earth - york university · from the earth’s core, which cause the plates to move across the...

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SC/NATS 1510, EarthSC/NATS 1510, Earth 11

EarthEarth

The Structure of the EarthThe Structure of the Earth’’s s EnvironmentEnvironment

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The History of the Environment on The History of the Environment on Earth Earth

The Earth itself is about 4.5 billion years old.The Earth itself is about 4.5 billion years old.When land finally emerged, it was distributed When land finally emerged, it was distributed unevenly across the globe on crusts or plates that sit unevenly across the globe on crusts or plates that sit upon molten rocks a few miles below the surface of upon molten rocks a few miles below the surface of the earth.the earth.There are convection currents caused by heat rising There are convection currents caused by heat rising from the Earthfrom the Earth’’s core, which cause the plates to s core, which cause the plates to move across the surface.move across the surface.


Continental DriftContinental Drift

The idea that the continents were not always as The idea that the continents were not always as they are now arose in the era of world they are now arose in the era of world exploration in the 15exploration in the 15thth to 17to 17thth centuries.centuries.The particular clue was the mirrorThe particular clue was the mirror--image match image match between the east coast of South America and the between the east coast of South America and the west coast of Africa.west coast of Africa.Was it possible that they had been broken apart?Was it possible that they had been broken apart?

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An Early Conception of Continental An Early Conception of Continental DriftDrift

In 1858, Antonio SniderIn 1858, Antonio Snider--PelligriniPelligrini made these two maps showing made these two maps showing how the Americas might have attached to Europe and Africa (on how the Americas might have attached to Europe and Africa (on the left) and then drifted apart (on the right).the left) and then drifted apart (on the right).


Alfred WegenerAlfred Wegener

The idea was further developed by the The idea was further developed by the German astronomer and meteorologist German astronomer and meteorologist Alfred Wegener around 1912.Alfred Wegener around 1912.Wegener not only noticed the close fit Wegener not only noticed the close fit of the coasts of South America and of the coasts of South America and Africa, but also similar fossil Africa, but also similar fossil distribudistribu--tionstions and mountain ranges.and mountain ranges.

E.g. Mountains that run EastE.g. Mountains that run East--West across South Africa West across South Africa continue across Argentina.continue across Argentina.

But Wegener had no explanation for why the But Wegener had no explanation for why the continents moved apart.continents moved apart.


The Earth as a MagnetThe Earth as a Magnet

In 1600, the English physician William Gilbert came to In 1600, the English physician William Gilbert came to the conclusion that the Earth itself was a giant magnet the conclusion that the Earth itself was a giant magnet with a northwith a north--south orientation.south orientation.Though not understood, the magnetic compass soon Though not understood, the magnetic compass soon became a useful navigational tool.became a useful navigational tool.

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The Earth as a Changing MagnetThe Earth as a Changing Magnet

By the midBy the mid--2020thth century, it was understood that century, it was understood that not only is the Earth a large magnet, roughly not only is the Earth a large magnet, roughly aligned to the north and south poles, but the aligned to the north and south poles, but the alignment changes regularly over vast periods of alignment changes regularly over vast periods of time.time.Molten metallic rock aligns itself to the magnetic Molten metallic rock aligns itself to the magnetic orientation of the Earth, and then when it cools orientation of the Earth, and then when it cools and hardens, preserves the orientation at the and hardens, preserves the orientation at the time it solidifies.time it solidifies.


The History of the EarthThe History of the Earth’’s Magnetic s Magnetic Changes Revealed in MagmaChanges Revealed in Magma

Volcanic eruptions produce large amounts of Volcanic eruptions produce large amounts of molten rock, molten rock, magmamagma, that then hardens into lava, , that then hardens into lava, revealing the magnetic orientation of the Earth revealing the magnetic orientation of the Earth at the time of the eruption.at the time of the eruption.

A record of the movement of large land masses is A record of the movement of large land masses is preserved in the magnetic orientation of rock preserved in the magnetic orientation of rock formations across the Earth.formations across the Earth.


The Atlantic RidgeThe Atlantic Ridge

There is an underwater ridge in the middle of There is an underwater ridge in the middle of the Atlantic, running norththe Atlantic, running north--south, that divides south, that divides the ocean into two symmetric pieces with the ocean into two symmetric pieces with alternating patterns of magnetic orientation.alternating patterns of magnetic orientation.

This could be produced by a slow spreading out of This could be produced by a slow spreading out of the ocean east and west from the ridge.the ocean east and west from the ridge.The continents appear to move apart, but it is really The continents appear to move apart, but it is really the large, rigid plates underneath that are shifting.the large, rigid plates underneath that are shifting.

What we call a continent is the bit that sticks out of the What we call a continent is the bit that sticks out of the water.water.

