History of geology

The history of geology is concerned with the development of the natural science of geology. Geology is the scientific study of the origin, history, and structure of the Earth. [1] Throughout the ages geology provides essential theories and data that shape how society conceptualizes the Earth.

Contents

Antiquity

A mosquito and a fly in this Baltic amber necklace are between 40 and 60 million years old

Some of the first geological thoughts were about the origin of the Earth. With a lack of knowledge and technology, ancient philosophers created mythical stories and proposed theories to explain how the Earth came to be. Ancient Greece developed some primary geological concepts concerning the origin of the Earth. Additionally, in the 4th century BC Aristotle made critical observations of the slow rate of geological change. He observed the composition of the land and formulated a theory where the Earth changes at a slow rate and that these changes cannot be observed during one person’s lifetime. Aristotle developed one of the first evidentially based concepts connected to the geological realm regarding the rate at which the Earth physically changes.[2]

However, it was his successor at the Lyceum, the philosopher Theophrastus, who made the greatest progress in antiquity in his work On Stones. He described many minerals and ores both from local mines such as those at Laurium near Athens, and further afield. He also quite naturally discussed types of marble and building materials like limestones, and attempted a primitive classification of the properties of minerals by their properties such as hardness.

Much later in the Roman period, Pliny the Elder produced a very extensive discussion of many more minerals and metals then widely used for practical ends. He was among the first to correctly identify the origin of amber as a fossilized resin from trees by the observation of insects trapped within some pieces. He also laid the basis of crystallography by recognising the octahedral habit of diamond.

Middle Ages

Abu al-Rayhan al-Biruni (973-1048 AD) was one of the earliest Muslim geologists, whose works included the earliest writings on the geology of India, hypothesizing that the Indian subcontinent was once a sea:[3]

"But if you see the soil of India with your own eyes and meditate on its nature, if you consider the rounded stones found in earth however deeply you dig, stones that are huge near the mountains and where the rivers have a violent current: stones that are of smaller size at a greater distance from the mountains and where the streams flow more slowly: stones that appear pulverised in the shape of sand where the streams begin to stagnate near their mouths and near the sea - if you consider all this you can scarcely help thinking that India was once a sea, which by degrees has been filled up by the alluvium of the streams."[3]

Ibn Sina (Avicenna, 981-1037), a Persian polymath, made significant contributions to geology and the natural sciences (which he called Attabieyat) along with other natural philosophers such as Ikhwan AI-Safa and many others. Ibn Sina wrote an encyclopaedic work entitled “Kitab al-Shifa” (the Book of Cure, Healing or Remedy from ignorance), in which Part 2, Section 5, contains his commentary on Aristotle's Mineralogy and Meteorology, in six chapters: Formation of mountains, The advantages of mountains in the formation of clouds; Sources of water; Origin of earthquakes; Formation of minerals; The diversity of earth’s terrain. Goodfield (1965), commented on Avicenna's contribution: "Around A.D. 1000, Avicenna was already suggesting a hypothesis about the origin of mountain ranges, which in the Christian world, would still have been considered quite radical eight hundred years later".[4]

"It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, a further was formed and piled, upon the first, and so on. Over each layer there spread a substance of different material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimantary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains."

In medieval China, one of the most intriguing naturalists was Shen Kuo (1031-1095), a polymath personality who dabbled in many fields of study in his age. In terms of geology, Shen Kuo is one of the first naturalists to have formulated a theory of geomorphology. This was based on his observations of sedimentary uplift, soil erosion, deposition of silt, and marine fossils found in the Taihang Mountains, located hundreds of miles from the Pacific Ocean. He also formulated a theory of gradual climate change, after his observation of ancient petrified bamboos found in a preserved state underground near Yanzhou (modern Yan'an), in the dry northern climate of Shaanxi province. He formulated a hypothesis for the process of land formation: based on his observation of fossil shells in a geological stratum in a mountain hundreds of miles from the ocean, he inferred that the land was formed by erosion of the mountains and by deposition of silt.

Industrial Revolution

During the 17th century the heated debate between religion and science over the Earth’s origin further propelled interest in the Earth and brought about more systematic identification techniques of the Earth’s strata.[5] The Earth’s strata can be defined as horizontal layers of rock having approximately the same composition throughout.[6]

Bust of William Smith, in the Oxford University Museum of Natural History.

The popular mining industry during the 18th century both increased social interest and drove scientists to form more systematic and detailed studies of the composition of the Earth’s strata. From the increased societal interest of geology, in 1741 it became a specific field of study to be taught at the National Museum of Natural History in France.[7] The controversial topic of the Earth’s origin continued to circulate between religious and scientific circles. Two feuding theories developed to explain the Earth’s origin with designated followers: the Neptunists whose theory supported that of the Bible’s Great Flood and the Plutonists who believed the Earth gradually formed over an immeasurable amount of time.[8]

The dialogue about the creation of the Earth occurring within the scientific community and the evidence being uncovered in Civil Engineering works during the 19th century drove the development of the stratigraphical column; many of the concepts behind this invention can be attributed to William Smith, Georges Cuvier and Alexander Broignart.[9] Also in this period, imperialism motivated countries to sponsor voyages of exploration to distant lands. Charles Darwin made geological observations on such a voyage, providing evidential support of his revolutionary theory of evolution.[10] Again a religious debate ensued; two conflicting groups, uniformitarians and catastrophists, argued over the age of the Earth.[11] Charles Lyell, an influential uniformitarian, published his book in 1830 the “Principles of Geology” which proposed that the Earth changes very gradually and is immeasurably old.[12]

The theory of Continental Drift was proposed in 1912 by Alfred Wegener.[13] This idea, unaccepted at the time, suggested a method of continental movement that occurred throughout history.[14] Supporting evidence of Continental Drift, including seafloor spreading and paleomagnetism, justified the theory of Continental Drift, which in the late 1960s was replaced and encompassed by Plate Tectonics.[15] In the latter half of the 20th century the approach to the study of geology changed to evaluating the Earth in a broader perspective.[16] To coincide with this perspective, satellites were first used in the 1970s and are still currently in use by the Landsat Program to produce images of the Earth that can be geologically studied.[17]

17th century

A portrait of Whiston with a diagram demonstrating his theories of cometary catastrophism best described in A New Theory of the Earth

It was not until the 17th century that geology made great strides in its development. At this time, geology became its own entity in the world of natural science. It was discovered by the Christian world that different translations of the Bible contained different versions of the biblical text. The one entity that remained consistent through all of the interpretations was that the Deluge had formed the world’s geology and geography.[18] To prove the Bible’s authenticity, individuals felt the need to demonstrate with scientific evidence that the Great Flood had in fact occurred. With this enhanced desire for data came an increase in observations of the Earth’s composition, which in turn led to the discovery of fossils. Although theories that resulted from the heightened interest in the Earth’s composition were often manipulated to support the concept of the Deluge, a genuine outcome was a greater interest in the makeup of the Earth. Due to the strength of Christian beliefs during the 17th century, the theory of the origin of the Earth that was most widely accepted was A New Theory of the Earth published in 1696, by William Whiston.[5] Whiston used Christian reasoning to “prove” that the Great Flood had occurred and that the flood had formed the rock strata of the Earth.

18th century

From this increased interest in the nature of the Earth and its origin, came a rise in the interest of minerals and other components of the Earth’s crust. Moreover, the increasing commercial importance of mining in Europe during the mid to late 18th century made the possession of accurate knowledge about ores and their natural distribution essential.[19] Scholars began to study the makeup of the Earth in a systematic manner, with detailed comparisons and descriptions not only of the land itself, but of the semi-precious metals that had such great value. For example, in 1774 Abraham Gottlob Werner published the book “Von den äusserlichen Kennzeichen der Fossilien” (On the External Characters of Minerals), which brought him widespread recognition because he presented a detailed system for identifying specific minerals based on external characteristics.[20] The more efficiently that productive land for mining could be found and that the semi-precious metals could be identified, the more money that could be made. This drive for economic success fueled geology into the limelight and made it a popular subject to pursue. With an increased number of people studying it, came more detailed observations and more information about the Earth.

During the eighteenth century, the story of the history of the Earth; namely the religious concept versus factual evidence once again became a popular discussion in society. In 1749 the French naturalist Georges-Louis Leclerc, Comte de Buffon published his “Histoire Naturelle” in which he attacked the popular Christian concepts of Whiston and other Christian theorists on the topic of the history of Earth.[21] From experimentation with cooling globes, he found that the age of the Earth was not 6,000 years as stated in the Bible, but rather 75,000 years.[22] Another individual who attributed the history of the Earth to neither God nor the Bible was the philosopher Immanuel Kant who published this concept in 1755 in his “Allgemeine Naturgeschichte und Theories des Himmels.”[23] From the works of these educated men, as well as others, it became acceptable by the mid eighteenth century to question the age of the Earth. This questioning represented a turning point in the study of the Earth. It was now possible to study the history of the Earth from a scientific perspective rather than a religious one.

With science as a driving force behind the investigation of the Earth’s history, the study of geology could now become a distinct field of science. First, the terminology and definition of what geological study consisted of had to be determined. The term geology was first used professionally in publications by two Genevian naturalists, Jean-Andre Deluc and Horace-Benedict de Saussure.[24] Geology was not well received as a term until it was used in the very popular encyclopedia, the “Encyclopedie,” published in 1751 by Denis Diderot.[25] Once the term was coined as the study of the Earth and its history, geology slowly became a more prevalent and recognized science of its own standing that could be taught as a field of study at educational institutions. In 1741 the most well-known institution in the field of natural history, the National Museum of Natural History in France designated the first teaching position specifically for geology.[26] This was an important step in the further development of geology as a science and in the recognition of the importance of widely distributing this knowledge.

After the designation of geology as a specific field of study in an institution, this subject flourished in educated society. By the 1770s two feuding theories with designated followers were established. These contrasting theories explained how the rock layers of the Earth’s surface had formed. The German geologist, Abraham Werner proposed the theory that the Earth’s layers, including basalt and granite, had formed as a precipitate from an ocean that covered the entire Earth, referring to the Deluge. Werner’s system was influential and those that believed his theory were known as Neptunists.[27] The Scottish naturalist, James Hutton, argued against the theory of Neptunism. Hutton proposed the theory of Plutonism; the Earth formed through the gradual solidification of a molten mass at a slow rate by the same processes that occurred throughout history and continues in present day. This led him to the conclusion that the Earth was immeasurably old and could not possibly fit within the limits of the Bible’s 6,000 years. Plutonists believed that volcanic processes were the chief agent in rock formation, not water from a Great Flood.[28]

19th century

Engraving from William Smith's 1815 monograph on identifying strata by fossils

The Neptunists and Plutonists supplied necessary data to help complete the stratigraphical column in the early 19th century. The stratigraphical column can be defined as “the sequence of rock formations arranged according to their order of formation in time.”[29] William Smith, Georges Cuvier and Alexander Broignart can all be recognized for their roles during this century in furthering the concept of fossil-based stratigraphy. The English mineral surveyor William Smith found empirically that fossils were a highly effective means of distinguishing between otherwise similar formations of the landscape. At about the same time, the French comparative anatomist Georges Cuvier realized that the relative ages of fossils could be determined from a geological standpoint; in terms of what layer of rock the fossils are located and the distance these layers of rock are from the surface of the Earth. Cuvier’s mineralogist colleague Alexandre Brogniart augmented Cuvier’s practices. Through the synthesis of these findings, Brogniart and Cuvier realized that different strata could be identified by fossil contents and thus each stratum could be assigned to a unique position in a sequence.[30] After the publication of Cuvier and Broignart’s book, “Description Geologiques des Environs de Paris” in 1811, which outlined the concept of stratigraphy, came a great interest in this new method.[31] Stratigraphy became very popular amongst geologists; many hoped to apply this concept to all the rocks of the Earth. During this century various geologists further refined the stratigraphical column to completion. For instance, in 1833 while Adam Sedgwick was mapping rocks that he had established were from the Cambrian Period, Charles Lyell was elsewhere suggesting a subdivision of the Tertiary Period;[32] whilst Roderick Murchison, mapping into Wales from a different direction, was assigning the upper parts of Sedgewick's Cambrian to the lower parts of his own Silurian Period[33]. The stratigraphical column was significant because it now supplied a method to assign a relative age of these rocks by slotting them into different positions in their stratigraphical sequence. This created a global approach to dating the age of the Earth and allowed for further correlations to be drawn from similarities found in the makeup of the Earth’s crust in various countries.

Geological map of Great Britain by William Smith, published 1815.

In early nineteenth-century Britain, catastrophism was adapted with the aim of reconciling geological science with religious traditions of the biblical Great Flood. In the early 1820s English geologists including William Buckland and Adam Sedgwick interpreted "diluvial" deposits as the outcome of Noah's flood, but by the end of the decade they revised their opinions in favour of local inundations.[34][35] Charles Lyell challenged catastrophism with the publication in 1830 of the first volume of his book Principles of Geology which presented a variety of geological evidence from England, France, Italy and Spain to prove Hutton’s ideas of gradualism correct.[36] He argued that most geological change had been very gradual in human history. Lyell provided evidence for Uniformitarianism; a geological doctrine that processes occur at the same rates in the present as they did in the past and account for all of the Earth’s geological features.[37] Lyell’s works were popular and widely read, the concept of Uniformitarianism had taken a strong hold in geological society.[38]

During the same time that the stratigraphical column was being completed, imperialism drove several countries in the early to mid 19th century to explore distant lands to expand their empires. This gave naturalists the opportunity to collect data on these voyages. In 1831 Captain Robert FitzRoy, given charge of the coastal survey expedition of HMS Beagle, sought a suitable naturalist to examine the land and give geological advice. This fell to Charles Darwin, who had just completed his BA degree and had accompanied Sedgwick on a two week Welsh mapping expedition after taking his Spring course on geology. Fitzroy gave Darwin Lyell’s Principles of Geology, and Darwin became Lyell's first disciple, inventively theorising on uniformitarian principles about the geological processes he saw, and challenging some of Lyell's ideas. He speculated about the Earth expanding to explain uplift, then on the basis of the idea that ocean areas sank as land was uplifted, theorised that coral atolls grew from fringing coral reefs round sinking volcanic islands. This idea was confirmed when the Beagle surveyed the Cocos (Keeling) Islands. Darwin's discovery of giant fossils helped to establish his reputation as a geologist, and his theorising about the causes of their extinction led to his theory of evolution by natural selection published in On the Origin of Species in 1859.[34][39][40]

Economic motivations for the practical use of geological data caused governments to support geological research. During the 19th century the governments of several countries including Canada, Australia, Great Britain and the United States funded geological surveying that would produce geological maps of vast areas of the countries. Geological surveying provides the location of useful minerals and such information could be used to benefit the country’s mining industry. With the government funding of geological research, more individuals could study geology with better technology and techniques, leading to the expansion of the field of geology.[19]

In the 19th century, scientific realms established the age of the Earth in terms of millions of years. By the early 20th century the Earth’s estimated age was 2 billion years. Radiometric dating determined the age of minerals and rocks, which provided necessary data to help determine the Earth’s age.[41] With this new discovery based on verifiable scientific data and the possible age of the Earth extending billions of years, the dates of the geological time scale could now be refined. Theories that did not comply with the scientific evidence that established the age of the Earth could no longer be accepted.

20th century

Alfred Wegener, around 1925

The determined age of the Earth as 2 billion years opened doors for theories of continental movement during this vast amount of time.[41] In 1912 Alfred Wegener proposed the theory of Continental Drift.[42] This theory suggests that the continents were joined together at a certain time in the past and formed a single landmass known as Pangaea; thereafter they drifted like rafts over the ocean floor, finally reaching their present position. The shapes of continents and matching coastline geology between some continents indicated they were once attached together as Pangea. Additionally, the theory of continental drift offered a possible explanation as to the formation of mountains. From this, different theories developed as to how mountains were built. Unfortunately, Wegner’s ideas were not accepted during his lifetime and his theory of Continental Drift was not accepted until the 1960s.[43]

In the 1960s new found evidence supported the theory of Continental Drift. The term Continental Drift was no longer used but was replaced by the concept of Plate Tectonics that was well supported and accepted by almost all geologists by the end of the decade. Geophysical evidence suggested lateral motion of continents and that oceanic crust is younger than continental crust. This geophysical evidence also spurred the hypotheses of seafloor spreading and paleomagnetism. The hypothesis of seafloor spreading, proposed by Robert S. Dietz and Harry H. Hess, holds that the oceanic crust forms as the seafloor spreads apart along mid-ocean ridges. Paleomagnetism is the record of the orientation of the Earth’s magnetic field recorded in magnetic minerals. British geophysicist S. K. Runcorn suggested the concept of paleomagnetism from his finding that the continents had moved relative to the Earth’s magnetic poles.[44]

Modern geology

By applying sound stratigraphic principles to the distribution of craters on the Moon, it can be argued that almost overnight, Gene Shoemaker took the study of the Moon away from Lunar astronomers and gave it to Lunar geologists.

In recent years, geology has continued its tradition as the study of the character and origin of the Earth, its surface features and internal structure. What changed in the later 20th century is the perspective of geological study. Geology was now studied using a more integrative approach, considering the Earth in a broader context encompassing the atmosphere, biosphere and hydrosphere.[45] Satellites located in space that take wide scope photographs of the Earth provide such a perspective. In 1972, The Landsat Program, a series of satellite missions jointly managed by NASA and the U.S. Geological Survey, began supplying satellite images that can be geologically analyzed. These images can be used to map major geological units, recognize and correlate rock types for vast regions and track the movements of Plate Tectonics. A few applications of this data include the ability to produce geologically detailed maps, locate sources of natural energy and predict possible natural disasters caused by plate shifts.[46]

See also

Notes and references

  1. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 7
  2. ↑ Moore, Ruth. The Earth We Live On. New York: Alfred A. Knopf, 1956. p. 13
  3. ↑ 3.0 3.1 Abdus Salam (1984), "Islam and Science". In C. H. Lai (1987), Ideals and Realities: Selected Essays of Abdus Salam, 2nd ed., World Scientific, Singapore, p. 179-213.
  4. ↑ Stephen Toulmin and June Goodfield (1965), The Ancestry of Science: The Discovery of Time, p. 64, University of Chicago Press (cf. The Contribution of Ibn Sina to the development of Earth sciences)
  5. ↑ 5.0 5.1 Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 118
  6. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 114
  7. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 219
  8. ↑ Frank, Adams Dawson. The Birth and Development of the Geological Sciences. Baltimore: The Williams & Wilkins Company, 1938. p. 209, 239
  9. ↑ Albritton, Claude C. The Abyss of Time. San Francisco: Freeman, Cooper & Company, 1980. p. 104-107
  10. ↑ Frank, Adams Dawson. The Birth and Development of the Geological Sciences. Baltimore: The Williams & Wilkins Company, 1938. 226.
  11. ↑ Peter, Bowler J. The Earth Encompassed. New York: W.W. Norton & Company, 1992. p. 404-405
  12. ↑ Frank, Adams Dawson. The Birth and Development of the Geological Sciences. Baltimore: The Williams & Wilkins Company, 1938. p. 226
  13. ↑ Charles, Drake L. The Geological Revolution. Eugene : Oregon State System of Higher Education, 1970. p. 11
  14. ↑ Charles, Drake L. The Geological Revolution. Eugene : Oregon State System of Higher Education, 1970. 11.
  15. ↑ Charles, Drake L. The Geological Revolution. Eugene : Oregon State System of Higher Education, 1970. p. 13
  16. ↑ "Studying Earth Sciences." British Geological Survey. 2006. Natural Environment Research Council. http://www.bgs.ac.uk/vacancies/studying.htm , accessed 29 Nov. 2006.
  17. ↑ Rocchio, Laura. "The Landsat Program." National Aeronautics and Space Administration. http://landsat.gsfc.nasa.gov , accessed 4 Dec. 2006
  18. ↑ Frank, Adams Dawson. The Birth and Development of the Geological Sciences. Baltimore: The Williams & Wilkins Company, 1938. p. 96
  19. ↑ 19.0 19.1 Jardine, N., F. A. Secord, and E. C. Spary. Cultures of Natural History. Cambridge: Cambridge University Press, 1996. p. 212-214
  20. ↑ Jardine, N., F. A. Secord, and E. C. Spary. Cultures of Natural History. Cambridge: Cambridge University Press, 1996. p. 212
  21. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 88
  22. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 92
  23. ↑ Jardine, N., F. A. Secord, and E. C. Spary. Cultures of Natural History. Cambridge: Cambridge University Press, 1996. p. 232
  24. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press. 1990. p. 8
  25. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 8
  26. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 219
  27. ↑ Frank, Adams Dawson. The Birth and Development of the Geological Sciences. Baltimore: The Williams & Wilkins Company, 1938. p. 209
  28. ↑ Albritton, Claude C. The Abyss of Time. San Francisco: Freeman, Cooper & Company, 1980. p. 95-96
  29. ↑ Frank, Adams Dawson. The Birth and Development of the Geological Sciences. Baltimore: The Williams & Wilkins Company, 1938. p. 239
  30. ↑ Albritton, Claude C. The Abyss of Time. San Francisco: Freeman, Cooper & Company, 1980. p. 104-107
  31. ↑ Peter, Bowler J. The Earth Encompassed. New York: W.W. Norton & Company, 1992. p. 216
  32. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 144
  33. ↑ Second J A (1986) Controversy in Victorian Geology: The Cambrian-Silurian Dispute Princeton University Press, 301pp, ISBN 0-691-0244-13
  34. ↑ 34.0 34.1 Herbert, Sandra. Charles Darwin as a prospective geological author, British Journal for the History of Science 24. 1991. p. 159–192
  35. ↑ Peter, Bowler J. The Earth Encompassed. New York: W.W. Norton & Company, 1992. p. 404-405
  36. ↑ Albritton, Claude C. The Abyss of Time. San Francisco: Freeman, Cooper & Company, 1980. p. 104-107
  37. ↑ Gohau, Gabriel. A History of Geology. New Brunswick: Rutgers University Press, 1990. p. 145
  38. ↑ Albritton, Claude C. The Abyss of Time. San Francisco: Freeman, Cooper & Company, 1980. p. 104-107
  39. ↑ Frank, Adams Dawson. The Birth and Development of the Geological Sciences. Baltimore: The Williams & Wilkins Company, 1938. p. 226
  40. ↑ Keynes, Richard ed.. Charles Darwin's zoology notes & specimen lists from H.M.S. Beagle, Cambridge University Press, 2000. p. ix
  41. ↑ 41.0 41.1 Jardine, N., F. A. Secord, and E. C. Spary. Cultures of Natural History. Cambridge: Cambridge University Press, 1996. p. 227
  42. ↑ Charles, Drake L. The Geological Revolution. Eugene : Oregon State System of Higher Education, 1970. p. 11
  43. ↑ Peter, Bowler J. The Earth Encompassed. New York: W.W. Norton & Company, 1992. p. 404-405
  44. ↑ Peter, Bowler J. The Earth Encompassed. New York: W.W. Norton & Company, 1992. p. 405-415
  45. ↑ "Studying Earth Sciences." British Geological Survey. 2006. Natural Environment Research Council. http://www.bgs.ac.uk/vacancies/studying.htm , accessed 29 Nov. 2006
  46. ↑ Rocchio, Laura. "The Landsat Program." National Aeronautics and Space Administration. http://landsat.gsfc.nasa.gov , accessed 4 Dec. 2006

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