presence of the solid inner core cause outer core convection to occur in cylinders aligned with the spin axis. The motion of the molten iron alloy causes free electrons in the iron to move, which generates an electric current. Any electric current produces a magnetic field. While the flow in the core is very complex geometrically, the net result of the constructive and destructive interference of magnetic fields is that there Earth has an overall dipolar field aligned with the spin axis. The field is generated by what is called Dynamo action in the core flow.
For several hundred years, humans had exploited the general dipolar geometry of the Earth's magnetic field, as observed at the surface, primarily for navigation. The symmetry of the dipole field, which has north and south magnetic poles very close to the north and south poles of the rotation axis, causes a systematic variation of magnetic field direction with latitude on the surface. Magnetized needles align with the field to point north-south at the surface, but there is also a varying dip to the magnetic direction with latitude. For example, the field is nearly horizontal at the equator, and steeply dipping at the poles. From centuries of observations, it was recognized that the magnetic field of the Earth actually varies with time, and there are small deviations from a perfect dipole geometry. This is the result of the complex dynamo generation mechanism. But, there are also variations of intensity of the field, and for example, for the past few centuries the field has grown progressively weaker. If it were to continue to weaken at the current rate, in a few thousand years, the field would drop to zero. Has this ever happened? The answer lies in longer term records than available from the history of navigation.
Earth rocks often include small amounts of magnetic minerals, rich in iron, which act like little compasses at the Earth's surface. When a rock forms, by cooling from molten lava, or by sedimentation processes, the local magnetic field caused by the dipole, tends to make the cooling magnetic minerals align with the field. In effect, the little compass needles get frozen into the rock as it cools or sediments. Once hardened and cooled, these magnetic orientations are preserved. The rocks then record the direction of the magnetic field at their source of origin, and if the rocks move laterally, we can tell, because their magnetization will differ from that where they formed (if the latitude changes). Also, if the magnetic field points somewhere other than north as in the present field, we can tell that the rocks have either been deformed (tilted, bent, folded, etc.) or the magnetic field has changed with time.
What was found by geologists in the 1950s is that in a layer of rocks at a given site, which have not undergone significant deformation, the orientation of the magnetic directions tend to flip direction with time. For example, a sequences of lava flows near a volcano may have layer upon layer, but in some cases the magnetite is aligned with a field oriented toward the north pole, and in other layers of different ages the rock magnetic field is oriented to point southward. This remarkable observation provided evidence that the Earth's magnetic field REVERSES, while maintaining a nearly constant north-south alignment with the rotation axis, the field has alternated with time interchanging the north and south poles. This does not happen on a regular basis, but with a very erratic history of reversals. By dating the rocks, geologists established that the ancient magnetic field has flipped with a unique sequence, imprinting a bar code of magnetic orientations onto vertical sequences of rocks.
In the late 1950s and early 1960s, there was massive military sponsored mapping of the sea floor, particularly in the north Atlantic and north Pacific. This was largely for submarine warfare purposes, but included mapping of the detailed bathymetry (water depth) of the oceans and mapping of the magnetic properties of the seafloor (for aid in detailed navigation of submarines). The main magnetic property measured was simply the intensity of magnetic field (not direction) of the sea floor, obtained by trailing magnetometers behind large ships that made many tracks over the ocean floor. This revealed an amazing banded structure of ocean floor magnetization, with long, quasi-linear stripes of high or low magnetization. The zebra stripes suggested that somehow the ocean crust acquired bands of uniformly magnetized rock. Key observations were made across the mid-Atlantic ridge, where it was recognized that the stripes parallel the submarine volcanic chain of the ridge, and are symmetric on either side of the ridge. The sequence of stripes bore strong resemblance to the unique bar code of magnetic field history seen vertically in layered rocks. This suggested that somehow the ocean floor has a horizontally varying age analogous to the vertically varying age of a layer of rocks.
This led to the notion of Sea Floor Spreading, by which new rocks in the oceanic crust are created at mid-ocean ridges, by upwelling of molten rock. This rock that sunders and pulls apart laterally, allowing new rock to form in the gap in between the continued rifting apart of the crust, symmetrically spreading on either side of the ridge results in youngest rock at the ridge and increasingly older rock laterally in directions perpendicular to the ridge. The spreading occurs fairly steadily, and during the millions of years that pass by the magnetic field has reversed many times. This causes rock formed at different times (in long central bands at the ridge) to acquire alternating magnetization. The magnetic stripes are preserved in the rocks. The intensity variations detected by magnetometers reflect the constructive/destructive interference of alternately polarized magnetic directions relative to the Earth's present magnetic field.
