History of the Earth

History of the Earth

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Index

Historical ideas about the age of the earth

the precambrian: the most distant past

actualism and uniformitarianism

the palaeozoic Era: diversity of life

What do fossils tell us?

the Mesozoic Era: the age of reptiles

measuring geological time

relative geochronology

the cenozoic Era: the age of mammals

historical geology

How do we know what dinosaurs looked like based only on fossils??

Palaeontologists study fossil remains. They learn about the past by studying fossilized bones, nests with eggs, and dinosaur tracks. Fossils can provide a wide range of information about dinosaurs. For example, did they walk on two legs or four? What did they look like? What did they eat?

Historical ideas about the age of the Earth

An old question

• Since ancient times, people have been studying how the Earth was formed. They also tried to calculate its age by observing nature and the sky.

• They combined this information with their religious beliefs.

• The first timelines were created by ancient Indians, Egyptians and Mesopotamians.

• They were closely linked to astronomy and the gods.

Beginning in the seventeenth century, human knowledge of nature began to increase significantly. Many new scientific disciplines developed, including geology, the study of the Earth's composition and structure. Geologists also study the processes that created and shaped the Earth.

Some approaches

In 1795, James Hutton published Theory of the Earth. He theorized that our planet must be very old. This was the only way he could explain processes such as mountain formation.

Charles Lyell expanded Hutton's ideas on his book Principles of Geology.

Using his knowledge of physics, Lord Kelvin calculated the age of the Earth based on the planet's cooling rate. He initially estimated 90 my, but soon afterward he recalculated it at 24 my.

Science and religion

Anglican arbishop James Ussher was a biblical scholar. He interpreted Middle Eastern and Mediterranean histories in conjunction with the Bible. In 1650, he concluded that the Earth was created on 23 October 4004 Before Common Era (BCE). Ussher's chronology had an impact on scientific thought until the end of the 19th century. Many naturalists were reluctant to present their research. They were afraid that they could be accused of heresy..

Radioactivity

• Radioactivity is the property exhibited by certain types of matter of emitting energy and subatomic particles spontaneously. It is, in essence, an attribute of individual atomic nuclei.
• An unstable nucleus will decompose spontaneously, or decay, into a more stable configuration but will do so only in a few specific ways by emitting certain particles or certain forms of electromagnetic energy.

Pierre and marie curie

• They researched radioactivity in the 20th century.

• They discovered that the radioactive elements within the minerals in rocks break down over time.

• This process of decay could be used like a clock to determine the precise age of the Earth.

Arthur holmes

• He developed the process of radiometric dating.

• This technique determines the age of rocks by analysing the decay of the radioactive elements comprising them.

• Applying this technique, scientists calculate that Earth is now 4,5 billion years old.

• The margin of error is less than 1%.

Actualism and uniformitarianism

Two important principles that enable scientists to interpret and date processes and materials on the Earth.

Actualism

Ripple marks

• Hutton proposed that the processes that shaped the Earth in the past continue to occur today.
• For example, erosion, volcanic activity and geological folding occur in the same way today as when the Earth was formed.

Uniformitarianism

the present is the key of the past

• Lyell agreed with Hutton that there is a uniformity of processes throuhout time. He also believed that there is a uniformity of process rates.

• Geological processes occur gradually over long periods of time.

• Uniformitarianism is also called gradualism, contrasted with the catastrophist ideas prevailling at that time.

• According to catastrophism, mountain ranges and valleys were formed by extraordinary catastrophic events.

• Today neocatastrophism is the most widely accepted theoretical model. It suggests that geological processes of different intensities and durations can coexist.

What do fossils tell us?

what is a fossil?

A fossil is the remains of a living organism or a trace of its activity. Fossils are preserved in rocks through a mineralization process that can take milllions of years. During this process, organic matter is converted into minerals.

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Types of fossils

PETRIFIED FOSSILS: hard parts of an organism are partially or completely mineralized. EJ. shells, bones and teeth.

PRESERVED REMAINS: some times soft parts are preserved in amber or in ice.

MOLDS: the imprint of an animal.

TRACES or ICHNITES: footprints and other evidence

CASTS: they are molds filled in and then turned into rock

CARBON FILMS: leaves imprints on rocks

CHEMICAL FOSSILS: chemical substances of organic origin preserved in sediment.

OTHERS: eggs, nests, coprolites, gastrolites...

Using fossils to study the past

• The study of fossils is called PALAEONTOLOGY.

• One of the first palaeontologist was Nicholas Steno (XVII).

• Fossils provide a range of information about the anatomy and habits of extinct organisms.

• They enable scientists to understand the changes that occured in species over time.
• They provide information about the ecological condition in past habitats.
• Fossils make it possible to determine the age of the rocks in which they are found.

