Transcript earth - Geo2

21 July 2010
1) What is the age of the oldest fossil?
2) It is famous for the exceptional
preservation of the fossils found
within it, in which the soft parts are
preserved. It is 505 million years
(Middle Cambrian) in age, making it
one of the earliest fossil beds to
preserve the soft parts of animals.
3) This results when original remains
of the organism have been
completely dissolved or otherwise
destroyed and all that is left is an
organism-shaped hole in the rock
4) This is a result of chemical
reduction of the complex organic
molecules composing the organism's
tissues
5) Give an example of trace fossil.
6) This galaxy is a collection of billion
of stars bound together by gravity,
including the solar system.
7)What is the discontinuity between
crust and mantle?
8) What is the relatively soft layer
below the lithosphere?
9) What is the deepest seafloor
depression in the world?
10) Who developed the Plate Tectonics
Theory?
11) Illustrate the Geologic Time Scale.
RADIOACTIVE DATING
It is the principal source of information
about the absolute age of rocks and
other geological features, including
the age of thE Earth itself, and can
be used to date a wide range of
natural and man-made materials.
Together with stratigraphic
principles, radiometric dating
methods are used in geochronology
to establish the geological time scale.
Among the best-known techniques are
radiocarbon dating, potassium-argon
dating and uranium-lead dating.
EARTH
An oxygen-rich and
protective atmosphere,
moderate temperatures,
abundant water, and a
varied chemical composition
enable Earth to support life,
the only planet known to
harbor life. The planet is
composed of rock and metal,
which are present in molten
form beneath its surface. The
Apollo 17 spacecraft took
this snapshot in 1972 of the
Arabian Peninsula, the
African continent, and
Antarctica (most of the white
area near the bottom).
EARTH’s QUICK FACTS
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Equatorial radius 6,378 km
Equatorial inclination 23.5°
Mass 5.97×1024 kg
Average density 5.5 g/cm3
Rotational period 0.997 days
Orbital period 1 year
Average distance from the Sun 149.6 million km
Perihelion 147.1 million km
Aphelion 152.1 million km
Orbital eccentricity 0.0167
Orbital inclination 0.0003°
Moons: 1
EARTH
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One of the planets in the solar
system
71% covered with water
Surrounded by layer of gases –
atmosphere
Venus – almost identical to Earth but
has too much heat-trapping carbon
dioxide in its atmosphere, extremely
hot surface (462 C)
EARTH, SOLAR SYSTEM,
GALAXY
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Earth is 3rd planet from sun (ave
dist:150 million km)
All planets revolve around the sun
due to force of gravitation
Earth’s orbital path : slightly elliptical
Solar system is part of Milky Way
Galaxy
Milky Way Galaxy – collection of
billion of stars bound together by
gravity
LAYERS of the ATMOSPHERE
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Troposphere – layer in which weather
occurs, extends from surface to about
16km (10 mi) above sea level
Stratosphere – upper boundary of 50
km (30mi)
Mesosphere – 50 to 90km (30-60mi)
Thermosphere – at 90km
Ionosphere – layers of ionized air
extending from almost 60km
Exosphere – beyond thermosphere
LAYERS OF THE
ATMOSPHERE
Divisions of the
Atmosphere
Without our
atmosphere,
there would be no
life on Earth. A
relatively thin
envelope, the
atmosphere
consists of layers
of gases that
support life and
provide protection
from harmful
DYNAMICS of the EARTH’S
INTERIOR
CRUST – consists of the continents,
other land areas and the basins, or
floors of the oceans
CONTINENTAL CRUST – dry land of
earth’s surface; 15 to 75 km (9 to 47
mi) thick; consists of lighter-colored
less dense igneous rocks, such as
granite and diorite; also includes
metamorphic and igneous rocks
OCEANIC CRUST – thinner than
continental crust; 5 to 10 km (3 to 6
mi); consists of dar, dense, igneous
rocks such as basalt and gabbro
PLATE TECTONICS
LITHOSPHERE – outer part of Earth,
consisting of the crust and upper
mantle; “rock layer”; 65 to 100 km
thick (40 to 60 mi); relatively cool,
outermost layer of the planet; rigid
shell; brittle; behaves as single mass
in the motion of the rocky plates that
make up the Earth’s surface
PARTS of LITHOSPHERE:
Crust – granite and basalt
Upper Mantle – peridotite (olivine and
pyroxene)
ASTHENOSPHERE – At depth of about
100 km (60 mi), seismic waves slow
down; underlying softer layer
*Motion of lithospheric plates result
from their attachment to the plastic,
slowly flowing asthenosphere, thus
the PLATE TECTONICS THEORY
PLATE TECTONICS – theory that outer
shell of the earth is made up of thin,
rigid plates that move relative to
each other.
