1. Introduction
1.1. The scientific method
Science is about understanding how nature works. It is carried out by making hypotheses basedon observations and/or experiments. Hypotheses are confirmed, revised or refuted as scientists
keep exploring nature, conducting new observations and performing new experiments. A set ofhypotheses explaining some aspect of nature is called a theory.
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Darwin’s theory of evolution by natural selectionOne fundamental scientific achievement that greatly improved our understanding of nature is Charles
Darwin’s (1809-1882) theory of evolution by natural selection. Darwin’s theory explains how newspecies can arise and how life evolved from simple microbial forms more than 3.5 billion years ago to
the complex animals and plants we see today. Darwin’s discovery is often refers to as the “DarwinianRevolution”. Stephen Jay Gould, a famous American paleontologist, summarizes Darwin’s theory inthree points: (1) organisms belonging to the same species display variations (color, size…) inherited bytheir offspring, (2) organisms produce more offspring than can survive, and (3) as a consequence of (1)and (2), organisms with more advantageous traits have a greater chance to survive and reproduce(they are naturally selected).
Darwin tells us that if a population of organisms of a particular species becomes reproductively isolated
from other members of the same species, the combination of new environmental constraints andrandom genetic mutations may lead to the formation of a new species after many generations. Darwin’s
understanding of evolution came in part from his observations of species geographic distribution during
his journey around the world on board HMS Beagle from 1831 to 1836. For instance, Darwin noticed aresemblance between the animals of the Galapagos Islands and those of the American continent. He
hypothesized that the Galapagos Islands must have been colonized by organisms from America andthat these organisms subsequently evolved in isolation to form new species by natural selection.
Darwin’s theory is also based on his study of domesticated pigeon breeds. He experimented with theartificial selection of different varieties of pigeons by crossing individuals with particular traits. He
surmised that given enough time (after many generations) nature could produce different species much
like a professional breeder can produce different varieties of domesticated animals, or farmers different
varieties of fruits and vegetables.
The issue of time was crucial to Darwin’s theory. There was no consensus at that time about the age of
the Earth and many still believed the Earth was very young, perhaps as young as several thousands ofyears, much too young for Darwin’s idea of natural selection. Darwin’s view of the geological time scale
was influenced by a book he was reading on board HMS Beagle called “Principles of Geology” written
by a prominent British geologist named Charles Lyell. This book recognizes the need of huge timeNote that the same idea was proposed independently by another British naturalist: Alfred Russel Wallace (1823-1913).

Darwin’s finches famously illustrate how birds originating from the same species can evolve into distinct species by adapting tovarious environments (e.g. species with pointed beaks for picking fruits, species with shorter, larger beaks for eating seeds on theground).

1spans to explain the slow and progressive changes shaping Earth’s landscape. This naturally appealed
to Charles Darwin whose theory of evolution by natural selection requires long stretches of time
(millions of years) to evolve new species by the slow and progressive accumulation of tiny
modifications.

_____________________________________________________________________________There are different ways of doing science. For instance, theoretical physicists do not draw
conclusions from a set of data in the same manner that Darwin did to elaborate his theory.
Theoretical physicists use the language of mathematics to explain how nature works. Theirfindings are then confirmed of refuted by observations and/or experiments. General, basic
principles explaining how nature works are referred to as physical laws. Examples of such laws
are Newton’s three laws of motion and his law of universal gravitation which explain therelationship between the motion of an object and the forces acting upon it.
When a natural system becomes relatively well understood, it is possible to create a scientificmodel explaining its behavior and predicting its response in different conditions. In geology forexample, materials with different densities can be used to simulate rock layers in the lab andexamine how they behave when exposed to different kinds of stress (analog modeling). Our
understanding of natural systems can also be translated into computer models. Computermodeling has become a major tool to simulate natural processes. Computer models which are themost familiar to us are probably those used to predict the weather. We see the output of thesemodels everyday in the news.

