When one mentions "geologic time" to a geoscientist, that person immediately starts to think in terms of millions to billions of years. The Earth organized as a large accreted body between 4.5 and 4.6 billion years ago. The unit of time measurement applied to geologic history remains the year, which probably has had only limited lengthening (from slowdown in Earth rotation) since the beginning. For practical purposes, we continue to work with the standard year of 365.25 days (although the days were once shorter as the Earth rotated faster).
The rocks at the Earth's surface or accessible (e.g., by drilling) in its shallow interior have a variety of ages as one moves from place to place. The question becomes: How does one know at least the approximate age, i.e., how much earlier in the past did this rock or rock unit (e.g., a layer) become initially emplaced (it may be moved by erosion or crustal deformation, etc.)? Two solutions to the question are: 1) determine its relative age and estimate its time of formation by approximation techniques; and 2) use some sort of absolute time dating method.
Relative age determination involves some important rules: 1) the Law of Uniformity (James Hutton):The Present is the Key to the Past; this means that processes we observe in today's world likely operated throughout the past, back even billions of years ago; 2) the Law of Superposition: Under most circumstances when sedimentary layers are deposited in a sequence, the lowest in that sequence was deposited first and is the oldest whereas the highest was last deposited and is the youngest (this may become spatially "untrue" when rocks are displayed by faulting or severe overfolding); 3) the Law of Cross-cutting: any geologic feature (igneous intrusion; fault, etc) that cuts into/across rock units must be younger than these units; 4) the Law of Faunal Succession: in a sequence of rocks of significant thicknesses, there may likely be remains of animals/plants (fossils) that show some systematic evolutionary progression; most commonly the change is towards complexity and diversity and follows morphological and taxonomic modifications.
Until the 18th and 19th Centuries, the age of the Earth was estimated more from biblical interpretations than from scientific determinations. Then, following Hutton, Lyell, Lamarck, Darwin and others two things were realized: 1) it takes thousands to millions of years to deposit layered sedimentary rock units of thicknesses in the 10s to 100s of meters realm; and 2) progressive evolutionary changes also take millions of years for significant advances in the development of genera and species to become recognizable. By the end of the 19th Century, Earth had "aged" from the 4004 BC estimate by Bishop Usher to millions of years. In the 20th Century the age passed two billion years and with more precise methods (radiometric; see below) zeroed in on the present value near 4.6 billion years.
The most indicative method for reaching the billion year age category was simply to measure the thicknesses of units in one place and estimate the time to deposit them (millions of years), then go elsewhere and find at least some of the same units that had different units below or above them, which had to be older or younger. This involves correlation of units (see second Grand Canyon diagram below) based on specifying in the sequence something in common from place to place, as for example certain animal/plant fossils that are the same or similar, or distinctive rock types in the same sequences. By roaming over a continent and between continents, sequences were recognized, described, and matched with sequences elsewhere that had new member units, within, up, or below. This leads to a general "worldwide" Geologic Column that tries to account for all the deposits laid down during given time intervals (spans) in various locations that can be matched and then expanded by overlapping correlation (see below).
Around the turn of the 18th Century into the 19th, various geologists working in different regions who were adept at stratigraphic analysis (recognition of layers that had characteristic time markers such as fossils) began to publish their descriptions of the sequences they studied. Others found different sequences and/or some of the same sequences in the descriptions. Combining the geographically separated observations, using correlation to tie units of the same age to the sequences, a composite sequence (which included filling gaps where rocks of an age in one region were missing) known as the Geologic Column was built up. As the Geologic Column grew, estimates of their ages were made using mainly deposition rates. Individual sequences within the column were assigned times in the past which resulted from the column estimates.