- jigsaw puzzle
by Alfred Wegener in 1912
(Continental Drift Theory)
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Volcanoes and Plate Tectonics
Scientists have determined that there is a
close connection between the formation of
volcanoes and the movement of the tectonic
plates. Nearly 80% of the earth’s volcanoes
are found near the tectonic plate boundaries
of the Pacific Ocean. The oceanic crust is
subducting, or plunging beneath, the
continental crust in this region. Volcanoes
can also be found at divergent plate
boundaries where tectonic plates are
splitting apart, or in the middle of tectonic
plates, such as the volcanoes of the
Hawaiian Islands in the North Pacific Ocean.
Large Plates: PACIFIC PLATE, NORTH
AMERICAN PLATE, EURASIAN PLATE,
ANTARCTIC PLATE
AFRICAN PLATE
Small Plates: COCOS PLATE, NAZCA
PLATE, CARRIBBEAN PLATE
PLATE MOVEMENT:
Geologists discovered absolute plate
motion when they found chains of
extinct submarine volcanoes.
Scientists use hot spots to measure the
speed of tectonic plates relative to a
fixed point. To do this, they determine
the age of extinct volcanoes and their
distance from a hot spot. They then use
these numbers to calculate how far the
plate has moved in the time since each
volcano formed. Today, the plates
move at velocities up to 18.5 cm per
year (7.3 in per year). On average,
they move nearly 4 to 7 cm per year (2
to 3 in per year).
“HOTSPOT”
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DIVERGENT PLATE BOUNDARIES occur where two plates are moving
apart from each other. When plates
break apart, the lithosphere thins and
ruptures to form a divergent plate
boundary. In the oceanic crust, this
process is called seafloor spreading,
because the splitting plates are
spreading apart from each other. On
land, divergent plate boundaries create
rift valleys—deep valley depressions
formed as the land slowly splits apart.
Rift valleys are long, deep valleys
bounded by parallel faults. They form
where Earth’s crust is being pulled
apart. Rift valleys can appear on land
or beneath bodies of water.
MID-ANLANTIC RIDGE - an underwater
mountain range created at a
divergent plate boundary in the
middle of the Atlantic Ocean. It is
part of a worldwide system of ridges
made by seafloor spreading. The
Mid-Atlantic Ridge is currently
spreading at a rate of 2.5 cm per
year (1 in per year). The mid-ocean
ridges today are 60,000 km (about
40,000 mi) long, forming the largest
continuous mountain chain on earth.
CONVERGENT PLATE BOUNDARIES occur where plates are consumed, or
recycled back into the earth’s
mantle.
3 TYPES:
1) OCEANIC-OCEANIC
2) CONTINENTAL-CONTINENTAL
3) OCEANIC-CONTINENTAL
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1) OCEANIC-OCEANIC: Chains of
active volcanoes develop 100 to 150
km (60 to 90 mi) above the
descending slab as magma rises
from under the plate. Also, where
the crust slides down into the earth,
a trench forms. Together, the
volcanoes and trench form an intraoceanic island arc and trench
system.
A good example of such a system is
the Mariana Trench system in the
western Pacific Ocean, where the
Pacific plate is descending under the
Philippine plate. In these areas,
earthquakes are frequent but not
large.
