As the 20th Century began, major unsolved problems in understanding the Earth’s geology included the distribution of rock type by age and by structural state, the causes of mountain building and other modes of deformation, the distribution of earthquakes and volcanoes, and the nature of the seafloor’s composition and geologic features. This map shows three fundamental rock units: 1) ancient Precambrian igneous-metamorphic rocks exposed as Shields (red), 2) mostly flat-lying sedimentary rocks (orange), and 3) folded/faulted rocks in mountain belts (brown)
As radiometric ages were determined for the shield-like rocks on the continents which were either exposed at the surface, underlay the flat rocks, or were within the interior of the mountain belts, patterns of age intervals were determined, shown in the next figure. These old rocks have been called the “basement”. This has been interpreted to mean that the continents had somehow grown (enlarged) by development (by addition?) of rock assemblages with these characteristic age parameters. The expanding continental nuclei are part of the “craton” which consist of both exposed and buried basement rocks.
By the latter half of the 19th Century, studies of the Appalachian Mountains and others led to this general picture of a linear, wide sequence of all three types of rocks that had been deformed and may still show topographic evidence of differential erosion producing present-day mountains.
One part of the mountain system consists of folded/faulted sedimentary rocks. These appear to have been deposited on the other part, deformed segments of the basement, much being Precambrian, both with younger granitic intrusions.
Attempts to explain this mountain structure led to various hypotheses, chief of which were put forth by James Hall and James Dana in the later 1800s from their studies of the Appalachians. In this model, there were two regions near a continental margin that downsank to form Geosynclines -troughs that could receive over time deposits of sediments that exceed 15000 meters (~50000 ft) in accumulated thickness.
From time to time these sediments (converted by burial to sedimentary rocks) would be squeezed by compressional forces causing uplift and folding of mountains over at least part of the length of the linear trough(s) (typically 1000-3000 miles; widths around 500 miles), followed by erosion (unconformities) and renewed deposition. Finally, the entire geosynclinal belt was subjected to intense compression leading to the main phase of orogeny (mountain-building) and general uplift that over time causes removal of some of the mountain units through erosion, in places exposing the basement.
However, as the 20th century progressed and new information about mountains and continents accrued, problems with this model were identified. Alfred Wegener, a European meteorologist, noted that if continental outlines (including submerged edges near shore) for Europe, Africa, North America and South America were placed next to each other (say, by cutting them out like “paper dolls), these continents show a remarkable fit, shown here.
Wegener hypothesized that at some time in the past those continents had been conjoined as a single supercontinent he called Pangaea. Then they broke apart (continental split) and starting moving away from one another until reaching their present positions. This was continuous but is shown here in three steps reprenting stages since breakup began near the end of the Paleozoic. He called the process Continental Drift and speculated that thermal currents in the mantle may provide the driving forces. Some evidence he cited to support the idea includes structural continuity (mountain systems on continental pairs match when the fit restores their original positions, glacial similarites, and, strongest of all, presence of the same animals/plants (as fossils) of types that could not swim across oceans or float far by air.
Pangaea itself further ruptured into two continents – Laurasia (north) and Gondwanaland (south), each of which split further into the present geographic layout.
By the 1950s the geosynclinal model had been discarded but continental drift remained in favor. A new paradigm was needed. A series of observations led to the general model of Plate Tectonics which became a major revolution in geological thinking about the realities of a dynamic Earth.
The first bits of explanatory evidence came from discoveries about the deeper ocean sea floor. Aerial geophysical flights across stretches of the ocean uncovered an unexpected magnetic phenomenon. Evidence found by magnetic properties analysis of the extruded oceanic basalt (which contains magnetite and other iron minerals that act like “miniature compass needles” that align so as to point to the Pole [arbitrarily called North] where magnetic lines of force in the Earth’s magnetic field enter the planet at the time of lava solidification) permits establishment of the polar directions at the time the basalt sample crystallizes. Studies of samples at different distances from the ridge crest found that the North and South Magnetic Poles reverse their polarity (South becomes the entry point for magnetic lines of force and North the exit point) over time intervals of less than a hundred thousand to a few million years (on average every 200000 years, leaving the field at minimum strength over about 3000 years) during the reversal period. When survey flights passed across Mid-Ocean Ridges, patterns like the one below were registered; each stripe indicates that for the time basaltic lava extrudes (at rates of 5 to 20 cm/year) the enclosed magnetic minerals for the full interval needed to produce the width of a stripe (10s to 100+ km) are pointed either to today’s magnetization (normal N-S) or to the opposite polarity (reverse N-S).