It became conventional to give a name to a sequence that seemed to represent a long span of time but had certain diagnostic properties (e.g., a collection of life forms that, while evolving, possessed traceable similarities). These sequences became Periods in a temporal-stratigraphic nomenclature and all rocks contained within the sequence made up a System. Subdivision of Periods into smaller time spans yielded Epochs, with their Rocks being Series. Broader divisions of time made up of a some number of Periods (each younger one overlying an older Period - Law of Superposition) were called Eras. Names given to Periods either had some geographic significance (the Cambrian Period was first described and measured in Cambria within the British Isles) or at the Era Level were defined by an appraisal of the dominant life forms and their stages of evolution (the Mesozoic means "Middle" "Life" [zo- is part of a Greek word pertaining to life). From this approach, a system for expressing geologic time and naming subdivisions has emerged, as shown in this diagram: This scale has many subdivisions over the last 600,000 million years, since these rocks are well preserved in parts of the world. All rocks older than the beginning of the Paleozoic ("ancient life"), whose oldest period is the Cambrian, are said to be Precambrian - a general term, that is now undergoing further subdivisions. The bulk of geologic time (about 87%) is Precambrian, as shown in this diagram With the introduction of this nomenclature, we can now look at two figures that use deposition rates, superposition, and correlation to build up a regional Geologic Column covering rocks in Arizona and Utah. The first shows the Column as determined from rocks exposed within the Grand Canyon. The Column units are well displayed in this photo showing the Grand Canyon from the stratigraphic base cut by the Colorado River to the canyon rim:
Several comments are in order: 1) the horizontal rocks all fall within the Paleozoic Era; since they are horizontal, they rest in the same positions as when deposited - they have not been folded; 2) some rocks within individual Periods are distinct from one another; they constitute Formations (mappable units that are named from where [geographically] they were first described); 3) representatives from some Periods or Epochs are missing - either they were never deposited in the ancient seas that produce the Formations or if deposited have since been eroded away - this gap represents a hiatus termed an Unconformity (this embraces both the missing strata and the surface of contact between the two rock age groups that together point to a break in depositional continuity); 4) rocks below the Paleozoic are Precambrian; the upper group are metasedimentary but tilted (folded) indicating they were deformed in some type of mountain building (Orogeny), beccame partially eroded and were then covered with the lowest Paleozoic units, making an Angular Unconformity ; 5) these in turn rest on metamorphic rocks (mostly schists and gneisses), much older with some upper units having been removed by erosion (reaching into lower levels of the crust) to form a Nonconformity; 6) the metamorphic rocks were intruded by granite, which by the Law of Cross-cutting must be younger; and 7) even now, rocks at the surface (top of the column) are being eroded and probably were undergoing erosion since the Paleozoic; this absence implies that Mesozoic and Cenozoic rocks are missing, either through erosion or absence of depositing seas; if at some future time seas roll into the region a new unconformity will develop on the gradually submerged land surface.
The next diagram shows how the Geologic Column has been compiled for this region using correlation:
The link between the Grand Canyon and Bryce columns is the Kaibab Limestone. Bryce contains Mesozoic units, of which the Jurassic Navajo Sandstone is most distinctive; the Navajo sst serves to link with the top of the column in Zion NP. That sandstone unit is near the base of the section at Bryce Canyon which preserves Upper (higher) Mesozic Units and an early Tertiary Formation - the Wasatch - in the Cenozoic. So the composite of the three columns or sections has representative deposits in Periods from the Precambrian through the Paleozoic and Mesozoic into the young Cenozoic.
The skills of stratigraphers over the last 200+ years has produced a general geologic column and a time scale that proves to be quite accurate. But this accuracy had to be confirmed by some independent method that measures absolute time. That had to await the discovery of radioactivity in the late 19th Century followed by the realization that radioactive elements (as isotopes) decay (their nucleus is changed to another radionuclide of the same element or more commonly to new elements) at very precise fixed rates. The decay is said to be exponential (for example, a series proceeding as 1-->2-->4-->8-->16-->....512-->1028-->... is exponential). For example, some given amount of the radioactive form of Potassium, K40 will have half its atoms come apart to form radioactive Argon A40 over a long finite time = 1.251 billion years. (An element with a superscript is an isotope whose mass number is given by the superscript; Argon has two isotopes [istopes have same number of protons {a particular atomic number that defines the element species} but differing numbers of neutrons], thus [A39 and A40], with the latter having one additional neutron). Consider this diagram:
Half the Potassium40 is gone after 1.251 billion years; half remains. Now if another 1.251 billion years elapses while the mineral containing the Potassium (several isotopes), then in 2.502 billion years only 1/4th of the original K40 will remain and a larger amount of A40has developed. The clock on dating begins when the original potassium is incorporated in the rock; it is assumed that no initial Argon, a gas, was present. Assuming none of the A40 escapes over time, then a geochronologist need only measure (using a mass spectrometer) the amounts of K40 and A40 now present in the rock to set up a ratio and to use the decay rate (given as a half-life, the time required for half of any of the radioisotope to decay) to determine the age. If that ratio is K40/A40 = 1/8th, the age would be 3753 million years - the time between incorporation of the Potassium in the rock (likely, in the feldspars making up a granite) and the present.
