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UNIT 4 EARTH HISTORY LECTURE AND STUDY GUIDE
EARTH TECTONICS AND SCIENCE
(Revised 7/11)
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Measurements of the Earth and Sun
- Earth’s diameter is approximately 8,000 miles.
- Earth’s circumference at the equator is about 24,000 miles.
- The sun’s diameter is approximately 800,000 miles.
Simple Compositional Model of Earth
- Earth has three distinct, concentric layers.
1) Iron core: is larger than ½ of Earth’s diameter.
2) Mantle: composed of ultramafic rocks and minerals.
3) Two types of Earth Crust:
a) Oceanic mafic crust (basalt).
b) Continental felsic crust (granitic).
Origin of Earth’s Three Concentric Layers
-Step 1: To derive a core and a mantle, heat required to melt the entire Earth was derived from:
1. Residual Formation Heat
a) Heat from gravitational energy.
b) Heat derived from kinetic energy generated by planetesimal collisions.
c) Heat generated from later meteor impacts (4.6 – 3.8 billion years).
2. Heat is also continually generated by radioactive decay.
- With melting, metallic iron and nickel sank to the center of the Earth to form the iron core (iron
catastrophe).
- The remaining “melt” was composed of mafic or “basaltic” rocks that surround the ultramafic Earth’s
mantle.
- FYI: We can replicate the above process with use of a multi-anvil press where rocks and iron powder
are mixed together. The mixture is then subjected to heat and pressure equal to that found in the
Earth’s interior. The resultant product will have iron metal core surrounded by a rock.
Step 2: Cooling of the Earth’s surface formed a basaltic crust (that will become oceanic crust).
- This mafic (low silica oceanic) crust is slightly different than the ultramafic (very lower silica)
mantle rocks.
Step 3: Creation of a continental crust. The rock cycle process (weathering, sedimentary rocks, etc.)
will produce a less dense rock that is richer in aluminum and silica (orthoclase feldspar and quartz).
- These less dense “granitic rocks” will form blocks of continental crust.
- It probable took early Earth about 200 million years to create scattered pieces of continental crust.
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Physical Properties of Earth’s Layers
- These properties are based on amount of contained heat and state: solid, plastic or liquid.
Lithosphere: Rigid outer layer containing “crust” and a frozen part of the upper mantle.
Asthenosphere: Partially melted upper mantle that has plastic-like properties.
Lower Mantle: (sometimes called the Mesosphere) Solid layer of rocks between the asthenosphere and
iron core.
Liquid Iron Core: Molten iron with some nickel and rocks (half diameter of Earth).
Solid Iron Core: + 6,000 0C
Figure above: Left is a compositional model and right is a physical properties model.
- (Note: we also call the mesosphere the lower mantle.)
Origin of Earth’s Atmosphere
- Frozen gas and water ices would be within the collection of planetesimals that collided together to
form the Earth.
- Most of Earth’s original hydrogen and helium would have escaped from Earth, because Earth did not
have enough gravity to hold these elements within its atmosphere.
Outgassing:
Outgassing: gases and water vapor escaping from the Earth’s interior by volcanoes
Earth’s Original Atmosphere: was composed of water vapor, methane, nitrogen, ammonia and CO2.
- There was almost no free oxygen with Earth’s original atmosphere.
- Earth’s original atmosphere may have been more like that of the Saturn moon Titan that has a thick
atmosphere of methane and nitrogen.
We can get over being poor, but it takes longer to get over being ignorant.
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Determining Earth’s early atmosphere that had little free oxygen:
- The earliest sedimentary rocks contain pyrite grains that were not oxidized thus no free oxygen.
- From 4.6 to 2.5 billon years, banded iron formations (that require abundant oxygen) are found to be
thin and scattered.
- Before 2.5 billion years, there are almost no beds of iron-stained (yellow and red) sandstone.
- From 2.5 – 2.0 billion years, there must have been more free oxygen because the banded iron
formations (that require abundant oxygen) are thick and cover vast areas. By this time, free oxygen
consumed all the dissolved iron in the oceans.
banded iron formation
Free Oxygen in Earth’s Atmosphere
- For more than ½ of Earth’s history, there was minor free oxygen on the planet.