This Sea Floor Spreading produces vast amounts of new ocean crust and underlying cooled and stiffened oceanic lithosphere that vary with age laterally from the sea floor volcanoes on the mid-Ocean ridge system. This system involves the mid-Atlantic, Indian, and Pacific ridges, which girdle the Earth. When rifting initiates, it may sunder overlying continents, as was the case for breaking apart South America and Africa, and the ongoing production of sea floor in between moves the continents laterally, resulting in Continental drift. The continents are not 'plowing through' the ocean floor, but are moving around as new ocean floor is created.
The production of new ocean floor must be balanced by destruction of old surface somewhere, if the net surface area of the Earth is to be conserved. Given the rapid acceptance of the overwhelming evidence for sea floor spreading, geologists looked for where the balancing mass flow could be located. Suddenly, it dawned on them that the areas of deep earthquakes, primarily in circum-Pacific regions represent areas of sinking old oceanic lithosphere. The unusually cold temperatures of the sinking lithosphere allow earthquakes to occur at depths where faulting does not normally exist. Regions where old oceanic lithosphere sinks into the mantle are called SUBDUCTION ZONES. These are areas with deep oceanic trenches, and large volcanoes exist in arcs on both continents and ocean islands, paralleling the deep trenches. These are the locations of the largest earthquakes and most explosive volcanic eruptions.
The long chains of earthquakes in oceans and on the Pacific margin are now recognized to outline the major chunks of the Earth's surface that constitute the plates of Plate Tectonics. These are large tracts of lithosphere that move coherently, growing on one side by the process of sea floor spreading, and being consumed on the other side by subduction. Where the edges of plates abut, there are different faulting processes that produce earthquakes, and there are melting processes that produce volcanoes. The entire surface of the planet is broken up into about 8 major plates and many smaller ones, all moving relative to one another.
Over time, plate tectonics has rafted continents around all over the surface of the Earth. We can use the magnetic record of seafloor to run the clock backward, closing up ocean basins such as the Atlantic and restoring past configurations of the continents. In doing this, we exploit the magnetic orientations preserved in continental rocks of varying ages to determine the latitude at which rocks formed. In addition, information about mountain building events, largely the result of collision between continents or subduction on continental margins, helps to constrain past positions of the continents. We can run the clock back with some confidence for a few hundred million years, and with diminishing confidence back to 600 million years. There have been intermittent aggregations of all of the continents into super continents such as Gondwanaland or Pangea, and also intervals of wide dispersal of the continents, much as we observe today. In addition, we can project the plate tectonic motions forward into the future for a few tens of millions of years, anticipating where new ocean floor will be produced at the mid-Atlantic ridge, and how the American continents will ride up onto the Pacific rise, subducting the ridge in the process.
The record of the large-scale lateral motions of plate tectonics is well established, and in fact can be actively measured today by space-based geodesy which allows instantaneous motions of the surface to be determined. The theory, now 30 years old has withstood many tests, and while some refinements have been made, it is the fundamental paradigm of the Earth Sciences, and guides most of our understanding of catastrophic processes such as earthquakes and volcanoes. The motions of the continents have also had direct effects on life, producing land bridges that allow lifeforms to expand from one continent to another, or long intervals of continental isolation, that allow one form of flora or fauna to come to dominate that may be distinct from other continents (as for the case of Australia and its dominance by marsupials). The connection of North and South America released a flood of highly competitive North American animals (tested in competition from Eurasian and African ancestors), which decimated more gentle forms that had dominated in South America. Plate tectonics has also affected climate, with continental collisions such as that which produced the high Himalayan mountains actually changing weather patterns and leading to the onset of the monsoon cycle which afflicts India and southeast Asia. When large continents are located at the high latitudes where ice can build up, sea level tends to decrease. There are many affects of plate tectonics that we will consider in the rest of this class. Our immediate attention will now turn to one of the most direct catastrophic by-products of Plate Tectonics: the occurrence of devastating EARTHQUAKES.
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