Very accurate ages of incorporation are possible using radiometric dating, provided none of the Potassium and Argon isotopes escape or no element contamination occurs. There are a number of radioactive element that have their own decay schemes. Radioisotopes of Uranium (U) decay at various rates to isotopes of other elements (e.g., Radon) and a sequence of one to several steps (Decay Series) and ultimately to isotopes of Lead (Pb). The element Rubidium (found in micas) decays to Strontium. Some elements decay over short half lifes and are limited to dating younger rocks (Carbon isotope decay works well if rock [wood; other organics] is less than a half million years old); others are especially suited to determining Precambrian times. If a shale is intruded by a granitic dike (narrow tabular cross-cutting body), the two rock types can be dated by different radioisotope methods, so that the time when each event took place can be fixed fairly accurately.
Thus, since the early 20th Century geoscientists have had a powerful tool to reconstruct when different specific events took place in a complex assemblage of rocks, so that a precise geologic history can unfold. The table below summarizes a generalized history of, mainly, the primitive Earth.
The oldest dated mineral, a zircon from Australia, is age-fixed at 4.1 billion years, but most early ages for rocks fall around 3.6 b.y. Thus most of the Earth's original, and some subsequent, crust has been destroyed (remelted; subducted; broken down by weathering). When oxygen was nearly absent from the atmosphere, the most characteristic rock type was BIF (Banded Iron Formation); its production consuming any oxygen released. As photosynthesis in plants emerged as a working process, oxygen increased, producing iron oxides in the form of Red Beds; then also carbonate rocks became commonplace in sedimentary sequences.
The most important events, from the human perspective, have been the origin and time of appearance of the first living forms, and the subsequent development of the major phylla and orders of life on land and sea. The first indications of life extend now to earlier than 3.5 billion years ago. Early life was single-celled - procaryotic. Multi-celled life - eucaryotic - appeared in the Precambrian. The greatest diversity ("explosion") of life occurred at the close of the Precambrian into the Cambrian Period. Since then at least 6 mass extinctions (significant fraction of all life types at the time diminishes or disappears) have occurred. The time sequence of life on Earth is depicted in this "cartoon", in which principles of evolution govern the progression and emergence of new phylla:
Much of the life topics above is examined in more detail on page 20-12.
Human and animal life, as well as many of the modern landscapes and landforms, have been strongly influenced in the last five million years by a series of continental glaciations. Because of this great surface-modifying phenomenon, several paragraphs and illustrations are incorporated here within the Geologic Time category.
About five million years ago, global temperatures began a slow, then faster decline, as indicated by the sensitive O18/O16 ratios in certain sediments and in ice. By about 2.0 m.y., the first signs of a worldwide Ice Age began to appear. From then until today, temperature measurements indicated many cycles of warming and cooling.
During this time, referred to as the Pleistocene (the most recent 15000 years to the present is called the Holocene), there were at least four major intervals of continental scale ice coverage over about 30% of the Earth's land masses. The duration of extensive ice coverage is between 50000 and 100000 years, with interglacial (warm) intervals of similar duration. In Europe, the glacial stages are given the names shown here:
Each glacial advance greatly modified landscapes it covered, and both glacial and interglacial times were influenced by climatic changes that affected surfaces well beyond the periglacial environment. (The tropics were altered in glacial times but during interglacial periods subtropical conditions were present in today's subarctic zones.) Most of the earlier glacial geomorphic effects were destroyed or buried by the last glaciation, which covered the northern hemisphere as shown here; below that is a more detailed look at North American glaciation during this Wisconsin time.
At least four glacial advances covered North America in the last 2,000,000 years. These and the intervening interglacials are named in this diagram
This map of the U.S. Midwest indicates that deposits from each of the advances can be found. This map also suggests that all the four glacial advances reach about the same southern limits.