Strategies to obtain information:

principle of correlation of parts

French zoologist Georges Cuvier suggested that each part of an animal's anatomy is closely linked. Therefore, palaeontologists can deduce what an animal was like based on one body part: a single bone or a tooth. The animal's anatomy is also related to its living habits.

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taphonomy

The context in which fossils are found provides other useful information. It indicates what happened from the time the organism was covered with sediment, and throughout the fossilization process. This information enables scientists to recosntruct the geological processes that ocurred in that context.

index fossils

These provide valuable information about the rock strata in which they are found. These fossils can be used to delimit short periods of geological time. They can confirm that two geographically distant are a similar age because they contain similar fossils.

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Measuring time

Rocks constitute an archive of information that can be used to reconstruct hte history of the Earth.

Geochronology is the science of determining the age of rocks

• RELATIVE GEOCHRONOLOGY

• It makes it possible to put rocks in order by age.

• This is done by studying two aspects:
• the materials that make up rocks.
• the geological processes that have affected the rocks.

• ABSOLUTE GEOCHRONOLOGY

• It determines the real age of a material, for example, using radiactive isotopes.

• There are several techniques: biological, stratigraphic and radiometric tecniques are the most important.

Stratigraphic dating methods: glacial varves. They are thin layers of sediment deposited seasonally on the bottom of glacial lakes.

Biological geochronology is based on the regular patterns of development of living thigs.

Ej. Growth rings in trees and corals.

Radiometric dating methods

Relative geochronology

It establishes a relative timeline of geological events, that is, which event happened first and which happened later.

Principles of relative geochronology

principle of superposition of strata

• The oldest layers of sediments are at the bottom.

• The youngest are at the top.

• This order can be altered by geological phenomena, such as folds or faults.

Principles of relative geochronology

principle of superposition of geological events

• An object or surface that crosses or affects another is always younger than the one affected.

• For example, a fold or a fault is always younger than the rock that is affected.

Principles of relative geochronology

principle of stratigraphic correlation

• It is possible to correlate strata that formed at the same time. This strata have similar characteristics and mineral composition.

• The strata may have formed in the same place and then been separated.
• The best stratigraphic correlation tools are index fossils but other unique characteristics are also used.

Concordance and discordance

When strata have been deposited continuously, they are separated by concordant stratification planes. This is still true even if they were later folded. Concordant strata usually belong to the same geological period. Sometimes, a concordant series undergoes folding or erosion. Discordance occurs if a younger series of strata is then deposited on top of the older series. In these cases, both series are discordant.

Geological history

It studies the evolution of theEarth.Nineteenth century naturalists used relative geochronology techniques to create a relative geo-calendar based on a geological cross-section of Europe. Modern geologists divide the history of the Earth into time intervals, using significant events to determine the boundaries.

The Precambrian

The most distant past

• The Precambrian Super Eon began when the Earth was formed 4,600 million years ago.

• It continued until the Cambiran Explosion, 541 million years ago (or 541 Ma Megaannum)

• Rocks from Cambrian Explosion onwards contain the remains of many living things.
• The Precambrian Super Eon represents almost 90% of the Earth's history.
• However, there is little evidence of events that occured during this period.
• It is divided into three eons: Hadean, Archean and Proterozoic.

The Hadean Eon (4600-4000 my)

• Meteorites bombarded the Earth preventing the surface of the planet from cooling and delayed the formation of a solid crust.

• Intense volcanic activity produced gases that formed a primitive atmosphere.

• Water vapour condensed, generating high precipitation leading to the development of the hydrosphere.

• The Mooon also formed during this Eon. Its gravitational force strongly affects the Earth.

The Archean Eon (4000-2500 my)

• First evidence of life marks. Evidence of biological activity in rocks 3800 my.

• Organisms may have been present earlier.

• Solid crust of the Earth formed and plate tectonic activity began.

The ProterozoicEon (2500-541 my)

• Fee oxygen started to accumulate in the atmosphere and the oceans.
• Oxygen accumulation enabled eukaryotic cells and multicellular organisms to develop.
• Fossils of this organisms have been found dating from the end of this eon.
• 1100 my ago, the supercontinent Rodinia formed. It began to break up aproximately 740 my ago.
• These phenomena coincided with the intense glaciation that transformed the Earth into a snowball.

The Palaeozoic Era

Diversity of life

Index fossils of the Palaeozoic era

Trilobites

Orthoceras

Calceola

Trilobites

The Great Permian Extinction

(299-252 Ma)

The Palaeozoic Era ended with The Great Permian Extinction. This largest known mass extinction event in the fossil record was caused by: - The Greenhouse Effect, which was induced by intense volcanic activity. - A reduction in coastal and continental shelf areas due to the formation of the continent Pangaea. This, in turn, led to the Hercynian Orogeny, a mountain building event.

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