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Piccard’s Bathyscaphe
In 1960 Swiss oceanographic
engineer Jacques Piccard and United
States Navy lieutenant Donald Walsh
descended to the Challenger Deep,
the lowest point on earth, located in
the Mariana Trench. They used the
bathyscaphe (a submarine vessel
designed for deep-sea exploration)
called the Trieste, pictured here.
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MARIANAS TRENCH - the deepest seafloor
depression in the world. It is located just east
of the Mariana Islands in the western part of
the ocean basin. The Mariana Trench is an arcshaped valley extending generally northeast to
southwest for 2,550 km (1,580 mi); its average
width is 70 km (40 mi). The Mariana is one of
many deepwater ocean trenches formed by the
geologic process of subduction. Near its
southwestern extremity, 340 km (210 mi)
southwest of the island of Guam, is the deepest
point on earth. This point, the Challenger Deep,
is estimated to be 11,033 m (36,198 ft) deep.
The Challenger Deep was named after HMS
Challenger II, the vessel of those who
discovered the point in 1948.
2)Continental-Continental : the
incoming plate drives against and
under the opposing continent. This
often affects hundreds of miles of
each continent and, at times,
doubles the normal thickness of
continental crust. Colliding continents
cause earthquakes and form
mountains and plateaus.
The collision of India with Asia has
produced the Himalayan Mountains
and Tibetan Plateau.
3) OCEANIC-CONTINENTAL: occur
between the ocean and land create
continental margin arc and trench
systems near the margins, or edges,
of continents. Volcanoes also form
here. Stress can develop in these
areas and cause the rock layers to
fold, leading to earthquake faults, or
breaks in the earth’s crust called
thrust faults. The folding and thrust
faulting thicken the continental crust,
producing high mountains.
Many of the world’s large destructive
earthquakes and major mountain
chains, such as the Andes Mountains
of western South America, occur
along these convergent plate
boundaries.
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TRANSFORM PLATE BOUNDARY
also known as a transform fault
system, forms as plates slide past
one another in opposite directions
without converging or diverging.
Geophysicist J. Tuzo Wilson studied
the direction of faulting along
fracture zones that divide the midocean ridge system and confirmed
that transform plate boundaries were
different than convergent and
divergent boundaries. Within the
ocean, transform faults are usually
simple, straight fault lines that form
at a right angle to ocean ridge
As a transform plate boundary cuts
perpendicularly across the edges of
the continental crust near the
borders of the continental and
oceanic crust, the result is a system
such as the San Andreas transform
fault system in California.
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TRIPLE JUNCTIONS
Rarely, a group of three plates, or a
combination of plates, faults, and
trenches, meet at a point called a
triple junction. The East African Rift
Zone is a good example of a triple
plate junction. The African plate is
splitting into two plates and moving
away from the Arabian plate as the
Red Sea meets the Gulf of Aden.
PLATE MOVEMENT changes the size of
our oceans and shapes of continents:
 Pacific plate moves at an absolute
motion rate of 9 cm per year (4 in
per year) away from the East Pacific
Rise spreading center
 India moves north at 5 cm per year
(2 in per year) as it crashes into
Asia, while Australia moves slightly
farther away from Antarctica each
year.
Causes of Plate Motion?
Although plate tectonics has explained
most of the surface features of the
earth, the driving force of plate
tectonics is still unclear. According to
geologists, a model that explains plate
movement should include three forces.
Those three forces are the pull of
gravity; convection currents, or the
circulating movement of fluid rocky
material in the mantle; and thermal
plumes, or vertical columns of molten
CAUSES OF PLATE MOTION:
A. Plate Mov’t caused by Gravity:
Geologists believe that tectonic plates
move primarily as a result of their own
weight, or the force of gravity acting on
them. Since the plates are slightly
denser than the underlying
asthenosphere, they tend to sink. Their
weight causes them to slide down
gentle gradients, such as those formed
by the higher ocean ridge crests, to the
lower subduction zones.