Yellow and green denote Nebraskan glaciation; blue Kansan; purple Illinoisian; and white with markings, the youngest, or Wisconsin glaciation.
The cause(s) of Pleistocene glaciation has(have) not been fully ascertained to most geoscientists satisfaction. Systematic (cyclic) variations in solar output (the Milankovitch cycles) seem to be a factor but these have been routine for millions of years. Changes in CO2 output, which alter temperatures, may be another input. Alternating freezing over, and then thawing, of the Arctic ocean could play a role. Perhaps decisive as a reason for the onset of glaciation in the past few million years is the joining of North and South America by the land bridge associated with the Isthmus of Panama - this was finalized about 3 million years ago. That completely changed the circulation patterns in the Atlantic and Pacific oceans, which could led to cooling of Arctic waters and initiation of northern hemisphere ice formation on a grand scale.
In the northern hemisphere at mid to high latitudes, most modern landforms owe some extent of their development to direct or indirect influences of Pleistocene glaciation. Minimal effects in Africa probably facilitated the evolution of the earlier hominids. The migration of the various "homo" species was partly directed by changing climates. Modern man, who first appeared about 100,000 years ago, was moved about by later stages of the Wisconsin advance. The beginnings of civilizations about 10000 years ago owe much to the onset of the present interglacial interval (see page 20-12).
A good review of these topics which supplements the coverage below has been prepared online by the U.S. Geological Survey.
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 worldwide 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)
Mountain belts are characterized by several distinctive geologic features: 1) they usually have significant topographic expression - the surface consists of peaks, ridges, and intervening valleys; 2) the sedimentary rock units are generally non-horizontal - the layers dip (incline); 3) there commonly are associated igneous and metamorphic rocks; 4) large parts of the crust have been broken into blocks and displaced along faults. Two hallmarks of many mountain belts (also called orogenic units) are greenstones (metamorphosed basalt that represents oceanic crust subducted below an overlying crustal block) and ophiolites (deep crustal or mantle rock brought upwards towards/to the surface). These are shown here as seen from space:
We will concentrate our discussion of Plate Tectonics and continental growth on North America as a definitive example. As radiometric ages were determined for the shield-like rocks within 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, as shown in the next two figures.
The rocks older than about 500 million years have been called the "basement" - a term that suggests they are found at the "bottom" of the accessible crust. Their age and distribution have been interpreted to mean that the continents had somehow grown (enlarged) around their oldest crustal components (nuclei) by addition of rock assemblages - in some instances these were crustal landmasses (terranes) developed elsewhere with their own characteristic age parameters. The additions are mainly through collisions of crustal masses resulting from plate tectonic movements throughout the ancient past; these accreted (or tectonostratigraphic) terranes are discussed in the second half of Section 17. The expanding continental nuclear units coalesce to become part of the "craton" which consists of both exposed (as a "shield") and buried basement rocks (mostly now metamorphic and igneous). Maps of these units shown above come from two sources, show different subdivisions, and hence do not perfectly agree in distribution and age among specific added components. The orogenic (mountain-building) units known as the Appalachians and the Cordillera make up the outer parts of North America and are the youngest (less than 500 million years) of the major components of the continent (which lies embedded in the North American Plate). In these maps sedimentary rocks deposited over the past 550 million years that lie between the eastern and western mountain belts in the continental interior have been "removed" so as to reveal the basement units they overlie.
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.
In the geosynclinal model, 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. Evidence he cited to support the idea includes structural continuity (mountain systems on continental pairs match when the fit restores their original positions, glaciation effects on individual continents being found continous when two continents are rejoined, 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. The next illustration is an oft-cited (this version came from a U.S.G.S site) panorama showing the positions of the continents from early Pangaean breakup to the Present.
The general time frames for specific separations and collisions involving Pangaea is summarized in this diagram (mya = millions of years ago):
One can ask the question of how Pangaea itself came into being. Most models draw upon paleomagnetic (see this Intro page, near bottom) and other evidence that suggest various earlier continents - with little resemblance to present-day ones - assembled by plate motions over the globe that caused them to converge into Laurentia and Gondwanaland and eventually into Pangaea. A group in New Zealand offers this history for a 320 million year span:
It is likely that several such assembly-breakup-reassembly-breakup.... episodes - each lasting hundreds of millions of years - have occurred ever since plate tectonics began (roughly 4 billion years ago). Those interested in more information on the sequences of drift positions over last half billion years that have now been reconstructed should visit this website produced by the Geology Department at the University of Wisconsin at Green Bay.