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Pangaea Pangaea

Around 225 million years ago, all the land surface of the Around 225 million years ago, all the land surface of the Earth had pushed together into a single large mass that Earth had pushed together into a single large mass that has been called has been called PangaeaPangaea..


Gondwanaland and Gondwanaland and LaurasiaLaurasia

Pangaea broke up into two massive continents, Pangaea broke up into two massive continents, GondwanalandGondwanaland, , situated over the South Pole, and situated over the South Pole, and LaurasiaLaurasia, near the equator., near the equator.


Breakup of Gondwanaland Breakup of Gondwanaland

Gondwanaland began to break apart about 160 million years ago Gondwanaland began to break apart about 160 million years ago into separate continents.into separate continents.

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North America North America –– Europe Split Europe Split

North America and Europe began to split 200 million North America and Europe began to split 200 million years ago, but most of the drift has been in the last 80 years ago, but most of the drift has been in the last 80 million years.million years.


PresentPresent--Day Configuration Day Configuration

The continents continue to drift apart at rates between The continents continue to drift apart at rates between 5050--90 mm per year.90 mm per year.


Natural Disasters Natural Disasters

Within continents large plates grind together and pull apart, wiWithin continents large plates grind together and pull apart, with th sometimes disastrous results for people in the wrong place.sometimes disastrous results for people in the wrong place.

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Fault LinesFault Lines

Places where the great continental plates meet Places where the great continental plates meet and jam against each other are called and jam against each other are called fault linesfault lines. .

The pressure from the meeting of the plates have The pressure from the meeting of the plates have often thrown up mountain ranges.often thrown up mountain ranges.These are among the most scenic and beautiful These are among the most scenic and beautiful places on Earth.places on Earth.

People flock to these places to live, but because People flock to these places to live, but because of the fault lines, they are dangerous.of the fault lines, they are dangerous.


The San Andreas FaultThe San Andreas Fault

The state of California The state of California bestrides the bestrides the San Andreas San Andreas Fault.Fault.““Silicon Valley,Silicon Valley,”” San San Francisco, and the Francisco, and the Stanford University Stanford University Linear Accelerator are all Linear Accelerator are all close to the San Andreas close to the San Andreas Fault.Fault.


EarthquakesEarthquakesEarthquakes are Earthquakes are caused by plates caused by plates grinding together grinding together and sometimes and sometimes releasing with a releasing with a tremendous jolt that tremendous jolt that can knock down can knock down buildings over a buildings over a large area.large area.Earthquakes are Earthquakes are common around common around fault lines.fault lines. Santa Rosa City Hall after the 1906

San Francisco earthquake.

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VolcanoesVolcanoes

Volcanoes are caused by eruptions of magma through cracks in Volcanoes are caused by eruptions of magma through cracks in the crust, pushed up by the colliding of plates beneath the the crust, pushed up by the colliding of plates beneath the surface.surface.


Great Natural Disasters in HistoryGreat Natural Disasters in History

Volcanic eruption of Volcanic eruption of TheraThera, possibly destroying , possibly destroying the Minoan culture on Crete.the Minoan culture on Crete.Eruption of Mt. Vesuvius in southern Italy, Eruption of Mt. Vesuvius in southern Italy, destroying Herculaneum and Pompeii.destroying Herculaneum and Pompeii.Earthquakes in Shensi province in China in Earthquakes in Shensi province in China in 1556, killing 800,000 people.1556, killing 800,000 people.And many more.And many more.


The Greenhouse EffectThe Greenhouse Effect

It is volcanoes that have made a viable It is volcanoes that have made a viable atmosphere and climate for life on Earth.atmosphere and climate for life on Earth.Millions of years of volcanic eruptions have Millions of years of volcanic eruptions have thrown enough carbon dioxide into the thrown enough carbon dioxide into the atmosphere to form a protective layer that traps atmosphere to form a protective layer that traps the heat from the Sun and keeps the the heat from the Sun and keeps the temperature on the surface of the Earth in a temperature on the surface of the Earth in a range to support life.range to support life.This is the soThis is the so--called called Greenhouse Effect.Greenhouse Effect.

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Distribution of Resources Distribution of Resources

Natural Resources were all laid down millions of Natural Resources were all laid down millions of years ago before the continents had drifted to years ago before the continents had drifted to their present locations.their present locations.The position of continents at an earlier stage The position of continents at an earlier stage explains the distribution of fossil fuels.explains the distribution of fossil fuels.