1.2. What is Earth Science?
Earth science is a multidisciplinary science and its purpose is to study the Earth. Geology is thestudy of the solid earth (e.g. its history, composition, internal structure, and surface features).
Geology is composed of various disciplines. For example, paleontology is the study of past life
based on fossils. Geophysics and geochemistry aims at understanding geological processes
using tools and principles of physics and chemistry, respectively. Geology is based on the study ofthe geological record, i.e. the information preserved in rocks formed at various times. Data ofgeological interest are primarily collected through field observations, rock sampling, lab analyses,
geological and topographic mapping, seismic surveys, aerial photography, and satellite remotesensing (e.g. gravimetry).
Other branches of Earth science include oceanography － the study of the oceans － ,meteorology －the study of the atmosphere－, glaciology －the study of ice and ice-related
processes － , and geobiology － the study of the interactions between the biosphere, thelithosphere and the atmosphere－. Note that all these disciplines are themselves multidisciplinary.
Over the past few decades, progress in space exploration has enabled us to look at the planets ofour solar system at an unprecedented level of details. Our understanding of the geological2

processes occurring on Earth combined with a wealth of data collected by satellites andland-based missions (e.g. Mars exploration rovers) are key to the study of the planets of our solarsystem (planetary science).

1.3. A brief history of geology
More than 2000 years ago, Greek philosophers were already trying to solve geological problems.
For instance, they had noticed the presence of seashells in sediment layers well above sea level.
Erastosthenes (3rd century BC) suggested that sea level may have fallen after the opening of theStrait of Gibraltar. Strabo (ca. 63 BC – ca. AD 24), on the other hand, proposed that catastrophic
events such as earthquakes, volcanic eruptions, or landslides, had resulted in repeated sea levelchanges.
Another question exciting the curiosity of Greek philosophers was related to the more fundamental
issue of the evolution of the Earth. By observing nature, they knew that rivers can carve deepvalleys. They surmised that the process of erosion of hard rocks by water must be slow andprogressive and must act over an extensive period of time to incise large mountains. Hence, Greek
philosophers had already a sense of the long duration of the geological time scale. Moreover,they pondered about the continuous destruction of rocks by erosional processes which implies thatmountains are meant to disappear on the long run unless they are regenerated by some otherprocesses. This idea relates to the cyclic nature of geological processes which is central tomodern geology (see plate tectonics and the rock cycle explained in the following chapters).
Different scenarios were imagined by different schools of philosophers to explain the evolution of
the Earth (see related slide). Although these ideas were inspired by observations, Greek
philosophers were not scientists because they did not try to test their hypotheses.
Centuries later, a French philosopher named René Descartes (1596-1650), best known for hismathematical legacy (Cartesian coordinates) and philosophical work (e.g. Discourse on theMethod and his famous “cogito ergo sum” －I think therefore I am－), elaborated a theory on theformation of the Earth and the origin of mountains and oceans. Based on his theory, the Universe
is composed of three types of matter. One is the matter that composes the stars. Another is thematter that composes terrestrial bodies like the Earth. The third is the matter that composes thesky. Stars evolve into planets by forming an external crust. The Earth is thus a former star andpossesses a central fire, remnant of its stellar origin. Various layers surround the central fire,including an internal ocean. The Earth’s landscape －mountains and ocean basins－ is the resultof the irregular collapse of the solid external layer. Although this idea has been completely refutedby later studies, it influenced generations of scientists who favored the idea of collapse rather thanuplift to explain the formation of mountains. We know now that mountain chains can form by upliftwhere two tectonic plates converge but the theory of tectonic plates and our modern understanding3