- FYI: Up to one billion years ago, the atmosphere contained 4% of today’s free oxygen level.
Up to 500 million years ago, free oxygen was only 60% of today’s level.
Creation of Earth’s Free Oxygen
- Ancient bacteria and algae were present at least 3.5 billion years ago that created free oxygen for
Earth.
- These life forms used photosynthesis to combine CO2 and water into organic molecules and release
free oxygen as a waste product into the atmosphere.
Today’s Atmosphere
- Composition today: approximately 78% nitrogen and 21% oxygen (rounded 80% - 20%),
- Only a small fraction of Earth’s CO2 gas was converted into oxygen.
- Seawater absorbs CO2 from the atmosphere.
- Microscopic plants and animals convert this gas into tiny skeletons which eventually form limestone.
- Because of limestone, we have very little CO2 in our atmosphere.
Earth’s Hydrosphere
- Most of Earth’s original hydrosphere (water) was derived from outgassing of volcanoes.
- Outgassing by volcanoes even today usually contains about 70% water vapor.
- Water vapor from volcanoes fell as torrential rains for a few million years, during Earth’s earliest
years.
- Rock records imply that the Earth’s oceans were fairly deep by 3.5 billion years.
- Rocks also indicate that the oceans reached their current salinity levels, early as the oceans formed.
- Zircon grains in Australia imply that the Earth had liquid oceans more than 4.4 billion years ago.
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PLATE TECTONICS AND CONTINENTAL DRIFT
(A review from Physical Geology)
Tectonic Definitions
Tectonics: the branch of geology that studies regional and global structural features of the Earth
Plate Tectonics: the theory of global dynamics in which the lithosphere is broken into individual plates
that move in response to convection within Earth’s upper mantle
- Plate tectonics explains the interrelationships of volcanoes, earthquakes, climates, mountains,
extinctions and evolution
Continental Drift
- Continental Drift Theory proposed by German Alfred Wegener in 1915
- Wegener was the first to compile detailed evidence that continents did not stay in the same area over
geologic time.
Continental Drift: theory that continents move in relation to one another
Pangaea: the last known super-continent in which all the present-day continents formed one, very large
continent.
Wegener’s Theory
- Continents are composed of less-dense rocks, thus they float on the asthenosphere above the oceans’
seafloors.
- The oceanic crust is denser and therefore forms basins (seafloors) for the oceans.
- Wegner pieced together a super continent model because:
- 1) Similarities in shoreline of continents
- 2) Distinctive rock formations on matching continents
- 3) Distinctive matching fossil groups
- Wegner pointed out that at one time the Appalachian Mts. of the Eastern North America and the
Caledonian Mountains of Great Britain – Scandinavia would form a continuous chain of similar
rocks.
- When placed together, the various Precambrian rocks of Africa and South America would form a
continuous match of rock types.
- An ancient period of glaciation and glacier movements (noted by glacial striations) could be
explained if the continents were placed together.
- The ancient location of Pangaea could explain the coral reefs in West Texas and the sand dunes in
Canada.
- Restricted Pangaea biodiversity: very large numbers of animals but limited number of species
- Before 145 MY, there was very little biodiversity in Pangaea as demonstrated by fossils of a freshwater reptile – Mesosaurus, and other animals including Cynognathus, Lystrosaurus and the planttree glossopteris.
- Antarctica at one time contained dinosaurs and had a climate that supported the development of coal.
- Wegener’s theory was rejected because he could not provide a mechanism to move the continents.
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Earth’s Core and Magnetism
- Earth has an outer liquid iron core and an inner solid iron core.
- The solid iron core spins 2/3rd of a second faster than the outer liquid iron core, which creates Earth’s
magnetic field.
- Earth’s magnetic field creates a huge magnetic shield that prevents the Earth from being blasted by
cosmic particles from the sun.
- The magnetic field creates a Magnetic North Pole and a Magnetic South Pole which we can detect
with a compass.
- The aurora borealis is evidence of Earth’s magnetic shield.
Earth’s spinning solid iron core
Magnetite and Magnetism in Rocks
- Magnetite: an iron oxide mineral that acts as a natural magnet
- Magnetite grains are a common constituent of the igneous rock basalt.
