Although Boltwood's ages have since been revised, they did show correctly that the duration of geologic time would be measured in terms of hundreds-to-thousands of millions of years. Boltwood, radiochemist of Yale University, published a list of geologic ages based on radioactivity. In 1905, the British physicist Lord Rutherford - after defining the structure of the atom - made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly shortly thereafter, in 1907, Professor B. The discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, the French physicist, opened new vistas in science. Nearly every name signifies the acceptance of a new scientific concept - a new rung in the ladder of geologic knowledge. The names used to designate the divisions of geologic time are a fascinating mixture of works that mark highlights in the historical development of geologic science over the past 200 years. Geologists have done the same thing to geologic time by dividing the Earth's history into Eras - broad spans based on the general character of life that existed during these times, and Periods - shorter spans based partly on evidence of major disturbances of the Earth's crust. Shorter spans are measured by the dynasties of ancient Egypt or by the reigns of kings and queens in Europe. We speak of human events as occurring either B.C. By comparison, the history of mankind is similarly organized into relative units of time. Such recurring events as mountain building and sea encroachment, of which the rocks themselves are records, comprise units of geologic time even though the actual dates of the events are unknown. The evidence of the pre-existence of ancient mountain ranges lies in the nature of the eroded rock debris, and the evidence of the seas' former presence is, in part, the fossil forms of marine life that accumulated with the bottom sediments. With the sinking of the land surface, sediments were deposited on the ocean floor. During the same interval of time in another part of the world, the land surface subsided and was covered by the seas. After the uplift of the land, the forces of erosion attacked the highlands and the eroded rock debris was transported and redeposited in the lowlands. Their studies show, for example, that during a particular episode the land surface was raised in one part of the world to form high plateaus and mountain ranges. Other rocks were so transformed by heat and pressure during the heaving and buckling of the Earth's crust in periods of mountain building that their original features were obliterated.įrom the results of studies on the origins of the various kinds of rocks (petrology), coupled with studies of rock layering (stratigraphy) and the evolution of life (paleontology), today geologists reconstruct the sequence of events that has shaped the Earth's surface. Some rocks were once lava flows or beds of cinders and ash thrown out of ancient volcanoes others are portions of large masses of once-molten rock that cooled very slowly far beneath the Earth's surface. Certain layers are in the form of sand bars and gravel banks - rock debris spread over the land by streams. Other layers, consisting of sand grains winnowed clean by the pounding surf, obviously formed as beach deposits that marked the shorelines of ancient seas. Some rock layers, containing clearly identifiable fossil remains of fish and other forms of aquatic animal and plant life, originally formed in the ocean. ![]() Careful studies by scientists showed that rocks had diverse origins. The full text the USGS publication titled, Geological Time is available online.Īt the close of the 18th century, the haze of fantasy and mysticism that tended to obscure the true nature of the Earth was being swept away.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |