Download doc Brandon`s (Precise Final Rev.)

Terrestrial Planets- Review Notes
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The Epic of Gilgamesh= the oldest known recorded story of man
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Archbishop Usher- Took the biblical story and recorded to be factual data and
used it to calculate the year of creation to be 4004 BCE
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Hindu: Calculated the length of one day and one night to be 8.4 x 10 to the 9
years (one cycle of the universe)
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Concordance Model- allows for acceleration of expansion driven by a mystical
dark energy
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Uranite rocks: 1.65 billion years
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Oldest materials found on earth were dated back to 4.5 billion years ago
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Becquerel- Discovered radioactivity- provided science with a clock with which
the “age” of rocks could be measured
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The half life of a particular isotope of carbon is 5730 years
- “age”- in the context of rocks ad minerals is the time since the radio decay clock
was last set
- Oldest mineral= Zircons, founded in Australia and has been dated by the
uranium lead decay sequence to approximately 4.4 x 10 to the 9 years
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Acasta Gneiss Complex- metamorphic rock
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Meteorites= 4.54 x 10 to the 9 years ago- formation of the terrestrial planets
- Tallish Lake Meteorite- fell on the lake in British Columbia in 2000, represents
the condensing of solar nebula
- Burbridge, Fowler, Hoyle= Big Bang
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Big Bang= when time and space emerged
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That moment is 13.5-13.9 x 10 to the 9 years ago (13.7 billion years ago)
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A hot, compressed ball of pure energy exploded our universe: The big bang
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As the explosion expands, it cools- adiabatic expansion
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At 1 second- temperature= 10 to the 12 K
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By 3 or 4 seconds, all of the baryonic matter of the universe has largely
formed into protons
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Neutrons begins to decay with a half life of 12 minutes into electrons and
protons
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Now we have a neutrally charged plasma of protons
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500 seconds- the universe is too cool and too expanded for any further
creation
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The hydrogen fusion reaction stops
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We will have to wait another 100 – 200 million years for a nucleosynthetic
process
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We are left with a hot plasma- electrons are not attached to the nuclei to from
normal atoms because the gas still remains too hot
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Mass has been converted to energy In an exothermic reaction
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As gravity compresses material together the material is heated and when it
gets hot enough and when the densities are high enough, the nucleosynthetic
process restarts- massive protostars form
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It is not believed that this process began about 200 million years if the Big
Bang
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The sun is in the hydrogen burning stage- produces LOTS of heat and light
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Metals= less than 2% of the sun’s mass
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Smallest stars- burn the slowest and live the longest
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Stars comparable to the sun or larger- helium sinks into the core. It can only
be ignited into the helium burning stage if the star is large enough that its
overlying mass compresses this core to temperatures exceeding 10 to the 10 K
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Expected end point of the sun’s evolution= 7x 10 to the 9 years
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Fusoin comes to an end when there is no longer an exothermic process
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Novae and supernovae- when the fusion fire burns down, a star collapses in
upon itself and if it is sufficiently massive, it explodes as a novae or a
supernovae- This is described by a massive star supernovae or a type II
supernovae
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Another mechanism that can create supernoval explosions. Stars that are
smaller than about 1.5M collapses into white dwarf stars after passing
through a red giant carbon-burning phase at the need of their lives. If these
stars have a close binary companion, or come close enough to another star,
these very dense white dwarfs can pull material away from their companion
onto themselves and become even more massive
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Stars can explode as what is sometimes called a carbon oxygen bomb, white
dwarf supernova or a type 1a supernovae
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Type 1a supernovae- their light spectra shows them not to contain the heavy
elements- this is the difference between supernovae type 2
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Supernovae type 2- there are explosions where energy is available for a fist
neutron-nucleus fusion- via r- process- r for rapid
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The energy released sustains the endothermic reactions
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If a star is not large enough to explode, it will collapse into a dwarf, a neutron
star, or a black hole
Our physics is based upon 2 main theories
1. Gravitational theory- Einstein in 1915 – large scale properties of nature
2. Quantum Mechanics- Bohr, Planck- small properties of nature- suggests that