Once the plate’s leading edge has
entered a subduction zone and
penetrated the mantle, the weight of
the slab itself will tend to pull the rest
of the plate toward the trench. This
sinking action is known as slab-pull
because the sinking plate edge pulls
the remainder of the plate behind it.
Another kind of action, called ridgepush, is the opposite of slab-pull, in
that gravity also causes plates to slide
away from mid-ocean ridges. Scientists
believe that plates pushing against one
another also causes plate movement.
B. Convection Currents:
In 1929 British geologist Arthur Holmes
proposed the concept of convection
currents—the movement of molten
material circulating deep within the
earth—and the concept was modified to
explain plate movement. A convection
current occurs when hot, molten, rocky
material floats up within the
asthenosphere, then cools as it
approaches the surface.
As it cools, the material becomes
denser and begins to sink again,
moving in a circular pattern.
Geologists once thought that
convection currents were the primary
driving force of plate movement.
They now believe that convection
currents are not the primary cause,
but are an effect of sinking plates
that contributes to the overall
Alfred
Wegener
In 1912 German
meteorologist Alfred Wegener
proposed that all the
continents were once joined
in a single landmass, which
he called Pangaea. Wegener
relied on geological evidence
and fossil records to support
his theory that the landmass
gradually separated through
continental drift. In the 1960s
researchers confirmed
Wegener’s theory when they
observed seafloor spreading
and other phenomena. This
research eventually led to the
theory and study of plate
tectonics.
EVIDENCES of POLAR WANDERING,
CONTINENTAL DRIFT and SEAFLOOR SPREADING:
1) Matching of Continental Outlines –
best fit between continents
2) Paleoclimatology – certain kinds of
rocks are found in areas where
present climates are not conducive
to their deposition.
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Cold climate at poles
Warm , moist climates near equator
3) Paleontological Evidence:
a) 64% of Carboniferous and 34%of
Triassic reptile faunas are the same
for the Southern continents
b) Freshwater reptile Mesosaurus has
been found only in South America
and Africa. Bone structure of this
salamander-like organism is such
that it would have not been able to
swim a large ocean
c) Numerous bones of terrestrial
reptiles have been discovered in
Triassic rocks in Antarctica which is
now completely isolated from other
continents.
d) Paleozoic marine invertebrate
assemblages are quite similar on
both sides of Atlantic Ocean
e) Fossil leaves of cycads and seed
ferns belonging to Glossopteris flora
of Permian age have been found on
all southern continents and India.
4) Stratigraphic Evidence: similarities;
stratigraphy of Antarctica is
remarkably similar to that of
southern continents.
5) Structural Evidence: continuity in
landforms (Atlantic)
6) Paleomagnetism
The direction of north and south we
take for granted, but magnetic north
and south have switched places in
the past and may do so again in the
near future.
The Earth's magnetosphere is what
generates the Earth's magnetic
poles. It also protects us from
harmful solar wind emanating from
the Sun and radiation from outside
our solar system. It sheathes the
Earth and extends outside the
atmosphere.
Origin of the Magnetosphere
At the center of the Earth is a core of iron
and other heavy metals. Iron is a magnetic
metal and this is what produces the
magnetosphere. The Earth is unusual to
have so much iron at it's core. It is now
widely believed from the findings of the
Apollo missions that this a result from a
collision with another object during the
Earth's formation. A majority of heavier
elements combined from both objects to
make the Earth, and a lot of the lighter
elements combined to make the Moon.
The Dynamic Magnetic Field
The magnetosphere is not quite a perfect
sphere. It's more like an apple, with the
poles where the stem of an apple is. It
constantly changes shape and strength,
and different patches can favor
different magnetic directions. The
magnetic field varies and shifts on a
small scale. If the field is chaotic
enough it is believed that a
geomagnetic reversal may occur.
History of Geomagnetic Reversal
Scientists know that reversals have occurred
many times in the past. The direction of
magnetic grains laid down successively in
the Earth's crust, particularly the sea floor
are a primary piece of evidence. When the
rock is new and molten the grains are free
to align themselves with the prevailing
magnetic field. As the rock cools, the grains
are frozen in time. As the sea floor expands
outward (in the Atlantic), it is regularly
striped with rock oriented in different
directions. This indicates that the magnetic
poles have reversed many times throughout
the Earth's history.