Fossil fuel sourcesFossil fuel sources

Coal, oil, and natural gas come from the Coal, oil, and natural gas come from the decomposition of vast tropical forests existing decomposition of vast tropical forests existing 250250--300 million years ago.300 million years ago.LaurasiaLaurasia, which became the northern , which became the northern hemisphere continents, was in the tropics and hemisphere continents, was in the tropics and had the forests that decomposed.had the forests that decomposed.Gondwanaland was over the Antarctic and was Gondwanaland was over the Antarctic and was too cold for tropical forests.too cold for tropical forests.


Distribution of Flora and Fauna Distribution of Flora and Fauna

Some plants and animals have evolved in Some plants and animals have evolved in isolation.isolation.Others have been driven to extinction by Others have been driven to extinction by competitors when suddenly brought in contact competitors when suddenly brought in contact with other parts of the world.with other parts of the world.

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Case in point: MarsupialsCase in point: Marsupials

Marsupial mammals had a Marsupial mammals had a worldwide distribution worldwide distribution about 80 million years about 80 million years ago.ago.

As continents drifted apart, As continents drifted apart, marsupials were replaced in marsupials were replaced in Eurasia by placental Eurasia by placental mammals.mammals.Survived in S. America Survived in S. America until it joined N. America until it joined N. America 30 million years ago.30 million years ago.They live on in isolated They live on in isolated Australia.Australia.


Evolution in Different PlacesEvolution in Different Places

Animals that were domesticated in Europe and Animals that were domesticated in Europe and Asia were not present in the Americas, e.g. Asia were not present in the Americas, e.g. sheep, goats, cattle, horses.sheep, goats, cattle, horses.Domesticated animals were not important in the Domesticated animals were not important in the Americas.Americas.

Thus agriculture and transport were both affected.Thus agriculture and transport were both affected.The wheel was known in the Americas, but could The wheel was known in the Americas, but could not be put to use without draught animals.not be put to use without draught animals.


Climate Climate

LongLong--term climatic trends affected ability of term climatic trends affected ability of humans to settle parts of the globe, and humans to settle parts of the globe, and influenced distribution of plants and animals.influenced distribution of plants and animals.Distribution of continents also influenced Distribution of continents also influenced climate.climate.

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The Ice AgesThe Ice Ages

Ice ages have dominated the worldIce ages have dominated the world’’s climate for the last s climate for the last 2 2 ½½ million years.million years.Where ice ages occurred depends on the distribution of Where ice ages occurred depends on the distribution of landmasses in the northern hemisphere.landmasses in the northern hemisphere.

In order for large ice sheets to develop and then later move In order for large ice sheets to develop and then later move south, it was necessary to have had continents clustering south, it was necessary to have had continents clustering toward the north pole, forming a landtoward the north pole, forming a land--locked Arctic Ocean 3 locked Arctic Ocean 3 million years ago.million years ago.The same conditions do not occur in the southern The same conditions do not occur in the southern hemisphere.hemisphere.


Astronomical Cycles Astronomical Cycles

The SunThe Sun’’s energy output has been increasing.s energy output has been increasing.Levels of carbon dioxide and methane in Levels of carbon dioxide and methane in atmosphere have trapped heat.atmosphere have trapped heat.


The earthThe earth’’s orbits orbit

Over 90Over 90--100,000 years the earth100,000 years the earth’’s orbit varies s orbit varies from nearly circular to more elliptical.from nearly circular to more elliptical.Presently it is becoming more circular.Presently it is becoming more circular.

This reduces the difference between times when This reduces the difference between times when sunsun’’s heat is at maximum and minimum.s heat is at maximum and minimum.

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Approach of Earth to the SunApproach of Earth to the Sun

The Earth changes its position relative to the The Earth changes its position relative to the sun on a cycle of 21,000 years.sun on a cycle of 21,000 years.The Earth is now nearest the sun in winter The Earth is now nearest the sun in winter (Northern hemispheric winter).(Northern hemispheric winter).

This reduces impact of seasonal climate change in This reduces impact of seasonal climate change in the north and increases it in the south.the north and increases it in the south.


Tilt of the Earth Tilt of the Earth

Cycle is 40,000 years.Cycle is 40,000 years.Tilt is now decreasing, reducing difference between Tilt is now decreasing, reducing difference between seasonsseasons

LongLong--term cycles affect distribution of sunterm cycles affect distribution of sun’’s s energy on earth.energy on earth.


Future Future GlaciationGlaciation

A 2% drop in heat from the sun during the northern A 2% drop in heat from the sun during the northern summer can start summer can start glaciationglaciation. .

Due to closeness of continents in northern hemisphere.Due to closeness of continents in northern hemisphere.

Cool summers allow winter snow and ice to survive to Cool summers allow winter snow and ice to survive to the next winter, and build up. the next winter, and build up. Snow cover increases reflectivity of earthSnow cover increases reflectivity of earth’’s surface.s surface.