- When basalt magma (lava) cools, the contained magnetite grains align themselves to Earth’s
magnetic field that is present at that time.
Paleomagnetism: the study of magnetic fields, as preserved in the magnetic properties of rocks
Magnetometer: an instrument used to detect Earth’s magnetism
- Magnetic fields that are preserved in ancient lava flows can be used to demonstrate that Earth’s
continents have moved over time.
Magnetic or Polarity Reversals: a 1800 reversal of the (N-S) polarity of Earth’s magnetic field
- This has happened at least 171 times in the last 71 million years
Geomagnetic Time Scale: matching geologic times with magnetic reversals found in rocks
- We might expect another magnetic reversal within the next 2,000 years or tomorrow
- Sun spots show us that magnetic reversals occur in the Sun every 11 years
Seafloor Spreading
- Dr. H. H. Hess (1906-1969) proposed in 1960 his theory of sea floor spreading as a method to move
and separate the continents:
Seafloor Spreading Theory: The seafloor is spreading apart, propelled by mantle convection currents.
- The theory proposes:
1) The seafloor is moving symmetrically away from a central mid-oceanic ridge.
2) The seafloor will be pushed to deep sea trenches (subduction zones) where oceanic crusts
descend into the mantle.
3) All oceanic crust will be regenerated about every 200 MY.
Good intentions are not good enough... ultimately we are measured by our actions.
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Describing the Seafloor
- Mapping the Earth’s seafloor determined that there was a mountain chain – almost 40,000 miles long
and 900 miles wide, in the center of most oceans.
- At the mid-oceanic ridge is a central rift valley and this ridge rises thousands of feet above the ocean
floor.
- Mathews and Vines (1963) tested magnetic patterns in seafloor rocks, along the mid-oceanic ridge.
- They found magnetic polarity stripes of magnetic reversals, in areas (ridges) of sea floor spreading
were symmetrical on both sides of the ridge. The polarity timing shows that the rocks get older away
from the ridge.
Seafloor magnetic stripes
- Deep seafloor trenches (+25,000 feet deep) are found alongside island and continent edges – not in
the middle of the oceans.
Submarine Trench: a very deep, elongated depression on the seafloor (adjacent to a subduction zone)
Mariana Trench: Earth’s deepest trench at more than 36,000 feet depth (about 7 miles)
- Core drilling by the Glomar Challenger ship and age dating of that core proved that none of the
ocean’s crust is older than 200 MY, even though parts of the Earth’s continental crust is more than 3
BY.
- The ship also drilled oceanic ooze that commonly deposits at a rate of 3 mm/1,000 years and all were
found to be less than 200 million years old
- The youngest oceanic crust is found at the ridges and the rocks get older away from the ridges.
- Significance: oceanic crust must be constantly destroyed and renewed (thus most oceanic crust is
much younger than most continental crust).
Tectonic Plates
- Heat derived from the Earth’s core creates convection cells within the mantle and/or asthenosphere.
- The rising part of a convection cell breaks the Earth’s crust (lithosphere) apart into pieces that we call
plates.
Lithosphere: oceanic crust or continental crust with an attached frozen mantle directly below these
crusts
Tectonic Plate: a single block of lithosphere that moves about the Earth’s surface
- Individual tectonic plates are based on structural features, not on continents and oceans.
- Plates are outlined by ridges, trenches and mountains.
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Lithosphere plate: most contain continental crust and some attached oceanic crust
- Today, we do not use the term continental drift because the continents are attached to plates that
move relative to each other.
- The “plastic” asthenosphere below the plates allows the plates to move about the Earth.
Lithospheric plates
Major Features of Plate Tectonic Theory
- There are three basic types of plate boundaries.
1. Divergent Zone – where lithospheric plates move apart
2. Transform Zone – where plates slide past one another
3. Convergent Zone – where plates come together
Plate Boundaries
1) Divergent Plate Boundary (spreading center, mid-oceanic ridge, continental rift zones)
a) Mid-oceanic ridge: also called a spreading center or central rift zone
- Most divergent zones are located within the ocean floor where of sea-floor spreading basalt
is creating new oceanic crust; this zone forms the mid-oceanic ridge that rises high above the
ocean floor
- This zone creates the largest, longest mountain chain on Earth
- Plates are being moved apart or away from each other (tension) where vertical injections of
basalt fill the fracture zones
- Underwater lava flows called pillow lavas form in the undersea divergent zone. These lavas
generally form unique ellipsoidal masses
-Divergent zones can create ocean basins over time.