Paleomagnetism
All rocks have magnetic fields owing
to the presence of magnetic
minerals, such as magnetite. By
measuring the direction and
inclination of the magnetic field, it is
possible to calculate the approximate
position of the magnetic poles of the
earth at the time of crystallization or
deposition of the rock.
Rock magnetism furnishes a ‘fossil
compass’.
Assumptions:
1) Magnetic axis of rock was oriented
parallel to the earth’s magnetic field
when the rock formed
2) Average position of magnetic pole is
assumed to approx. that of rotational
pole.
3) Earth’s magnetic field is assumed to
have always been dipolar, with one
north and one south magnetic pole.
‘Natural Remanent Magnetism’ may be
due to:
1) Thermoremanent magnetism – rocks
cool past their Curie temperature.
Magnetite has Curie temp of 578 C.
2) Chemical Remanent magnetism –
produced during formation of ironbearing minerals in low temp. Hematite
in arkosic red beds forms by
weathering in place of such minerals as
biotite and hornblende. Such beds
would have magnetsim parallel to
earth’s magnetic field at time of
RESULTS:
Paleomagnetic pole positions have
been determined for formation of
various ages and from various
continents.
Positions of rotational poles have
changed relative to the continents:
i) Paleozoic poles were generally
located near equator but late
Paleozoic poles were near mid
latitudes
ii) Mesozoic and Cenozoic poles were
located near the present
geographical poles.
SEA-FLOOR SPREADING:
Harry Hess was the first to propose
that blocks of sea-floor are moving
relative to one another in response
to the motion of convection currents
within the mantle.
Marine Magnetics – magnetism in
ocean floor; presence of a series of
north-south trending magnetic
anomalies.
‘magnetic anomaly’ – deviation from
average intensity of earth’s magnetic
field. In area of positive anomaly,
earth’s magnetic field has a greater
than average intensity, whereas in
an area with negative magnetic
anomaly, intensity is below average.
RESULT:
Studies of direction of magnetization
within sequences of rocks have shown
that in some, the direction of
magnetization is exactly the opposite of
that in adjacent strata.
Samples of volcanic rocks that have been
dated radiometrically as less than
690,000 years have the north-seeking
pole of their magnetic axis pointing
approximately in the direction of the
magnetic North Pole.
Normal Polarity, were formed when
earth’s magnetic field was
approximately the same as it is
today.
Rock samples between 690, 000 yrs to
890,000 yrs old have an opposite
direction of magnetization (Reverse
Polarity) and were formed during a
time when the earth’s magnetic field
was the reverse of what it is today.
Oceanic Volcanoes – earliest indication
is general increase in age of volcanic
islands and seamounts away from
the Mid-Atlantic ridge.
THE ORIGIN OF EARTH AND ITS
EARLY HISTORY
Early Theorization on universe:
Early man : earth consisted of a
saucer balanced on the back of a
monstrous reptile
Thales (550 B.C.), Greek: earth was a
flat disk floating on water
Anaximander (550 B.C.), Greek:
universe was a cylinder in a great
void
Aristarchus (third century B.C.),
Greek: heliocentric universe
Hipparchus (second century, A.D.),
Greek: the geocentric universe
Ptolemy (second century, A.D.),
Egyptian: the geocentric model with
stars and planets on fixed spheres
around the earth.
Copernicus (1543) – revived the
heliocentric model of the solar
system and proposed that earth was
one of several planets orbiting the
sun.
Galilieo (1609) – constructed first
astronomical telescope; gathered
evidence that supported Copernican
model
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BIG BANG THEORY – (by Georges
Lemaître)
is the cosmological model of the
initial conditions and subsequent
development of the universe that is
supported by the most
comprehensive and accurate
explanations from current scientific
evidence and observation..
As used by cosmologists, the term Big
Bang generally refers to the idea that
the Universe has expanded from a
primordial hot and dense initial
condition at some finite time in the
past (currently estimated to have
been approximately 13.7 billion
years ago), and continues to expand
to this day
HYPOTHESES ON ORIGIN of the
SOLAR SYSTEM:
Based on close encounter:
Protoplanet Theory. Buffon, 1979: a
passing star or comet caused the sun
to shred rings of matter, which
ultimately condensed to form planets
orbiting the sun.
Planetismal Hypothesis. T.C.
Chamberlin and F.R. Moulton, 1895,
a passing star and eruptive activity
in the sun caused solar material to
go into orbit and form the planets.
Tidal Hypothesis. J. Jean and H.
Jeffreys, 1917: a near collison
between the sun and another star
drew out of the sun and another star
drew out of the sun a tide of gases
which broke up to form the planets.
Double Star Hypothesis. Lyttleton,
1936: a companion star of the sun
collided with a third star to form the
solar system.
Based on Contraction of Nebula:
Nebular Hypothesis. Kant, 1755:
Laplace, 1796: the nebula contracted
gravitationally and by conservation
of angular momentum began to
rotate, separated into rings with
centers of gravitation which became
the planets and their satellites.
Turbulent eddies hypothesis. Carl von
Weizsacker, 1944: the cosmic dust
and gas contracted into a rotating
disk of turbulent eddies, of which the
smaller, denser eddies produced the
plantes.
Dust cloud hypothesis. F. Whipple,
1946: the solar system formed
through contraction of globule.
Protoplanet hypothesis. G. Kuiper,
1950: a condensing and rotating
globule flattened and its dense
central mass formed the sun. The
protoplanet which were larger that
the modern planets, developed from
orbiting eddies and their lighter
gases (hydrogen, helium) were
driven off by the sea.
THEORIES OF EARTH-MOON
RELATIONSHIPS:
1) Moon was once part of the earth and
separated from it as a result of rapid
rotation of the earth.
2) Earth and moon formed nearby in
space at approximately the same
time and from the same raw
materials.
3) Moon formed elsewhere in the solar
system and was subsequently
captured by the earth.
ASTEROIDS, METEORITES AND EARTH
HISTORY
Asteroids – small bodies, probabl
representing remains of disrupted
planets
Meteorites – asteroids that strike the
earth
 Stony meteorite – silicates
 Metallic meteorite – nickel-iron
4.5 – 4.7 billion years – age of most
meteorites based on U-Pb, K-Ar, RbSr
4.66 billion years – age of oldest lunar
sample
4.53 billion years – age of earth based
on Pb isotopic ratios in oceanic
basalts
SEPARATION OF EARTH’S CORE,
MANTLE, CRUST
As the earth contracted in its
formative stages, its interior began
to heat up through release of grav’l
energy and decay of radioactive
isotopes; the nickel-iron melted and
the resulting liquid sank toward the
center of the earth.
SUN : 86% hydrogen
: 13.7% helium
: <1 % elements heavier than
helium
Jovian planets are further from the sun
and therefore colder, having lost less
of their original mass.
Earth: 78% Nitrogen
: 21% Oxgen
: 1 % Carbon dioxide and
Water
A.I Oparin (1936) – proposed that
early atmosphere of the earth was
reducing and was rich in methane
and ammonia.
S.L. Miller (1953) - produced organic
molecules
experimentally from the components
suggested by Oparin as comprising
the earth’s primitive atmosphere.
Miller passed an electric spark through
a closed chamber filled with a
mixture of ammonia, methane, water
vapor and hydrogen. Spark produced
the necessary energy to bring about
the reaction (lightning or UV rays).
Resulting organic molecules were
amino acids, building blocks of
protein.
P. Abelson (1957) – added carbon
dioxide, carbon monoxide and
nitrogen to Miller’s original mixture
and was able to produce all amino
acids found in living matter, plus
some proteins.
BUT it should be remembered that
even the simplest forms of life are
far more complex than the proteins
that have been synthesized and man
is still a long way from creating living
molecules in the lab.