In 1654, Archbishop Usher (Ireland), based on genealogy in Bible, determined that Earth was created October 26, 4004 BC, 9:00am (PST). Therefore, the Earth was 6000 years old.

During the late 18th and early 19th century, a German mineralogist, Abraham Gottlob Werner, proposed that all of the Earth's rocks were formed by rapid chemical precipitation from a "world ocean," which he then summarily disposed of in catastrophic fashion.

Though not directed toward the genesis of landforms in any coherent fashion, his catastrophic philosophy of changes of the Earth had two major consequences of geomorphic significance.

This led to the theory of catastrophism, that the Earth was shaped by series of giant disasters and that they had to fit many processes into a short time scale.


A French scholar, Bernard Palissy who lived from 1510-1589 believed the Earth was much older based on his observations that rain, wind, and tides were the cause for much of the present-day appearance of the Earth. He wrote that, these forces could not work over such a short period of time to produce the changes. He was burned at the stake in 1589. A bad time for scientific inquiry.

In 1770's, James Hutton, Father of Geology (Scotland, 1726-1797) published `Theory of the Earth' in 1785. Demonstrated that Hadrian's Wall was built by Romans and that after 1500 years there was no change. Thus, he suspected that Earth was much older than 6000 years.

This is the theory of uniformitarianism, that slow processes shape earth. Mountains arise continuously as a balance against erosion and weathering. "Present is key to the past". The physical and chemical laws that govern nature are uniform.

In the mid 1800's, Scottish geoglist Sir Charles Lyell expanded on uniformitarianism to develop gradualism, the view that all features of the Earth's surface are produced by physical, chemical, and biological processes through long periods of geological time.

His system was based on two propositions: the causes of geologic change operating include all the causes that have acted from the earliest time; and these causes have always operated at the same average levels of energy. These two propositions add up to a "steady-state" theory of the Earth. Changes in climate have fluctuated around a mean, reflecting changes in the position of land and sea.

Lyell's position suggested that the world had always been (roughly) similar to its current state. In particular, Lyell believed that the species composition of the world remained unchanged, with at least some members of all classes of organisms existing throughout the history of the earth.

How old is Earth? (scientific methods)

  1. In 1897, Lord Kelvin assumed that the Earth was originally molten and calculated a date based on cooling through conduction and radiation. The age of Earth was calculated to be about 24-40 million years based on the laws of thermodynamics.

    Unknown at the time was that the Earth has an internal heat source (radioactive decay)

  2. In 1899, John Joly (Irish) calculated the rate of delivery of salt to the ocean. River water has only a small concentration of salts. Rivers flow to the sea, therefore, evaporative concentration of salts can be calculated.

    By this method, the age of oceans will equal the total salt in oceans (in grams) divided by rate of salt added (grams per year)

    Age of Earth was calculated to be 90-100 million years.

    Problems: no way to account for recycled salt, salt incorporated into clay minerals, salt deposits.

  3. In 1860, the thickness of total sedimentary record is divided by average sedimentation rates (in mm/yr) and calculated to be about 3 million years old. In 1910, the same measurement yields about 1.6 billion years old.

    Early measurements of maximum thickness of sediment ranged from 25,000 m to 112,000 m. With more recent mapping, thickness of fossiliferous rocks is at least 150,000 m. The average sedimentation rates are about 0.3 m/1000 years. At this rate, the age of the first fossiliferous rocks is about 500 million years.

    Problems: This calculation does not account for past erosion or differences in sedimentation rates; also ancient sedimentary rocks are metamorphosed or melted.

  4. Charles Lyell (1800's) compared amount of evolution shown by marine mollusks in the various series of the Tertiary System with the amount that had occurred since the beginning of the Pleistocene. He estimated about 80 million years for the Cenozoic Era alone.

  5. Radioactivity is discovered by Henri Becquerel in 1896. In 1905, Rutherford and Boltwood used radioactive decay to measure the age of rocks and minerals. Uranium decay produces He, leading to a date of 500 million years for the oldest rocks.

    In 1907, Boltwood suspected that lead was the stable end product of the decay of uranium and published the age of a sample of urananite based on Uranium-Lead dating to be 1.64 billion years.

    Mass spectrograph was used after WWI (1918). Led to the discovery of over 200 isotopes. Many radioactive elements can be used as geologic clocks since each element decays at its own nearly constant rate. Once this decay rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent and the amount of stable daughter elements.

So far, oldest dated Earth rocks are 3.96 billion years. Older rocks include meteorites and moon rocks, where Moon rocks from the Lunar highland are about 4.5 billion years old, mare basalt rocks are 3.2 - 3.8 billion years old. Meteorites are all older than 4.5 billion years.

Most of the evidence for an ancient Earth is contained in the rocks that form the Earth's crust. The rock layers themselves -- like pages in a long and complicated history -- record the surface-shaping events of the past, and buried within them are traces of life --the plants and animals that evolved from organic structures that existed perhaps 3 billion years ago.

Thus, the results of studies of rock layers (stratigraphy), and of fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), attest to a very old Earth.

Why is the Earth younger than the moon and meteorites?:

Current estimages of the age of the Solar System are 4.5 billion years, but the oldest rocks on the Earth are only 3.6 billion years old. This is younger than the oldest rocks from other solar system objects, why?