Of special significance is that the patterns on either side are mirror images of each other. This can be explained by assuming that new ocean crust pours out at the ridge and spread away in both directions over the span of time in which one polarity – normal or reverse is operative. Spreading rates to either side are about equal. The series of normal-reverse polarities alternate over time giving the symmetric pattern observed.
Then, deep sea dredging and later drilling brought up samples of the basaltic ocean crust which could be dated radiometrically. Over the years enough parts of the oceans’ floors were reached, sampled, and dated. The general trend, when data points were plotted, was for (magnetized) stripes of basalt to be youngest at the ridges and oldest where ocean floors meet continents. This is the general picture:
This surprising mechanism of adding new material at ridges and having surficial layers move away from the Mid-Ocean Ridges (found in the Pacific and Indian Oceans too) was independently, and almost simultaneously named by Dr. Harry Hess (Princeton) and Dr. Robert Dietz (NOAA) as Sea Floor Spreading. It started others to thinking about how it works and the consequences applied to the Earth’s exterior. As new data from geophysics on earthquake epicenters (surface projections of source areas at depth) and better plots of volcanic activity were shown on maps, this general pattern became obvious
The way to explain these observations now opened fast for geoscientists. No one individual is credited with “thinking up” all the basics of Plate Tectonics; many contributed vital evidence and innovative operational models during a relatively short period in the 1960s onward. The essential idea starts with this assumption in an attempt to explain the earthquake and volcanic distributions: The present-day Earth outer shell is broken into 6 major plates (cover large areas) and some smaller ones. They have several types of boundaries (see below) and are about 200 km thick. (If a large plate could be “plucked” from the Earth it would resemble an orange peel, being curved as a segment of a sphere). The plates consist of a sequence of rock types, either basaltic crust and iron-magnesium upper mantle or continental crust overlying some basaltic crust and mantle, which makes up relatively rigid rocks in the Lithosphere. Below the lithosphere is mantle rock soft enough (through heat) to allow the lithospheric plates to “glide” laterally across parts of the globe. This map shows today’s major and minor plates now identified as separate moving bodies; over time in the past and projected into the future, the plates size and location will vary as individual plates grow or are consumed:
Four types of plate boundaries or margins have been recognized:
Boundary type A is diverging; at a Mid-Ocean Ridge, lava extrudes in two directions as it adds to adjacent plates. This is the region of the main driving force that moves plates. Boundary B occurs where two ocean type plates (no nearby continental crust) converge head on. One plate is forced under the other, this is called subduction in which the underthrust plate gradually melts and dissipates (becomes part of the mantle rock) when pushed to increasing depths. The process leads to indentations of the crust that oceanographers call trenches; deepest on Earth today is the Marianas Trench in the Pacific, whose bottom is nearly 35000 feet (10 km) beneath sea surface. The C Boundary refers to a converging margin where continental crust meets continental crust on the second plate. Boundary D is somewhat different – it does not develop at a diverging or converging boundary but is either at a plate edge where two plates slide past along transform faults or is one of a series of transform faults that aid movement within a plate.
We are now ready to define the interaction of plates through this schematic diagram:
Melting of the mantle, mainly in the heated asthenosphere, causes lava to move upwards into a long linear fracture system that builds up as a Mid-Ocean Ridge; the two plates on either side are diverging. To the left one of these ocean plates meets another and subducts. Frictional and residual heat produces magmas on the up plate side that reach the surface as lavas which accumulate into volcanic structures. These produce Island Arcs, constructed around the volcanoes; Indonesia, Japan, and the Aleutians are three examples. To the right, the other plate meets a continent-bearing plate and also subducts. Melting again produces magmas that intrude near the continental margin and surface a volcanic lavas (either flows or volcanoes); the American Cascades are of this nature. Finally, within the continental upwelling convection currents may be forcing the continent to pull apart as a rifting zone which in time may split the continent into two or more parts (Pangaea).