time is incremented with fixed steps between which in time exists- time step is
called the Planck time
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String theory may be what integrates physics
Physics describes interactions between things: strong interactions, weak interactions,
the electromagnetic force and gravity
- Our physics holds a 4 dimensional space-time- A string theory will require a 10
or 11 dimensional space-time
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380,000 years- a rapid expansion had to have taken place- this is called the
inflation
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In inflationary big bang theory remains the best physical description of the
early universe
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Sumerian Animal round- first known astronomical instrument
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En Hendu anna- Moon goddess of Babylon- and is the first poet
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Polynesians- navigated by the stars, sun and moon
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Copernicus- he recovered the heliocentric view- point
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Tycho Brahe- made important measurements- had not embraced the
Copernican explanation
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Kepler- measurements that contributed to the laws of planetary orbital
movement. Book- The new astronomy: commentaries on the motion of mars
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Galileo- supporting evidence of Kepler’s work
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The earth orbits around the sun with orbital diameter of 300 x 10 to the 6 km,
or 2 AU
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Light emitted from a source spreads over an area which is proportional to the
distance from the source, squared
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Hipparchus’ Original definition- curious scale to describe how bright a
source of light appears to be
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The Sun’s apparent magnitude would be 26.8
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Bolometric magnitude measures the total brightness by all wavelengths of
light emitted by stars
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We would have to fill the entire sky with full moons to produce the day light
brightness provided by the sun
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Centauri- second closest of all stars
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Proxima Centauri- closest star
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Betelguese- star in the late red giant phase
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Birghtest stars shine bluest and the dimmest the reddest
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For most materials, compression brings them to solidify
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The higher the pressure, the higher the temperature at which they solidify or
freeze- except for water
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The pressure freezing of iron releases its latent hear of fusion
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J. Marvin Herndon- required additional heat is now being produced in a
small fission reactor deep within the frozen inner core
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The warming and melting iron in the upper regions of the earth, sinks into the
depths, displacing lighter materials towards the surface… this is
differentiation!
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Iron is differentiation
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Iron= 35% of earth’s mass
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Oxygen= 30% of earth’s mass
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As the iron reaches the deep interior pressures begin to compress it into a
solid. A solid inner core forms surrounded by a still liquid outer core, both
largely composed of iron. The pressure freezing of the iron in the inner core
releases heat- the latent heat of fusion. This heat raises the temperature of the
liquid iron outer core and helps to maintain its liquid state
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If a conductor is move through a magnetic field, an electrical current is
generated: Faraday’s law
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Current flows in closed loops and a loop of current generates a magnetic
field: Ampere’s law
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The earth’s spin helps to align the new field with the original field, thus
maintaining the magnetic field of earth
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The convection is powered by the escape of the heat generated in the
differentiation and freezing of the inner core
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The geodynamo is the sources if the earth’s magnetic field
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Magnetic felids can be frozen into a rock as it cools through the curie
temperature
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He moon is les rich in iron that the earth
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Zircons can dissolve into acidic aqueous solutions
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The earth’s surface was already cool and wet by 4.4 x 10 to the 9 years ago
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Continental cratons= the earliest masses of rock
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As heat from the earth flowed towards the earth’s surface a process of
convection began
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The granatic or granite- like crust formed the continents
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The denser basaltic crust formed the ocean basins
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The denser basalt floated deeper on the plasric mantle and the cooling magma
ocean and gave room for waters of the oceans to fill it
Where did the ocean water come from?
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Through out gassing of hydrogen and oxygen
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Cemetery bombardment brought vast amounts of water onto its surface
Why no oceans on the moon or other planets?