Not in the Southern hemisphere because there is insufficient Not in the Southern hemisphere because there is insufficient land near the pole and too much water.land near the pole and too much water.

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Ice Ages and Interglacial Ice Ages and Interglacial PeriodsPeriods

Interglacial periods have been short compared to ice ages:Interglacial periods have been short compared to ice ages:250,000 years out of last 2 million250,000 years out of last 2 millionThe warmest interglacial period was 120,000 years ago with tempeThe warmest interglacial period was 120,000 years ago with temperatures ratures 22°° C warmer than today.C warmer than today.


Ideas about GeologyIdeas about Geology

In the late 18In the late 18thth and early 19and early 19thth centuries, there was centuries, there was much speculation about how the Earth came to much speculation about how the Earth came to have its particular form: mountains, rivers, have its particular form: mountains, rivers, valleys, oceans, plains, etc.valleys, oceans, plains, etc.There were two basic rival theories:There were two basic rival theories:

UniformitarianismUniformitarianism andandCatastrophismCatastrophism


UniformitarianismUniformitarianism

The chief proponent of The chief proponent of uniformitarianismuniformitarianism was was Charles Lyell, who held that Charles Lyell, who held that every different formation we every different formation we see can be caused by natural see can be caused by natural combinations of wear and combinations of wear and tear and occasional local tear and occasional local eruptions, such as volcanoes.eruptions, such as volcanoes.

At the right: Mount Etna in At the right: Mount Etna in Sicily erupting. Lyell spent Sicily erupting. Lyell spent several years studying the several years studying the pattern of Etnapattern of Etna’’s eruptions.s eruptions.

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CatastrophismCatastrophism

The chief proponent of The chief proponent of catastrophismcatastrophism was Georges Cuvier, was Georges Cuvier, who held that the only way to who held that the only way to explain the strange bones of huge, explain the strange bones of huge, extinct creatures (the dinosaurs) extinct creatures (the dinosaurs) was if there had been vast was if there had been vast catastrophes in the past unlike catastrophes in the past unlike presentpresent--day events.day events.

Cuvier reconstructed the skeletons of Cuvier reconstructed the skeletons of dinosaurs from bones just then being dinosaurs from bones just then being discovered.discovered.


UniformitarianismUniformitarianism Wins the DayWins the Day

On the whole, the uniformitarian explanation On the whole, the uniformitarian explanation proved to fit better with other facts. All the proved to fit better with other facts. All the normal geological formations, including normal geological formations, including continents themselves, are explainable by natural continents themselves, are explainable by natural laws and processes.laws and processes.


But there were catastrophes, tooBut there were catastrophes, too……

Cuvier was not all wrong, however.Cuvier was not all wrong, however.The extinction of the dinosaurs occurred about The extinction of the dinosaurs occurred about 6060--65 million years ago, for reasons not entirely 65 million years ago, for reasons not entirely understood.understood.But there does seem to have been a rather But there does seem to have been a rather sudden event on Earth that changed the sudden event on Earth that changed the environment radically and made life impossible environment radically and made life impossible for the large reptiles.for the large reptiles.

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Attack from Outer SpaceAttack from Outer Space

The likely cause of the catastrophes that Cuvier The likely cause of the catastrophes that Cuvier imagined was the Earth being struck by cosmic objects imagined was the Earth being struck by cosmic objects hurtling through space: comets, asteroids, etc.hurtling through space: comets, asteroids, etc.It would not take a very large cosmic object to crash It would not take a very large cosmic object to crash into the Earth and throw up such a cloud of dust into into the Earth and throw up such a cloud of dust into the atmosphere that it would radically alter the climate.the atmosphere that it would radically alter the climate.There are many large craters visible on Earth that could There are many large craters visible on Earth that could have been caused by such impacts.have been caused by such impacts.


The Environment Changes The Environment Changes ConstantlyConstantly

Regardless of human actions or inactions, some of the Regardless of human actions or inactions, some of the most crucial changes to the Earthmost crucial changes to the Earth’’s environment have s environment have been caused by natural forces far beyond our control.been caused by natural forces far beyond our control.

Some are the result of normal geological processes, such as Some are the result of normal geological processes, such as continental drift, volcanoes, earthquakes, ice ages.continental drift, volcanoes, earthquakes, ice ages.Some are features of the EarthSome are features of the Earth’’s orbit in the Solar system, s orbit in the Solar system, such as the tilt of the Earth, the proximity to the sun, the such as the tilt of the Earth, the proximity to the sun, the shape of the Earthshape of the Earth’’s orbit.s orbit.Some are caused by events originating outside the solar Some are caused by events originating outside the solar system, such as cosmic objects striking the Earth.system, such as cosmic objects striking the Earth.

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