The diagram below ties this type of plate margin into the rock cycle.
The nature of the driving forces seems to be tied to slow movements something like currents (analogy: in a boiling pot of water) of very hot, plastic-like mantle rock. These involve heat transfer by convection. Some evidence suggests these convection currents (shown below) originate near the mantle/core boundary. Other signs indicate shallower origins or perhaps a secondary set of currents in the upper mantle only.
Just to emphasize the characteristics of the plate tectonics model, this is the third variant we have shown on these two pages. The upper diagram follows the full mantle convection hypothesis; other versions show the cycle to have the major flow to be sea floor spreading in the upper half and flow of the upper mantle towards the ridge exits within or below the level of the asthenosphere.
So, how does the Plate Tectonic Model tie in with the notion of Continental Drift? Or, more to the point, what is the evidence for drift? The chief proof comes from Polar Wandering. At the time rocks containing magnetite solidify on the continent, the magnetic grains act like tiny magnets and point to the north pole as does the needle on a compass. Assuming that the magnetic poles remain constant in position (but not in polarity) over vast time periods – for which there is good evidence, these grains serve as markers suited to locating the pole at the time they were encased in cooled rock (usually basalts). If the polarity is determined in rocks of different ages, the positions can be plotted, as follows:
This resulting Polar Wandering plot is explained as follows: On the left diagram are a pair of curves made by connecting the geographic location of the pole in North America and in Europe yielding points at different times – the progression is from 300 million years to the Present. Note that the two curves do not fit; each was constructed from pole position data acquired on North America alone and Europe alone. On the right diagram, in which the continents have pushed together as they are claimed by Continental Drift to have been prior to breakup of Pangaea, the two curves now coincide. This is convincing proof that at that time the continents were conjoined.
We have already alluded to the possibility (actually it is common) of two plates each bearing a large landmass (up to continental size) colliding. If little oceanic crust is involved at later stages, the continents will collide head on, will probably weld to each other, and one may override the other, with the result that the now combined continents in the collision zone actually thicken. This has happened in the case of the Indian subcontinent heading into the “underbelly” of Central Asia, as sequenced in this diagram. The result is the Himalayan Mountains, highest on Earth.
This process suggests one means by which continents (which contain more silica-rich rocks like granites and usually have extensive sedimentary rock cover) can grow in size. The various plates in modern times have not only continents on them but many smaller features such as island arcs, ruptured continental fragments, and even spreading ridges are within a plate. When collisions occur, some of this “flotsam” may subduct but commonly it is shoved on and welded to the continental margin. These additions are called Terranes and the assemblage of individual terranes that arrived at separate times make up what is term Accreted Terranes. The western edge of North America has been built up by a succession of accreted terranes, as indicated in the next two figures:
The eastern part of North America also has a large number of terranes added both before and after the Pangaea split. Continents may in fact build up largely by terrane accretion, as suggested in the time map of Provinces in North America appearing earlier on this page.
Finally, we want to indicate in some detail how mountain belts are produced. We will look at the Appalachians – the group that led to early ideas of mountain-building in the 1800s. In this five panel illustration, the major steps and events are shown from Precambrian (top) to the present (bottom).
This sequence is likely: 1) In Late Precambrian, a plate east of the North American block (already existing for a long time) subducted, causing am island arc and a back-arc depositional basin; 2) In Cambrian times, a rupture near the margin produced a second subduction zone pointing in a direction opposing the first; volcanism and deposition continued; 3) In the Ordovician and again in the Devonian, more mini-subduction occurred producing the Taconic and Acadian mountains – precursors to the present; 4) As the main westward movement of the African plate continued to subduct under North America, the African continent itself approached; the Iapetus Ocean between the two continents progressively closed; 5) in time the African continet crashed against North America, closing the Iapetus, but by Triassic times the two continents split and the Atlantic Ocean opened; there no longer is an active subductive zone next to either continent. This running description helps to demonstrate that plate tectonic action can lead to some complicated sequences of events.
Primary Author: Nicholas M. Short; Collaborators: Code 935 NASA GSFC, GST, USAF Academy