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Gravity was not great enough on the moon or Mars to hold the water onto its
surface
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Mercury and Venus are too hot for the water to remain on the surface
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Mars did have surface waters which slowly evaporated into space because its
gravitational force was not sufficient
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The present oceans on earth are saline
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The earth would be hard frozen everywhere if it were not for the greenhouse
effect trapping heat near the surface
The present solar system
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9 planets- all but 2 have associated satellite moons
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A belt of at least 6000 and probably 900 asteroids
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A kuiper belt perhaps 200 million comets
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Most comets that enter the inner solar system from the Kuiper Belt or the Oort
Cloud along highly eccentric elliptical or parabolic orbits
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Most asteroids are gathered in their orbital ring beyond the orbit of mars
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The Chicxulub impact event- 65 million years ago a large asteroid struck
earth, in Mexico. The impact caused a crater 180 km in diameter and splashed
up minerals, which are found in the Canadian Yukon. The atmosphere was so
filled with dust that solar insulation was blocked for years and the earth
became very cold. Dinosaurs starved to extinction. When the dust settled, the
greenhouse shedding of escaping heat provided by this gas caused the earth to
warm to very high temperatures. Whatever life survived the asteroidal winter,
faced a scorching greenhouse summer, Life was challenging and only the
Darwinian’s won
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Comets= why our solar system condensed. There is water and methane ice
which have trapped dusts and gases. They are “dirty snowballs”
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Asteroids were thought to be debris left behind when a planter was unable to
coalesce from planetsmal pieces due to periodic perturbations of Jupiter’s
gravitational field
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Smaller pieces of rocky and metallic debris not organized into semi stable
orbits are called meteorites. They are smaller asteroids.
Iron Meteorites
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Composed of iron- 20% nickel
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It is a siderophile element- meaning it likes to be associated with iron
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In order to be protected from loosing heat, these meteorites must have formed
under a thick layer of insulation
Stony Meteorites
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They may represent an insulating mantle which enclosed an iron core of a
fragmented planetesmal
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They are sub- classes as chondrites and achondrites
Chrondrites
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Characterized by the glassy chrondrules like very time beads
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Glass forms when solids freeze too quickly to organize a crystal structure.
These chondurules were melted and they solidified before they could
crystallize
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Another explanation is that they formed from rapid cooling or rains of melted
minerals within the dust cloud
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The rarer carbonaceous chondrites can contain amino acids, the protein
building blocks of the terrestrial life
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2000 degree magma ocean- would destroy earth
Achondrites
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Look like igneous rocks, those which have cooled from a melted state on earth
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Heliocentric= the theory that our universe is sun centered
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This was not accepted until Copernicus and Galileo promoted the heliocentric
viewpoint
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The mass of the sun is so enormous compared to other masses- It’s orbit is not
affected by the gravitational forces
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Sun= the centre of mass
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The centre mass of the solar system, though is so close to the centre of mass
of the sun itself that the heliocentric viewpoint serves well in revealing the
dynamics of the solar system most simply
Kepler’s Laws- Based on observation, not fundamental theory
1. The orbit of a planet about the sun is an ellipse with the sun at one focus
2. A line joining a planet and the sun sweeps out equal areas in equal intervals of
time- meaning that planets travel faster in their elliptical orbits when they are near
the sun, then when they are far away
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January 4th- earth is closest to the sun- aphelion 1.52 x 10 to the 8th
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July 5th- earth is furthest from the sun- Perihelion 1.47 x 10 to the 8th
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In the Northern hemisphere the shortest day of the year in our winter solistice,
Dec 21 or 22
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The earliest sunrise precedes the summer solstice by about 1 week and the latest
sunset follows by about 1 week
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The elliptical orbit of the earth has effected the local solar noon, that time which
would be measured as noon by a sundial
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Earth’s obliquity, the inclination of its rotation axis from the normal to the orbital
plane, that most contributes to the varying length of daylight and to the annual
cycle of seasons
3. The square of a planet’s sidereal period is proportional to the cube of the length
of its orbit’s semi major axis.
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Kepler’s laws were studies by Newton, who recognized that a force which he
called gravity was fundamentally responsible for the ordered Keplerian motion of
the planets
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In his philosophae naturalis principia mathematica, he showed how forces of
gravity contained planets into precisely Keplerian orbits.
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For a circular orbit, it is easy to obtain Kepler’s 3rd law: we need only compare
2 forces: the one provided by the gravitational attraction of the sun on a planet,
and the other the tension required to maintain the planet along a circular path with
the sun at the centre
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The tension is called the centripetal force
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The sun provides this centripetal force through its gravity
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Andrea Ghez- she determined that there is an “invisible” supermassive
gravitational centre
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If we were to look at the orbital velocities of stars within galaxies we
could determine the mass distribution within them
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Astronomers have called this unseen mass “dark matter”
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Once each lunar month, the moon comes to full moon phase. This happens
during each month at the moment when the line from the sun-to-earth- tomoon is most nearly a straight line
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If the alignment of the earth, moon and sun is just right is casts a shadow
over the face of the moon. This is a lunar eclipse.
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New Moon- occurs at the moment when the line from the sun to moon to earth is
nearly a straight line. If the alignment is just right one may observe a solar eclipse
at New moon
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Lunar month- the period between phases of the moon
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The lunar month has a period of 30 days
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Siderial Month-
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The moons orbit is inclined to earth’s equator by 18.3% and processes about the
earth with a period of 18.6 years and so, month by month it doesn’t return to
exactly the same place in the night sky against fixed stars.
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Tropical month- the actual revolution period is that period between fixed in
declinations, which are the angle, measured from the earth’s average rotation axis.
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Can we use this to determine the earth’s mass? Yes, but remember the following:
while the moon orbits the earth, the earth also co0orbits the moon. The best fixed
point for observing these orbits is the centre of mass of the earth-moon system
which is displaced towards the moon
Tides
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Differential gravitational forces on the earth caused by the sun and mood raises
tides.
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Lunar tides
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Solar tides
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Beating of the tidal periods- when the sun and moon are aligned, tidal amplitudes
are highest as solar and lunar tides conspire to add up in effect
Rotational dynamics of the planets and their satellites
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Planets rotate about their own axes
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The earth wobbles as it rotates
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Part of this wobbling is caused by the seasonal movement of mass over the earth,
the 12 month seasonal wobble
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The other part of the wobbling is a so called free wobble, called the chandler
wobble, which has a 14 month period
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The moon liberates as it orbits the earth
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Venus is inclined by almost 180 degrees and Venus rotates backwards!!!! (So
cool)
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Uranus and Pluto rotate with their axes lying almost in their orbital plane
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We define the North direction according to the what is called the “right hand
rule”- Venus’s North points is the almost opposite direction of earth’s
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A rotating body tends to stay in rotation unless acted upon by an external
torque
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The quality and quantity, which determines the continuing tendency to
rotate, is called angular momentum and usually designated by a vector.
The direction of the vector is established by a convention called the right
hand rule
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When the mass is concentrated towards the centre of the planet, its moment of
intertia is lower than when mass is distributed at distance from the centre. A
moment of intertia determines that the planet’s mass is relatively concentrated
towards the centre. The lower the moment of intertia, the more mass is
concentrated at depth (only thing that makes sense! yaya)
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Still the moment of intertia can be determined for some planters in response to
astronomical torques and forces
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When a torque is applied to the axle (on the rotational axis) a reactive torque 90
degrees to the applied torque results in maintaining torque balance
Torque on earth due to moon
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The moon produces a torque on the axis of rotation and in reaction to this torque;
the earth precesses with a precessional rotation in the same sense as the spin. This
precession is cyclical with a 26000-year period and an average half-life angle of
about 23 degrees.
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Physical analysis of the gravitational force induced torques on the earth due to
moon and our observation of the period of the rate of precession allowed us, to
determine the moment of interia
The other terrestrial planets
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Mercury has no moon- it rotates very slowly
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Mars rotates quickly
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The moon rotates slowly under the enormous gravitational torque applied by the
earth. Its liberation best determines the axial moment of intertia
Information
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For the earth, the moon and mars, the moment of inertia are less than those of a
sphere of uniform density having equal size and mass
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This tells us that density increases towards their centers
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This agrees with our expectations for Earth: the earth must contain about 35% of
its mass in iron and that the amount of iron is not seen in the crust or in mantle
materials which have been brought to the surface. The iron must be at depth
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Moon’s relative moment of intertia is somewhat less suggests that it contains
much less iron and accords with its smaller size which would not allow as much
compression to high density of interior materials at depth. Mar’s moment in
intertia supports an iron core to about ½ its radius
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The high density of Mercury suggests that it contains a very large core
Planetary Differentiation
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Geophysics refers to the physics of the earth and geochemistry to the chemistry of
the earth
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Science of chemistry is sometimes better called Cosmo chemistry
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Selenophysics to describe the physics of the moon
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Buoyant materials rise and dense materials sink… The temperature cause
buoyancy of mantle materials and fluid core is largely due to he slow freezing of
the iron inner core and the release of the latent heat of fusion
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Earth is now well differentiated
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The thin crust extends from the surface to an average depth of 33km
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Below the crust, the mantle which comprises 85% of the volume of earth
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Moon- less iron than earth and a smaller iron core
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Venus- Differentiated as earth, with an iron core which is either entirely frozen or
still completely liquid
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Mars- is thought to have an iron core comprising perhaps of 30%
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Mercury- has a large mass
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Earth- has the highest density mass for its volume.
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Its high density accounted for by the compression of overlying materials
squeezing the deep iron core to high density
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Jupiter probably has a small core of rocky silicate composition materials and a
deeper iron core just like earth and perhaps of about the same size as earth
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Quartz is a relatively low density mineral- it is the most common type of sand is
the chemical composition of ordinary glass
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Granites rocks float high above the oceans, forming the continents
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Isostasy largely explains differences of evaluation of the earth’s surface
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J.H Pratt suggests that the reason for high elevation so that the light materials
“float” higher than do dense materials and that the rock of areas of high elevation
are of low density
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G.B Airy- proposed another hypothesis: the high standing regions are
compensated by deeper roots but their densities are similar to those of low
standards
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The Laurentian ice sheet covered almost all of Canada
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It had depressed the earth’s surface even deeper below average sea-level
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If the fluid upon which these depressed continental areas were floating could have
moved very quickly, it would have infiltrated under the lowered load quickly. The
fluid has very high viscosity
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Mantle fluid flows extremely slowly
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Glass at normal temperature in an amorphous solid- it behaves like a fluid
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The fluid mantle is stickier than glass
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Almost all regions of the mantle are harder and stronger than any known materials
on the surface
Mantle Convection
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Continents move across the surface of the earth
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Sir Francis Bacon- recognized that the coastline of North America could fir
against the coastlines of Europe and Africa
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Von Humboldt- the same symmetry existed between the coastline of America
and Africa
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It seemed as though the continents, separated by the Atlantic Ocean, had once
been joined. The hypothesis was clearly stated by A Snider
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F. Taylor H.D Baker and Alfred Wegener described the theories of
“Continental Drift”
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Expanding Earth Hypothesis- Earth’s volume had increased over geological
time and that the earlier surface was no longer large enough in area to cover the
surface of the greater volume
Earth’s Mantle is Fluid
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Mantle is fluid and it can be transport heat from depth by a process called
convection
The Adiabatic Gardient
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If we compress a cube very quickly, it doesn’t have time to thermal equilibrium;
if we insulate it from its environment so that heat energy can neither flow in nor
out of it, it cannot come to equilibrium. This is called adiabatic compression; what
is preserved in adiabatic compression is the entropy of the cube material
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Heat capacity at constant pressure determines what temperature increase would
be caused by an inflow of heat
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The material of the earth’s mantle behaves accordingly
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If we know temperature at the top of the mantle and we know the thermodynamic
constants appropriate to mantle material, we can determine the exponential
adiabatic temperature profile to the base of the mantle
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The temperature at the centre of the earth’s core is at least 5100K
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Because the mantle is fluid-like, the mantle is in constant convection and this
brings the mantle temperature profile towards the adiabatic temperature profile
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Heat can be carried through fluids by convection but can only be carried
through solids by conduction
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The outer elastic shell of the earth, is lithosphere, acts as a heat-conductive layer
with a very poor heat conductivity
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If a volume of a mantle material is moved along the adiabatic temperature profile,
its temperature decreases as it expands to remain in exact temperature equilibrium
with the lower local pressure
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When a volume of a material is moved upward, it cold following the adiabatic
gradient- it follows an adiabat. But in cooling it finds itself warmer than other
material
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It is relatively buoyant
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That is, if we even infinitesimally move a volume of material upwards through a
temperature gradient which is steeper than the adiabatic, it wants to move even
further upwards under buoyancy
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This is the process of convection
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The whole material of the mantle is set into continuing motion as heat is carried
from depth towards the surface by the convecting fluid
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The motion would eventually stop if the actual temperature profile cooled to settle
down to the adiabatic. That is, when there is no more excess hear in the deep
interior maintaing the steep temperature gradient, convection will cease
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Much of the heat emanating from the core to heat the base of the mantle is being
released by the freeing of iron onto the inner core
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The rest is the residual heat from earlier decay of radionuclei and from the
gravitational potential energy released as heat during the density differentiation of
the earth
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When the whole core is frozen, convection of the mantle will stop
The Rayleigh Number
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R= buoyant forces/ (divided by) viscious forces
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Leon Knopoff determined that for a spherical shell bounded by a rigid surface
contact with a spherical hot core and above by a rigid surface, with the inner
radius being just ½ the outer, convection is maintained
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This model approximates the geometry of Earths mantle
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The evidence for this is the very rapid motion of tectonic plates
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The buoyancy forces are high when the temperature gradient is high, that is,
if the deep interior of the earth is at temperatures much higher than could be
accounted for by adiabatic compression
Seismic and tectonic boundaries
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Concerned with earthquakes and tectonics
The Seismic boundaries
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Crust- shows relatively how velocity for seismic sound waves
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Mantle- most of the volume of the earth comprises the silicate, rocky mantle
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Outer core
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Inner core
The tectonic boundaries
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Lithosphere- this is he outer elastic shell of the earth
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Asthenosphere- this layer is the softest or most easily deformedable and flowing
region of the upper mantle
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Mesosphere
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Core mantle boundary zone
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The rigid lithosphere is broken up into 11 major and several minor plates. They
are floating on the athensosphere, driven into circulation by the convection heat
engine
Earthquakes, volcanoes and tectonics
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Ring of fire- characterizes boundaries between colliding and laterally sliding
tectonic plates
Plate Margine
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Convergent margins- oceanic plates ertr converging towards a continental plate
and subducting or moving downwards under it
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Divergent margins, spreading ridges
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Transformative margins- on a sphere, plates cannot simply spread apart on one
boundary and come together on another. They have edges that somewhere must
slip against each other
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San Andreas fault and Anatollian fault are transform faults that are slipping
laterally
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Hot spots= are isolated volcanoes which are neither associated with ridges nor
rims
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All of these motions suggest that the underlying mantle is convecting
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There is a now well understood variation of density within the mantle due to
variations in the geochemistry and mineralogy of the mantle with depth
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The power driving the convective heat engine of Earth dervies largely from the
heat fusion given up at the inner core outer core boundary as the inner core
freezes. In carrying this heat towards the surface, the mantle is forced into
convection circulation. This circulation pushes the floating lithosphere plates
across the earth’s surface
Earthquakes, volcanoes and tectonics on other planters and moons
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“Moonquakes” were observed due to shallow, near surface fractures
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The Viking Landers of 1976- landed on Mars. Marsquake- it is thought to have
been caused by a fracture due to long standing stresses in the crust of Mars, or a
landslide
Heat Flow
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The elastic lithosphere rides oer the mobile, fluid asthenosphere
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The lithosphere is tick. Heat flow through the cores of continents is lessened by
the thickness of this insulating layer
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Through the thinner lithosphere (underlying the ocean basins) heat flows more
easily to the surface
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The rate of heat flow can be used in determination of lithosphere thickness
Loads of the lithosphere
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When a line load (a long line of mass) lies on the elastic lithosphere, the load
depresses the lithosphere directly under the line load but allows it to bulge
upwards at some distance lateral from the line load
Paleomagnetic evidence for plate motions
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The earth’s magnetic field looks like one produced by an enormously strong
permanent magnet buried at great depth
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The earth’s geomagnetic field is generated by a magneto-hydrodynamic dynamo
called the geodynamic
Magnetization of Rocks
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When a hot rock cools through the curie temperature of its comprising minerals,
whatever natural magnetic felid exists at that time and place is frozen into the
rocks
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They take on the magnetic field direction, which holds at their time of freezing.
When they cool to crystallize minerals they establish their age
Polar Wander
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We can determine how the magnetic pole has apparently wandered over time
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Polar wander offered the first geophysical evidence for the relative separation of
Europe and North America and the spreading open of the Atlantic ocean
Magnetic stripping of the ocean basins
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When the rocks issue from the interior of rhea earth and solidify along the ocean
ridge axes, they quickly cool and take on (freeze in) the momentary direction and
intensity of the earth’s geomagnetic field
Motion of plates over “plumes”: Hot spot vocanism
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At several places on the earth, the lithosphere seems to be penetrated at hotspots,
where magma issues to the surface from somewhere deep within the mantle.
