The Age of the Earth

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.

• First, it indirectly led to the formulation of an opposing, less extreme view by the Scottish scientist James Hutton in 1785.
• Second, it was in some measure correct: catastrophes do occur on the Earth and they do change its landforms. Asteroid impacts, Krakatoa-type volcanic explosions, hurricanes, floods, and tectonic erosion of mountain systems all occur, may be catastrophic, and can create and destroy landforms. Yet, not all change is catastrophic.

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.

Uniformitarianism:

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.

   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?

• Earth is geologically active.
• Earth has a hot, molten interior.
• Rocks are remelted and their internal clocks are reset.
• Also, rocks on Earth's surface are acted on by erosion and weathering. Rocks on Earth surface are not as old as the Earth, they are "recycled" rock materials. Rocks are broken down into sediment (gravel, sand, silt, clay). Sediment will turn into sedimentary rock over time. Older rocks are buried deeply under younger rocks.

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.

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.

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)