If present day trends continue, one can extrapolate the drifting of continents into the future such that in about 250 million years a new super-Pangaea will have reassembled. This is an "educated guess" as to the configuration:
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 permitted establishment of the polar directions at the time the basalt sample crystallizes. (The basalt 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.) Studies of samples at different distances from the ridge crest found that the North and South Magnetic Poles reverse their polarity (i.e., the geographic South polar region 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; positive) or to the opposite polarity (reverse N-S; negative).
More about this and other relevant information applicable to Geology has already been reviewed for you in the subsection of the Introduction that dealt with Geophysical Remote Sensing
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 (not really black and white - that is for depiction purposes).
About that time, 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 overall 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 (see also the two relevant illustrations on page Intro 2-1c):
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 the relatively rigid rocks that comprise 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:
Today, the plates are moving in different directions and at velocities of a few millimeter to centimeters per year (see this page.)
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 where the main driving force that moves plates apart is applied. 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; the deepest on Earth today is the Marianas Trench in the Pacific, whose ocean floor top 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 near the top of the heated asthenosphere (zone where the rock is very hot, but soft [like tar], yet remains solid), 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 produce magmas on the plate's up 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 as 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 (example: 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: those that stir up the surface in a swirling cup of hot chocolate) of very hot, plastic-like (but still solid) 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; the lower diagram show the part of the cycle that includes the major upward flow that drives sea floor spreading in the upper half.
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 start to solidify on the continent, the magnetic grains are able to move freely in the mush. Theyact like tiny magnets and point to the Earth's North pole as does the needle on a compass. Assuming that the Earth's magnetic poles remain constant in position (but not in polarity) over vast time periods - for which there is good evidence - these grains, aligned in a fixed direction by the terrestrial magnetic field, 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 of the North Pole at each age can be plotted, yielding polar wandering paths, appearing 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 on a map that shows today's location of each continent; the curves were constructed from pole position data acquired on North America alone and Europe alone. On the right diagram, the continents have been pushed back together as they are deduced to have been before the current onset of Continental Drift and prior to breakup of Pangaea. The two offset curves now coincide. This is convincing proof that at that time the continents were conjoined.
We have already alluded to the process (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 [supracrustal assemblage]) can grow in size. The various plates in modern times have not only continents embedded in their upper parts but many smaller features such as island arcs, ruptured continental fragments, and even spreading ridges overriden ane caught 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 (also Exotic Terranes; Suspect Terranes; and more formally Tectonostratigraphic Terranes (see Section 17; pages 7-6 ff). The western edge of North America has been built up by a succession of accreted terranes, as indicated in the next figure:
The eastern part of North America also has a large number of terranes added both before and after the Pangaea split. There are many terranes making up the Piedmont Belt in the Appalachians. Here is that belt in Virginia, with each unit being an accreted terrane:
Whole 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. The ancient crystalline rocks that make up the basement of the North American craton (huge continental mass that serves as the nucleus for later continental growth), now covered by younger horizonal sedimentary rocks, have been shown from penetration by drilling to consist of slices of exotic terranes accreted to that craton in its formative stages. Here are terranes in the Kansas basement:
Finally, we want to indicate in some detail how mountain belts are produced. We will look at the Appalachians - the orogenic complex that led to early ideas of mountain-building in the 1800s. First, the setting: Here are the main structural (and correlated landforms) subdivision of today's Appalachian Mountains in the eastern U.S.
The Piedmont is mostly metamorphic and igneous rocks (including terranes) that were heated up both prior to the final collision (end of Permian) and during that juncture. They continue to the east but are covered by younger coastal plains sedimentary rocks. The Blue Ridge is an assemblage of older rocks that resist erosion and thus rise topographically above the Piedmont. The Valley and Ridge unit is a sequence of folded anticlines and synclines deformed during the Taconic (Ordovician), Acadian (Devonian), and Alleghenian (Permian) orogenic events (perhaps related to terrane accretion). The Appalachian Plateau is a thick sequence of trans-Paleozoic sedimentary rocks that filled basins against the North American protocontinental margin, which was not crumpled by the main folding but was uplifted (then, and by rejuvenation several times since). The Northern Appalachian unit is mostly metamorphic/igneous rocks to some extent correlative with the Piedmont. The Appalachians continue northward into Nova Scotia and thence across the since opened Atlantic Ocean into Ireland/western England/Scotland/Norway. To their southwest, these mountains are now buried by the Mississippi Embayment sedimentary rocks but appear again in Arkansas/Oklahoma and westward to reappear in Mexico.