- In the oceans, this zone forms the mid-oceanic ridge that rises high above the ocean floor.
- Hot water escapes in fountains along these zones called “black smokers” which have some
very exotic marine life forms that never see sunlight.
b) Rift (Zone) Valley: a divergent zone on land, where the continental crust is arched up and
extended and pulled apart into fault blocks creating valleys. The Rio Grande Rift in New
Mexico is an example rift zone
Top: rift zone; Bottom seafloor spreading
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2) Transform Boundary (sliding plate boundary)
- Where plates slide past one another (because Earth is a sphere)
- These are directly associated with divergent zones.
- The San Andreas Fault is a transform fault
3) Convergent Plate Boundary (subduction zones)
- This is the area where the plates comes together (compression). These zones are where
continental crust is created; most metamorphic rocks are created here
a) Convergent zone where two oceanic crusts meet: One plate will be subducted into the
mantle and the other plate will form a chain of volcanic mountains called an island arc.
b) Convergent zone where an oceanic crust is subducted into the mantle under a continental
crust. This will create mountains alongside the continental crust.
c) Convergent zone where two continental crusts collide creating gigantic mountains (but
neither plate is subducted into the mantle); an example collision created the Himalayas
- Associated with subduction zones
- Subduction zone: a locality where oceanic crust is being pushed or subducted into the mantle
(directly associated with deep-sea trenches or convergent zones)
- Plate motions are complex involving geometry of a curved plate moving on a sphere.
There are three types of people in this world: those who make things happen, those who watch
things happen and those who wonder what happened. We all have a choice. You can decide which
type of person you want to be. I have always chosen to be in the first group.
Mary Kay Ash
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Driving Mechanisms of Plate Tectonics
1. Convection cells within the asthenosphere or mantle can cause plates to move
2. Slab Pull theory: weight of the slab entering the mantle will pull the crust remaining into the mantle
- The slab pull theory is most favored today
Colliding Terrains
- Conditions set up when an island arc collides with a continent. The island arc will be scraped off and
will become a part of the continent (sometimes called accreted terrain or exotic terrain).
Hot Spots and Mantle Plumes
- These tectonic features are usually not found at a plate boundary
- Mantle plumes (hot spots): magma that rises from deep inside the mantle, with a continuous supply of
magma that may last for 100 million years, even as the overlying plate moves
- Hawaii is the result of a mantle plume or hot spot
Supercontinent Cycle
- About every 500 million years, most of Earth’s continents collide together, then later split apart.
- Pangaea was only the latest supercontinent.
- The ultimate driving force for plate tectonics is the heat that causes the convection cells within the
Earth.
Don't be fooled by the calendar. There are only as many days in the year as you make use of. One
man gets only a week's value out of a year while another man gets a full year's value out of a week.
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Science and the Use of Scientific Principles
- The study of Earth History involves the use of science.
- What is science? How often do people accept explanations to questions that were answered by using
the scientific method?
- Science is often unpopular with many groups because scientific theory often conflicts with their core
beliefs
- We have seen that insisting that everyone be “politically correct” can cause conflicts with science and
culture
Scientific Endeavors
- Science is different from other fields of human endeavor because science searches for regularities and
patterns in the natural world.
- Science then tries to express these regularities as scientific laws, principles, theories and models.
- Science provides a special path of learning about the inner workings of our world and universe
- Many aspects of being human are not always amenable to scientific description (because of culture
and opinion)
- Physical Geology and Earth History (normal sciences) try to carefully address phenomena of the
natural world and does not address subjective “sciences” such as:
-- Justice and honor that varies by culture – not science
-- Esthetics and manners, etc. – these vary by culture and not by science
Questions that Science Can Address Are Limited by:
1. The properties of the natural world (which would leave out the paranormal)
2. The instruments available to extend human sense
- We have to have something to measure and a way of measuring it
- There is no way to scientifically measure something that always “conveniently disappears”, thus
you cannot do a scientific study on these subjects (Bigfoot?)
