Geological Change (1830s)

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In his most famous and most influential multivolume work “Principles of Geology,” Scottish lawyer-geologist Charles Lyell (1797 - 1875) advocated the controversial doctrine of uniformitarianism, an assumption that geological and natural processes that occur in the past are the same as those in operation in the present. It is based on the idea that the Earth is constantly being shaped and reshaped by forces gradually acting on it over a long period of time. The doctrine is in contrast to the then accepted idea of catastrophism, which proposes that the Earth undergoes from time to time sudden and violent changes that can be worldwide in scope, such as earthquake and flood. Today’s scientists hold a more consolidative view of both doctrines.

Troposphere and Stratosphere (1890s)

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With his pioneering use of unmanned balloons equipped with scientific instruments, French meteorologist Leon Teisserenc de Bort (1855 - 1913) observed that air temperature steadily decreases up to an altitude of 11 kilometers (7 miles), but remained constant above that height. Following at least 200 balloon experiments, he indicated that the atmosphere is made up of two layers called the troposphere (the lowest part of the Earth’s atmosphere) and the stratosphere (the second layer).

Magnetic Field Reversal (1900s)

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In his paleomagnetic study of rocks hardened by Miocene lava flow around 13 million years ago, French geophysicist Bernard Brunhes (1867 - 1910) discovered that magnetic direction of Earth then was nearly in direct opposite to present-day magnetic field, that is, the Earth’s magnetic field has reversed direction. However, it would take another fifty years before his discovery was fully acknowledged as correct by the scientific community.

Radiometric Dating (1900s)

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American chemist Bertram Boltwood (1870 - 1927) calculated the age of rocks by measuring the rate of its radioactive decay from uranium to lead. The result of his calculations placed the age of the Earth at 2.2 billion years. Much older rocks or minerals have recently been discovered and have been dated at 4.4 billion years old, an age that is very close to the best estimate of the age of the Earth determined by radiometric dating of lunar samples. His method has been found to be compatible with other radioactive elements, including carbon-14, which is used extensively by geologist, paleontologist and archeologist to date historical artifacts.

Earth’s Core (1900s)

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Irish geolologist Richard Oldham (1858 - 1936) discovered that seismic waves passing through the center of the Earth move much slower than through the mantle around it; and deduced the fact that the Earth has a liquid core as seismic wave cannot travel as rapidly through liquids as through materials of lesser density.

Earth’s Inner Core (1930s)

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In 1936, Danish seismologist Inge Lehmann (1888 - 1993) verified that some seismic waves do not pass through the Earth’s core but instead are reflected back. Based on these observations, she concluded that the Earth has an outer core composed of molten iron that surrounds an inner core of solid iron sphere.

Continental Drift (1910s)

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Struck by the similarities of fossils in geological strata oceans apart and the seeming fit of the of the eastern bulge of South America with the bend of Africa’s west coast, German meteorologist Alfred Wegener (1880 - 1930) advance the theory that all of the continents in the world once formed a single giant continent that ultimately split apart around 200 million years ago in a process called continental drift. The theory was not generally accepted until it is incorporated into the theory of plate tectonics and seafloor spreading developed in the 1960s.

Seafloor Spreading (1950s - 1960s)

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Integrating his accumulated data on the changes in seafloor depth with the discovery of other geologists, American geologist Harry Hess (1906 - 1969) proposed the theory that the Earth’s crust moved sideways away from volcanically active oceanic ridges, a process called seafloor spreading, resulting in continental drift. He hypothesized that when molten magma beneath the Earth’s crust seeps through the Mid-Ocean Ridge, it pushes the plates away from the rift as the hot magma cools, bringing about the widening of the Atlantic Ocean over time.

Plate Tectonics (1960s)

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The work of many scientists reveals that the Earth’s outermost layer or crust, called “lithosphere,” is made up of at least seven interconnected plates, the movement of which results in continental drift and mountain building. Some plates diverge at ocean ridges, pushed apart by molten magma, forcing its way up between the plates. This process can throw up volcanic mountains above sea level. Where plates converge or collide, one plate can go under another, thrusting the other upwards. This is called a subduction zone, and these zones encircle the Pacific in a so-called “ring of fire,” where volcanoes erupt and earthquakes happen with great frequency.

Cosmic Radiation (1910s onward)

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In 1912, Austrian physicist Victor Hess (1883 - 1964) personally flew by hot air balloon, outfitted with measuring instruments, to a height of 5.3 kilometers (3.3 miles) and discovered that levels of radiation rise with altitude. With further meticulous experiments using equipments of higher precision, he became deeply convinced that the radiation came from outer space and penetrated the Earth’s atmosphere. His discovery was subsequently confirmed in 1925 by American experimental physicist Robert Andrews Millikan (1868 - 1953), who gave the radiation its name, and led to many advances in the field of nuclear physics.

Periodic Ice Ages (1930s)

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Serbian astrophysicist Miultin Milankovic (1879 - 1958) expounded on the theory that the Earth goes through periodic cycles of ice ages and long-term climate change due to variations in its motions, such as the orbital shape (eccentricity), axial tilt (obliquity), precession (wobble) and orbital inclination, resulting in different levels of solar energy reaching the Earth.

Global Warming (late 20th century)

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Many scientists believe that there is increasing evidence that the Earth is warming up due to the increasing concentration of greenhouse gases, such as carbon dioxide and methane. Levels of carbon dioxide in the atmosphere have been steadily rising since the Industrial Revolution took effect in the late 18th century. Global warming theory states that temperature of the Earth’s atmosphere will increase if these greenhouse gas emissions continue, resulting in unpredictable effects on the climate and on sea level.

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