How? plate tectonics. Note that we have not found a way to determine the exact age of the Earth directly from Earth rocks because Earth's oldest rocks have been recycled and destroyed by the process of plate tectonics. If there are any of Earth's primordial rocks left in their original state, they have not yet been found.

Continental Drift:

Any major new idea in science appears to lead instantly to a search of the past for those who might once have proposed similar concepts and with whom the current proponents should therefore share the credit. In the case of plate tectonics, the primary candidate is obvious: Alfred Wegener of Germany, who explicitly presented the concept of continental drift for the first time at the outset of the 20th century. Though plate tectonics is by no means synonymous with continental drift, it encompasses this idea and derives much of its impact from it.

avant.gif apres.gif

In 1858, geographer Antonio Snider-Pellegrini made these two maps showing his version of how the American and African continents may once have fit together, then later separated. Left: The formerly joined continents before their separation. Right: The continents after the separation.

There might have been predecessors even to Wegener. The outlines of the continents bordering the Atlantic Ocean are so similar that many probably noticed the correspondence, and some might have drawn the conclusion that the lands on both sides were once joined together. The earliest reference to this peculiar geographic feature was made by the English philosopher Francis Bacon. In his Novum Organum (1620), Bacon pointed out the correspondence but did not go beyond that. Such was also the extent of the contribution of the great French naturalist Georges-Louis Leclerc, Count du Buffon, a century later. Neither can Franois Paget qualify as a forerunner of continental drift theorists: even though he stated in 1666 that an undivided continent existed before Noah's flood, he explained the creation of the Atlantic Ocean by having part of that continent sink into the sea.

As noted by Snider-Pellegrini and Wegener, the locations of certain fossil plants and animals on present-day, widely separated continents would form definite patterns (shown by the bands of colors), if the continents are rejoined.

The first credible proponent of continental drift was Antonio Snider-Pellegrini, a belated advocate of catastrophism who, in 1858, ascribed the biblical flood to the former existence of a single continent that was torn apart to restore the balance of a lopsided Earth. More recent and much more sophisticated was the work of the American geologist Frank Taylor, who, disdaining the then-prevailing contraction model of mountain building, postulated in 1908 that the arcuate mountain belts of Asia and Europe resulted from the equatorward creep of the continents. His analysis of tectonic features foreshadowed in many ways modern thought regarding plate collisions, and he anticipated Wegener's publications by only a few years. Curiously, however, his work instantly sank into oblivion.

Plate Tectonics:

The evolution of species on the land is linked to and driven by various climatological and geological changes that operated on the land surface of the earth.

As we will discuss later, the earth currently has significant climate variations on a timescale of 100,000 years. In addition, over the last 200-250 million years the earth is experiencing an era of global tectonic motion which makes the land surface a Dangerous Place to Live:

Plate Tectonics means that the crust of the earth is divided into large connected units, all of which are moving relative to one another and colliding with one another in various ways. The idea of Plate Tectonics was first published by the German geologist, Alfred Wegener in 1915 but this theory was largely ridiculed until magnetic mapping of the ocean floor was done in the late 50's.

Summary of Evidence for Plate Tectonics:

Now it is recognized that the surface of the earth can be divided up into roughly 10--12 large scale plates and perhaps a number of smaller ones as in the case of the Pacific Northwest.

The plates move with typical velocities of 50 to 100 mm/yr, which is slow, but considering the mass of the plates means a great deal of energy is stored in the kinetic energy of plate motion => earthquakes.

Paleomap Project

Thermal Plate Tectonics:

The driving mechanism of plate tectonics is a network of convective heat currents, generated by the hot core of the earth and which circulate in the mantle. The heat is provided from the decay of Uranium-238 which is an R-process Supernova element. The overall transport of heat from the core through the mantle is quite inefficient so it takes a long time for these convective heat currents to become established. Hence, plate movements are something which occurs late (i.e. now) in the geological history of the earth.

The earth's crust is actually a two-component layer. The lithosphere is a thin layer of rock (average density of 2.7 grams per cc) and "floats" on top of a plastic-like layer called the asthenosphere. Plastic-like materials are weird - they deform under stress but don't really break. A glacier is a good example of a material that moves and flows plastically. The convective heat currents in the mantle impinge on the asthenosphere causing deformation and subsequent movement of the lithospheric plates.

This process can be simulated in your kitchen by putting some jello in a bowl and putting some peebles on top of the jello. As you shake the bottom of the bowl, the jello deforms but doesn't break and the rocks that float on the jello collide. (apologies to real geologists for this analogy).

As a result of plate movements, interesting things occur at plate boundaries. In general you don't want to live near a plate boundary as the earth is active there. About 75% of the world's population does live near these boundaries.

There are three types of plate boundaries:

Local Manifestations of Plate Tectonics:

The Pacific Northewest is an active tectonic zone. One of the prime hazards of active volcanoes is the heavy mudflows which can result from the sudden melting of their heavily glaciated slopes.

As early as the 1920s, scientists noted that earthquakes are concentrated in very specific narrow zones, now known to be plate edges . In 1954, French seismologist J.P. Roth published this map showing the concentration of earthquakes along the zones indicated by dots and cross-hatched areas.

An example of tectonic activity in the form of volanic activity on the Earth = Mt. St. Helens:



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