3. Socially imposed missions; an example is the restricting of human cloning
4. The paradigm that guide scientific inquiry
- Paradigm: a method to sort, organize and classify information
- For example, today it is now acceptable to use computer simulations to help explain natural
phenomena
In Science, We Learn by – Using the Scientific Method
- The scientific method involves the following organized steps
1) Observations of phenomena or events in the natural world includes the gathering of facts and
data
2) Making a deduction about the possible causes and effects related to the observed facts
3) Create a hypothesis that could explain the observed facts and deductions
4) Test your hypothesis by experimentation and additional observations
5) Create a scientific theory when tests and observations repeatedly confirm the hypothesis
- A scientific theory is much different from “theories” as used in a wild speculation or non-scientific
sense (i.e. some people have a theory that all Corpus Christi drivers are reckless)
- Remember that a scientific theory is an explanation, derived by the scientific method for regularities
or patterns in the natural world
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Theories (Properties of Theories)
- Theories are not based on speculation and must have the following characteristics:
a) A theory must explain the data.
b) A theory must be capable of prediction.
c) A theory is capable of being proven wrong.
- Example – the theory of evolution. It can be overturned if you can prove that early mammals (a
higher life form) appeared on Earth before early fish (which are a less advanced life form)
The great advantage of the scientific method is that it is unprejudiced: one does not have to
believe a given researcher; one can redo the experiment and determine whether his/her results are
true or false. The conclusions will hold irrespective of the state of mind, or the religious persuasion, or
the state of consciousness of the investigator and/or the subject of the investigation. Faith, defined as
belief that does not rest on logical proof or material evidence, does not determine whether a scientific
theory is adopted or discarded (sciencebuddies.org)
Scientific Knowledge:
a) is not absolute
b) is tentative, approximate and subject to revision
c) also applies to the Earth sciences
- In reality, many of our scientific theories and geologic principles are changing, because of our
continual use of science and scientific investigations
- The shift from continental drift to plate tectonics is a prime example
- We discussed how Alfred Wegener completed an exhaustive study of “known” geologic data for that
time. We saw how he methodically presented detailed evidence for continental drift, which looked
very compelling at that time for only a few scientists.
- But the scientific community rejected his theory because some key components were not critically
addressed to explain his hypothesis. He could not satisfactorily explain how to move the continents
over Earth’s surface – a devastating flaw to his otherwise interesting theory and undeniable geologic
evidence.
- In science, a theory is not sacred – it can be overturned.
- A major long-held theory (or rejected theory) can be overturned by evaluating new data to old data
and re-evaluating – a paradigm shift.
-- Example: studies in the 1960’s used most of the components of the continental drift theory and
found a way to move the continents. We now have the “acceptable” Plate Tectonic Theory.
“Political Science”
- A lot of “science” today is used to achieve political goals
- We did the impossible by using science to put twelve men on the moon. Even though everyone was
aware of the consequences and dangers, the benefits to mankind from that scientific project was
beyond our wildest imaginations.
Just the Facts
- For any of us to “win” with science, we must be presented with all the scientific facts and not just the
information that leads to judgment in one direction or another.
- Details that could throw doubts on the interpretations of your theory must be given (if you know
them)
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- You must include all the facts that disagree with your theory, as well as those that agree (no hiding of
the reasonable possibilities)
- Giving all of the information will help us to question and judge the value of the theory
Historical Note
- The early 1960s were concerned about global cooling.
- Then overpopulation became the next scientific crisis
Global Warming and Climate Change
- Yesterday we had the “political science” of global warming (now climate change) where mostly one
side of the scientific picture was presented.
- Any natural phenomena changes were blamed of global warming (if you did not write this, you won’t
get much publicity)
- There are many geologists that want to see how mere man can change the world climate when studies
show this process has occurred without man for billions of years.
People are always blaming their circumstances for what they are. I don't believe in circumstances.
The people who get on in this world are the people who get up and look for the circumstances they
want, and, if they can't find them, make them.
George Bernard Shaw
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