of mountain formation came to light only 300 years after the death of René Descartes!
An important step in the history of geology is marked by the achievements of a Danish anatomist
named Nicolas Steno (1638-1686) who is considered to be the first “true” geologist. In the 17thcentury, the biological origin of fossils was not yet accepted. Steno identified a previously
mysterious fossil called Glossopetrae as being the remains of the teeth of once-living sharks. Hewas also the first to formulate two fundamental principles of geology: the principle ofhorizontality and the principle of superposition. The first states that sediments are originallydeposited horizontally; the second that older layers are at the bottom and younger layers at the top.
Steno also realized that different assemblages of fossils represent different environments, andtherefore that fossils can be used to reconstruct past environments. This is a fundamental concept
underpinning modern stratigraphy, the study of layered rocks.
A hot debate which kept geologists busy in the 18th and 19th century opposed the neptunists andplutonists. The first school of thoughts was led by a German geologist named Abraham Gottlob
Werner (1749-1817) and the second by a British geologist named James Hutton (1726-1797).
Based on the neptunistic viewpoint, the Earth was originally covered by a global ocean. Most rocks
found on Earth formed under water, including granites and basalts. We know now that granites andbasalts form instead by crystallization of a cooling magma. The neptunists believed that mountains
were formed by a combination of erosional and depositional processes in the ocean. For them,Earth’s internal heat had little influence on the evolution of the landscape. Conversely to this view,
the plutonists considered Earth’s internal heat as the main driving force behind the formation ofmountains and ocean basins. They correctly recognized the link between granites, basalts, andvolcanic activity.
Another popular theory of the 19th century is called contractionism. It is based on the assumptionthat the Earth’s interior is progressively cooling; as the Earth cools and contracts, the rigid externalcrust deforms. This deformation results in the creation of mountains and ocean basins, much likethe wrinkles forming at the surface of a drying apple.
Another question that was central to geology in the 19th century was to know whether we can use
our knowledge of present-day geological processes to understand the past. More simply put: isthe present the key to the past? Two very different schools of thoughts emerged:
uniformitarianism and catastrophism. The uniformitarianists, led by Charles Lyell (1797-1875), held
the view that the history of our planet is characterized by geological processes that are still acting
today. Hence, it is possible to understand the past based on a careful study of the processes
shaping our modern landscape. The catastrophists, led by the French naturalist Georges Cuvier(1769-1832), emphasized the importance of catastrophic events which have no analogue withpresent-day phenomena to explain the evolution of our planet (cataclysmic earthquakes, volcanic
eruptions or floods). Our modern understanding tells us that the basic idea behind
uniformitarianism is true. We can indeed understand a great deal about the past based on our4

knowledge of geological processes occurring today. For instance, we can recognize beachdeposits or river beds in the fossil record by analogy with modern sediments. We can recognize oldmountain chains (e.g. Appalachian Mountains) and understand how they formed based on thestudy of active plate boundaries (e.g. Andes, Himalayas). However, the catastrophists were notentirely wrong because we know now that exceptional events like the impact of a very large
meteorite or huge volcanic eruptions never witnessed in historical times have happened in thedistant past and will happen in the future. For instance, the most widely accepted and best
documented cause of the disappearance of the dinosaurs 65 million years ago is the impact of avery large meteorite off the Yucatan Peninsula.
The Newtonian revolution in physics took place in the 17th century. Geologists also have theirrevolution but it happened only recently in the mid-20th century. Although various theories
explaining the formation of mountains and ocean basins, earthquakes and volcanoes have been
proposed since the Greek antiquity, it is only in the 1950s that we began to understand themechanisms behind these phenomena. The story begins with the discovery of continental drift byAlfred Wegener (1880-1930). Wegener compiled evidence supporting the fact that the position ofcontinents is not fixed. However, he had no clue regarding the mechanism driving this movement.
The time of Wegener’s discovery is also the time at which radioactivity was discovered by HenriBecquerel (1852-1902). Soon after, Ernest Rutherford (1871-1937) came up with a technique todate rocks based on the decay of radioactive isotopes which confirmed the old age of the Earth.
The link between Earth’s internal heat and radioactivity was soon established, which led a British
geologist named Arthur Holmes (1890-1965) to propose that heat produced by radioactivity could
generate convection movements beneath the Earth’s crust and drive continental drift. The final
pieces of the puzzle were put together in the mid-20th century when bathymetric surveys of theocean floor led to the discovery of mid-ocean ridges. Dating of the ocean crust on both sides ofthese ridges showed that the age increases with the distance from the ridge. Based on this andother supporting evidence, geologists formed a theory known as the theory of plate tectonics (fordetails about plate tectonics, see chapter 3 of this course). Based on this theory, the Earth’s
external rigid layer is divided into plates. Plates are moving relative to one another. New ocean
plate is formed at mid-ocean ridges where two plates are pulled apart. Mountain chains form where
two plates converge. Volcanoes and earthquakes are primarily located along plate boundaries
where tectonic forces are concentrated. The theory of plate tectonics is at the core of moderngeology and has revolutionized our understanding of the Earth and its history.

ReferencesGohau, G., 1990. A History of Geology (revised and translated from the French by A.V. Carozzi and M.
Carozzi). Rutgers University Press, New Brunswick, New Jersey, 259 pages.
Gould, S.J., 1977. Ever Since Darwin – Reflections on Natural History. W.W. Norton, New York · London,
285 pages.
Gribbin, J., 2002. Science: A History (1543-2001). Penguin Books, 647 pages.
Hallam, A., 1992. Great Geological Controversies. Oxford University Press, 244 pages.

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