In this five panel illustration, the major steps and events in Appalachian Mountains history are shown from Precambrian (top) to the present (bottom); the Iapetus Ocean is ancestral to the Atlantic.
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 (welded on as a terrane) 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 boundary-subduction, with attendant terrane accretion, occurred producing the Taconic and Acadian mountains - precursors to the present; 4) As the main westward movement of the African plate continued to subduct crust and embedded rocks under and onto North America, the African continent itself approached; the Iapetus Ocean between the two continents progressively closed; 5) in time near the end of the Paleozoic the African continent 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. An informative review of the growth of the northern Appalachians, including terrane accretion, is worth a visit to this web site.
We have touched upon volcanoes on these two Geology tutorial pages. They are the most spectacular of the major geologic pages (although earthquakes can be more detructive). Here we will consider several of their general aspects (some types of volcanoes, especially as seen from space, are examined on page 17-3).
When one thinks of a "volcano", this photo is typical of what comes to mind:
This volcano, Mt. Rainier in Washington state, is a classic stratovolcano, whose structure is shown here:
A volcano, then, is a surficial structure that is built up of melted rock that reaches the surface as lava, which has one or more features that connect to a magma chamber in the crust, or possibly the upper mantle. A series of eruptions (usually many) lead to a gradual buildup of the structure as the component material piles up as ash or as outflows of molten rock. Stratocones generally are composed of lavas that have high to intermediate silica content.
There is a second major type of volcanic structure, the shield volcano, whose slopes are gentler (5 to 15 degrees). Mauna Loa in Hawaii is an example:
There are other, smaller volcanic types and landforms (again, see page 17-3). But for this tutorial we will place volcanoes in their tectonic setting. These broad classes are set forth: 1) those that occur within a continent in a rift zone where the continent is splitting apart; 2) those found along spreading ridges of upwelling magma; 3) those located above a subduction zone; 4) those produced above an active hot spot (a column of upwelling magma; and 5) those that appear to be isolated from main zones of tectonic and/or magmatic activity.
This next map shows the distribution of the five classes:
Class 1 is illustrated by the East African Rift Valley, where an arm of a triple junction (three zones of upwelling magma roughly 120 degrees apart) is breaking the ontinent to the east apart from the main mass of Africa. Consider these two illustrations:
The Atlantic Ridge is a good example of volcanoes along a spreading zone. The number of volcanoes is rather small, and some are underwater (submarine volcanoes). There are clusters of volcanoes in the Canary Islands, the Azores, and Iceland. This is a photo of Icelandic volcanoes:
Long lines of volcanoes, or chains, are typical of those found along subduction zones (Class 3). This (rather busy) map of Indonesian volcanoes, on the up plate side of a vast subduction zone, and a photo of a line of volcanoes from space, show this class:
The Hawaiian Islands are the most frequently cited example of Class 4 - hot spot volcanoes. (Yellowstone Park sit on top of a hot spot.) Another, less familiar, example is Cheju Island, off the coast of Korea in the Yellow Sea:
We will not show any examples of the Class 5 isolated volcanoes. The Jemez mountains in New Mexico fall in this group.
We have said little about how the Earth was formed as a planet and developed during its first three billion years. For that, go to page 19-2a.
So there you have it: In these two pages some of the rudiments of Geology - some but not all of its principal ideas - have been reviewed to give you a working background to appreciate material in this and other Sections. To learn more - hunt through the Internet or take a course in college. And, a plug given here to reach any who, like the writer (NMS), may aspire to Geology as a career: I point with the following photo to a "trademark" of this profession - the joy of being outdoors doing field work, as illustrated in this trip in 1966 which I participated in during an Oregon Conference on Volcanism:
