The ten most important discoveries in the saga of discovering our planet
In earth sciences we included the various branches of geology (geophysics, geochemistry, stratigraphy), and atmospheric sciences - meteorology and oceanography. We tried to map mainly discoveries that shaped a worldview, and that became part of the main scientific dogmas. The choice was made by proposing the main topics by the exercise center (YK), and during a very large number of conversations we tried to focus on the most important ones. We enjoyed the exercise quite a bit, because it focused the attention of all of us on marking central and important issues in the science in which we are engaged.
In 1785, James Hutton presented a lecture that was later published in his book Theory of the Earth, which is considered the beginning of modern geology. Hutton formulated the principle of uniformitarianism, which states that "the present is a key to the past", in other words - knowledge of processes and their results in nature today explains similar phenomena in the geological past.
Hutton realized that rocks such as granite and basalt were formed from hot silicate melt (magma) and were not deposited from water, as had been thought until then. He treated rocks as an archive, a record of the processes that created them. He concluded that Kada is much older than what is implied by the biblical dating. Here is the beginning of the use of the term "geological time" - a very long time.
Hutton's insights opened a period of about fifty years of revolutionary discoveries in geology, the peaks of which are:
A. George Cuvier (1811), who used fossils to build a stratigraphic order (order of layers) of sedimentary rocks; This is how the concept of time received its one-way meaning - determining early and late in the order of events.
B. William Smith (Smith), who identified with the help of fossils relations (correlation) between layers in different places, and in 1815 he published the first geological map, which is a graphic summary of the different conditions that prevail in different areas, and of the changes that apply to these conditions.
third. The culmination of the period was the publication of Charles Lyell's book "Fundamentals of Geology" in 1830, which summarizes the knowledge accumulated up to that time, and prepares the ground for the appearance of the giant - Charles Darwin.
2. Deciphering the internal structure of the earth
In 1798, Henry Cavendish measured the average density of KDA and found it to be 5.45 g/cm2.7 (compared to about 1848 g/cmXNUMX of surface rocks). Since then it has been clear that there is no "inside of the ball already". The way to find out the internal structure of the sphere was opened in XNUMX when Robert Mallet established the hypotheses of Thomas Young and Joseph Gay-Lussac that the energy of earthquakes travels in waves, and measured their speed.
In 1883, John Milne determined that this energy was so great that it could be recorded in distant places on Earth's surface. When the seismograph was built, the basis for seismology was created, a new science that greatly contributed to understanding the Earth's interior structure and the study of earthquakes.
At the beginning of the 20th century, Richard Oldham showed that seismic energy propagates in several types of waves, with different speeds in different mediums. In the center of the Earth there is a liquid core, through which no shear waves pass, whose radius is about half the radius of the sphere and surrounded by a shell, and each region - the core, the shell and the crust - has its own seismic wave speed. George Airy (Airy) and John Pratt (Pratt) explained the topography of the country with a model of a crust of hard rock floating on top of a mantle of dense and soft material.
In 1906, Andrei Mohorovičić verified the suggestion that between the crust and the mantle beneath it there is a sharp change in seismic velocities. In 1936 Inga Lehman showed that the inner part of the nucleus is solid and only the outer part is liquid. In this way, the "spherical model" of KDA was presented - the first approximation that assumes that the structure of the sphere is the same in all directions and changes only with depth. Many studies in the 20th century dealt with the second approximation - the structural changes in different regions. These discoveries were central to the development of plate tectonics. A major challenge facing seismologists today is mapping the temperature and flow patterns in the mantle.
3. Plate tectonics
Plate tectonics is currently the main theory of geology. The harbinger of this theory was Alfred Wegener, who in 1915 published his book "The Formation of the Continents and the Oceans" and in it a series of "continental migrations", which explains many phenomena on Earth: the correspondence between the descriptions of the various continents that can be connected to a supercontinent One, the appearance of identical terrestrial fossils in identical periods across continents that are now separated by oceans, the location of mountain ranges, and more.
Wagner's theory was almost overwhelmingly rejected, and for more than three decades few supported it. Science returned to its approach thanks to the development in several areas: the mapping of the simulated magnetic "pole migration", which is different for Europe and America, was well explained by continental migration. The mid-ocean ridges that extend for thousands of kilometers led Hess and Robert Dietz to propose in 1962 that the ocean floor is "spreading" (seafloor spreading) from the mid-ocean ridges and "carrying" the continents on it.
Acceptance of this concept, along with the mapping of the magnetic field in the oceans, and the understanding that this field undergoes periodic reversals from time to time, led to the revolutionary article by Fred Vine and Drummond Matthews (Vine and Matthews, 1963) who presented an almost complete theory, which connects apparently unrelated phenomena: the outline of the continents, their age The younger of the sea floor rocks and the distribution of the volcanoes.
Mapping the epicenters of the earthquakes led to the formulation of the Torah in its entirety: a number of plates moving in relation to each other, with most of the distortion concentrated at the borders, and only some of it inside the plates. The new Torah made it possible to link phenomena that occur on the surface, such as the rising of mountains and the eruption of volcanoes, with the flow of heat inside the earth.
Geochemical differentiation in the interior of the earth is related to the rolling of material that is affected by magmatic processes on the surface, and the transport ("subduction") of sedimentary rocks to the face of the globe. A very large series of ocean floor drilling (DSDP, ODP), which began in 1968 and continues to this day, confirmed the predictions of the plate theory.
4. Determining the age of the earth - 4.55 x 109 years
The age of the planet we live on is of central importance in building our view of the material world. In the second half of the 19th century, Lord Kelvin (William Thomson - Lord Kelvin) began attempts to estimate the age of the sphere using physical methods: assuming that the sphere was entirely molten at the beginning, he calculated the time needed to cool it, and arrived at an age estimate of between forty ms and 400 MS. The geological community was united in its opinion that this estimate is much lower than the time needed to complete the structure of the landscape and the development of the fauna of Kada.
With the discovery of radioactivity a new method of time measurement was found. The first to raise this possibility in 1906 was Ernest Rutherford, who was to discover the atomic nucleus. By measuring the uranium content in minerals, as well as that of the decomposition product of uranium - the element helium (whose nuclei are alpha particles) - Rutherford for the first time directly estimated the age of a mineral - about 1500 m.s. This is how the science of geochronology was born. If it turns out that two isotopes of uranium, 238U and 235U, decay into two isotopes of lead, 206Pb and 207Pb, with different half-lives, dating will be possible based on determining the isotopic composition of the lead only. In these measurements, the age of KDA was estimated at about 3000 m.s.
The decisive step was taken in 1956 by Claire Patterson, who determined from the ratios of 206Pb, 207Pb, and 204Pb (the non-radiogenic isotope) in meteorites and oceanic sediment that the age of Earth (and the solids in the solar system) is 4.55 billion years. This age estimate has not been significantly changed since then, and has been verified by other dating methods. Geochronological methods have led to the ability to date events over vast time spans: from archaeological objects and various rocks to determining the age of the moon rocks (as the age of the earth) and the age of the elements in the universe. The use of radioactive isotopes later led to a great understanding of the composition of the deep matter in DHA and its behavior - the degree of its uniformity and the speeds of its movement.
5. The thermohaline flow in the ocean and the Earth's climate
Already in the fifties of the 20th century, Henry Stommel estimated, based on changes in water depth in the oceans, that there are only two sites on the surface of the Earth where water can sink from the surface to the depths. These sites are in the Atlantic Ocean: one in the north, east of the island of Greenland, and the other in the south, near the continent of Antarctica.
The cold water at the edge of the frozen Arctic Sea and in the Southern Ocean at the edge of the Antarctic continent sinks deeply because of its great density, and fills the bottom of the oceans: the Pacific, the Indian, and of course the Atlantic. The descent of dense water into the depths of the ocean in the high latitudes must be accompanied by the flow of warm surface water from the tropics to the high latitudes, and this causes a recession of the weather on the European coasts of the North Atlantic.
Stommel's discovery was joined in the XNUMXs to a more perfect picture of the oceanic conveyor belt, which carries enormous amounts of heat towards the high latitudes on the surface, and transports colder water from the high latitudes towards the equator in the depths of the ocean. The oceanic heat convection, as well as the density of the sea water, depends not only on the temperature of the water but also on the concentration of salts in the water: high salinity increases their density.
The strength of the oceanic conveyor belt is therefore affected both by the cooling of the water in the higher regions (which increases the flow in the conveyor belt) and by the increase in the salinity of the water in the lower regions, due to the increased evaporation there (which decreases the flow in the conveyor belt). It became clear to the climate researchers that a violation, even a small one, of this delicate balance of evaporation-cooling can significantly reduce the strength of the conveyor belt, and significantly change the climate of Kada.
6. The regime of winds in the atmosphere and the importance of the rotation of the earth
Already in the 17th century, the sailors who transported goods in sailing ships between Europe and America knew that the westward movement in the equatorial latitudes is faster than in the subtropical regions, and this is because of the winds ("trade winds"). An explanation for this was proposed at the end of the same century by Edmund Halley, who linked the heating by the sun in the tropics with the rise of the heated air upwards and its flow on the surface from the subtropics to the equator.
Halley's proposal did not explain the observation of the trade winds, and it was only in the 18th century that George Hadley connected the rotation of the equator with the deviation of the trade winds from the pure meridional direction. Hadley explained the formation of the regional component (from east to west) in the trade winds by the fact that the air coming from high areas has a smaller eastward velocity component than that present in low areas, due to the difference in the circumference of the circle that these winds travel during the day, and which therefore the winds must develop a component directed westward in relation to some fixed point.
About a hundred years later, Gustav Coriolis formulated the laws of motion in a rotating system, and mathematically proved the existence of the force that still bears his name: the Coriolis force. This force is caused by the rotation of the system, and its magnitude is proportional to the speed of the moving body relative to the system. This formulation was the basis for predicting winds based on measurements of the pressure drop, by assuming an equilibrium between the pressure drop force and the Coriolis force, and confirmed Hedley's conclusions.
Only in the modern era did complex numerical calculations take the place of the simple-approximate calculation, which resulted directly from Coriolis' formulation. Today it is known that the cell of the bucket does not extend to the poles, and that the agitators of the medium spans have an important and complex role in transferring the momentum and heat from the low spans to the high spans.
7. Understanding the impact of man on his environment and climate change
Paul Crutzen proposed to see these days as the beginning of a new geological era, the Anthropocene, in which man's influence on Earth increased. About a century ago, Svante Arrhenius estimated that doubling the concentration of carbon dioxide in the atmosphere due to the burning of fossil fuels would lead to a 5ºC increase in global temperature, and indeed the accepted estimate today is 1.5 to 4.5 degrees.
CO2 is second only to water as a "greenhouse gas" - transparent to the incoming solar radiation, and absorbs a significant part of the heat radiation emitted from the surface towards space. Another "greenhouse gas" is methane, (CH4) whose concentration in the atmosphere has increased almost threefold in the last 150 years as a result of natural gas utilization, increasing the area of rice fields, cattle breeding and fires. The increase in the concentration of greenhouse gases was recorded both directly in the atmosphere and indirectly, in air bubbles trapped in glaciers.
Global warming causes glaciers to melt and increase the amount of water vapor in the air (which act as greenhouse gases and increase warming); The melting of the ice decreases the brightness of the surface (albedo) - that is, the amount of solar radiation returned to space, thus increasing the warming. The melting of the glaciers is also manifested in the rise of ocean levels and the flooding of coastal areas. On the other hand, the increase in particulate air pollution in the atmosphere causes cooling, both by increasing brightness and by creating clouds, which return more solar radiation to space.
Man's influence on the atmosphere is also evident in the emission of additional gases: a. Sulfuric oxide and nitric acid, which cause the formation of acid rain b. Various gases, mainly nitrogen oxides (NOx) and sulfur, which contribute to the formation of smog and c. Contribution of artificial gases, such as chlorine-fluorine-carbon compounds (ClFC and among them the freon used for refrigeration. These stable gases rise to the stratosphere, and there - in a chain of photochemical reactions - contribute to the destruction of the ozone layer ("the hole in the ozone"). Understanding the process led to the "Montreal Protocol ”, which indeed contributed to the partial restoration of the ozone layer.
8. Ice ages and paleo-climate
Back in the middle of the 19th century it was clarified that during the Pleistocene period (about the last million years) glaciers spread over large areas of the Northern Hemisphere. The evidence for this came from typical glacial deposits and unique landscape forms found in Europe and North America at low geographic latitudes.
A revolutionary step in paleoclimate research was made in 1948 when Harold Urey and then Samuel Epstein and his colleagues (Epstein et al.) showed that the distribution of oxygen isotopes (the ratio (18O/16O between water and calcium carbonate (CaCO3) (such as in fossil shells) is a function of temperature. This made possible isotopic paleothermometry - measuring the past temperatures of oceans.
From an isotopic test in the shells of foraminifera (a group of single-celled) in marine sediments, it became clear that in the last 700,000 years the appearance of glaciers was cyclical: about ten ice ages and interglacial periods in between. During the cold periods, a lot of ice accumulated at the poles, the sea level dropped and the isotopic composition of the ocean water changed. The dating of the ice ages made it possible to re-examine Milotin Milankovitch's proposal from 1912 according to which they are caused by changes in the solar radiation reaching the earth, following cyclical changes in the geometry of the earth's rotation around the sun and around an axis. From this it was possible to predict the length of the glacial cycles, a prediction that can now be checked against the timings of the Pleistocene cycles.
In the last three decades, deep ice drillings have been drilled in Greenland and Antarctica. The ice cores included several glacial cycles identified by the isotopic composition of the oxygen and hydrogen in the ice. Ancient air was also trapped in ice, the examination of which showed the connection between global temperatures and the concentration of the greenhouse gas - CO2 - in the air.
9. Understanding the way rocks are formed from physical and chemical principles
The sciences are first and foremost observational sciences. Many observations can be analyzed using methods developed in the framework of geology, but knowledge from the fields of physics and chemistry allows for further understanding. Victor Goldschmidt (Goldschmidt) and Norman Bowen (Bowen) stood out for the breakthroughs they achieved in the 20s and XNUMXs.
Bowen and other researchers developed systems for laboratory experiments at high temperatures and pressures, and used them and the principles of physical chemistry to investigate the creation of igneous rocks. Bowen began his investigations with field observations, simplified the observational systems so that they could be investigated in the laboratory, and used the results of the experiments to formulate a new understanding of the formation of trending rocks in nature. His book on the evolution of trend rocks (1928) established the understanding of these rocks as products of processes of melting, formation and chemical interaction between the melt and the rocks it penetrates.
Goldschmidt studied the behavior of rare chemical elements and their integration into crystals common in nature. His main achievement was the division of the elements into four "geochemical families" according to their tendency to form compounds in nature: those concentrated in the atmosphere, those that tend to appear as silicates, those that appear mainly as sulfides, and those that tend to appear as metals or metal melts.
Goldschmidt combined the study of the chemistry of crystals and the measurement of the concentration of the various elements in minerals, rocks, meteorites and main reservoirs in DHA. And based on these studies and the understanding that meteorites come from small planetary bodies, Goldschmidt proposed a chemical identification for the concentric shells of Kadaha: the inner one is rich in iron and its "lovers" (the core), above it is a layer rich in sulfur and its "lovers" (a disproved hypothesis), another layer of silicates Rich in magnesium (the mantle) and a crust that contains the "slag" of an ancient smelting process.
The picture as a whole is close to the accepted structure today, except for the understanding that the crust was built in a continuous process. Goldschmidt laid the foundation for the fertile field of trace element geochemistry.
10. The history of life on Earth: evolution, catastrophes, extinctions
The animal world on Earth has undergone enormous changes from the beginning of life about 3.5 billion years ago until today. Explosiveness of fossils in different periods was explained by most researchers in slow evolutionary development, in accordance with Darwin's theory. An exception to this current of thought was the early 19th century Frenchman Georges Cuvier, who argued that the major changes in the fossil world are the result of catastrophes - the mass destruction of populations and their replacement by others.
In the sixties and seventies of the 20th century, with the development of radioactive dating methods, it became clear that many changes in the fossil world were very rapid: at the boundary between the Permian and Triassic periods (about 250 ms ago), more than ninety percent of the oceanic species became extinct, and at the boundary ) that between the Cretaceous and Tertiary KT (about 66 ms ago) about fifty percent of all species and about 15 percent of all families became extinct, including the dinosaurs. It turned out that these limits are global extinction events, following which completely new groups of creatures flourished at once. This is how the "warm-blooded" mammals, which before were a small and marginal group, developed at the KT border.
Climate changes, volcanism, changes in temperature and sea level have been proposed as reasons for the extinctions. The most fascinating proposal is that of Luis Alvarez, winner of the Nobel Prize in Physics, his geophysicist son Walter and their friends in 1980. This group found that clays in the KT boundary layer are almost tenfold enriched in the element iridium (Ir), whose concentration in the crust is very low, but in meteorites its concentration is much higher. They suggested that the enrichment in iridium is the result of a meteorite impact on DHA. The impact created huge clouds of dust, blocked sunlight, radically changed the climate and wiped out most of the animal world.
Much supporting evidence was found for Alvarez's hypothesis: they identified quartz crystals that indicate an impact, they identified an iridium anomaly in many other places in the world, and finally the identification of the specific place of the impact that caused the extinction at the end of the Cretaceous: the buried crater of Chicxulub, on the Yucatan Peninsula in Mexico.
The authors of the article are members of the KDA Institute of Science at the Hebrew University of Jerusalem, researchers and teachers in geology, atmospheric sciences and oceanography
