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.
See these guys who claim to have done better
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.
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)
Unknown at the time was that the Earth has an internal heat source (radioactive decay)
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.
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.
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.
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.
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.
Why is the Earth younger than the moon and meteorites?:
Current estimages of the age of the Earth are 4.5 billion years, but the oldest rocks are only 3.6 billion years old. This is younger than the oldest rocks from other solar system objects, why?
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.
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.
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.
Thermal Plate Tectonics:
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.
The outflow of heat from core is transfered to convective motion in mantle. The motion is converted in linear motion of the crustal plates. There are 12 plates all floating on the mantle with speeds of a few cm per 100 years.
There are four (4) types of boundaries between the plates which give rise to particular surface features. For example, colliding plates form mountains. Young mountain system are sharp and irregular (e.g. Himalayas), old mountain systems are low and rounded (e.g. Appalachians)