Where a plume penetrates the lithosphere, its magma forms great shield volcanoes
Age and depth of the ocean floor
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By dating the rocks of the deep ocean floor using radiodecay clocks, we find that
the ocean basins are, as expected, oldest when farthest from their generating
ridges axis
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Deepest= the oldest
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As the oceanic lithosphere ages and cools, it freezes even thicker; the frozen
lithosphere is denser than the magma and so it “floats” deeper in accord with
isostatic adjustment
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The ocean basins subduct into the mantle at convergence
Other measures of plate motions
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Radio interfermontry- the qusars, known in the universe with radio telescopes
which are sitting on different tectonic plates, the baseline distance between the
telescope could be measured
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GPS- determine their positing with accuracy- and also to synchronize clocks on
the surface
Continents and ocean basins
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The young ocean plates collide with the continents as the continents float out
across the surface. Where these plates collide, the oceanic plate is driven down- or
is pulled down under its own weight- into the mantle. The ocean basins are being
continuously recycled through the earth’s interior
Earthquakes and Sesimotectonics
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They follow a trend which makes an angle of about 45 degrees with the surface
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The zones of deep-trending earthquakes were the first recognized in the 1930’s by
Huho Benioff
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Plate tectonics, the science of the deformation of motion of earth’s surface plates,
explains the phenomenon: subduction
Earthquakes on subduction zones
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Earthquakes in Benioff zones occur within a subducting lithosphereic plate
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The Nazca Plate is subducting under the South American Plate
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- The Tonga Trench defines subduction zone where an ocean plate subducts
beneath another oceanic plate
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Largest event known to have ever occurred on earth= the Cascadia Subduction
zone
Volcanism on subduction zones
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As frequent and very large earthquakes characterize the subduction zones, they
are also characterized by volcanism. In a down dip direction along the subducting
plate, light continental granitic materials which have been slowly sedimented over
eons on the ocean floor and which are carried down with the subducting plate to
depth, eventually melt and break their way back to the surface causing volcanoes
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Tonga Island is formed of such volcanism
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Wherever you find volcanoes issuing andesitic or rhyolytic rock wherever you
find earthquakes aligning along Benioff zones, we have convergent margins
Earthquakes and continent- continent collisions
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Earthquakes occur at zones of continent-continent convergence
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Large earthquakes occur, typically in the Indian subcontinent, as a consequence of
this collision; recall the Gujarat earthquake of January 26, 2001
Earthquakes on Transform Margins
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While it might not be clear, which plates move in relatively straight-line
directions over the surface of a spherical earth, it is necessary that a style of
faulting normal to the spreading axis of the ridges occur. These are the transform
faults. Such faults often arise along long convergence margins
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A famous and highly active transform margin is known as the San Andreas Fault
Seismology and the Internal structure of earth and planets
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Earthquakes are caused by a fracturing or faulting of the elastic rock of the
lithosphere
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Focus- the point at which an earthquake starts to fracture or at which
mineralogical phase begins to change is called the focus
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Epicenter- the place on the surface directly above the focus is called the
earthquake’s epicenter
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Fault plane- the surface over which the fracture occurs
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Seismic waves- by measuring the field of seismic waves that issue from an
earthquake. One can determine the plane of faulting, the amount and direction of
slip along that fault plane and the amount of energy released in the event and so
understood the mechanism and tectonic stresses involved in precipitating an
earthquake
Seismic waves
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Much of our knowledge about earth’s deep interior has been obtained by studying
the wave fields generated by earthquakes
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P- waves- those waves which travel most rapidly from an earthquake source to a
distant seismograph receiver are called p=waves
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P-waves are actually sound waves which travel through the body of the earth
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The modulus of rigidity is determined by a material’s resistance to shearing
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S-waves- a slower traveling body wave type is also generated by earthquakes. It
follows the P-wave along a path through the interior of the earth. It is a shear
wave
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S-waves do not travel through liquid
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The body waves that travel from an earthquake don’t travel very deeply into the
earth
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Surface waves- earthquakes also produce disturbances on the surface which
causes waves to travel out over the surface of the earth
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Rayleigh waves- travel out over the surface like ripples travel across the surface
of the water. But because elastic forces rather than gravity forces dominate in the
“springing” of the Rayleigh wave, its ripple motion is a little different from that of
water ripples which are sometimes called gravity waves
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What forces a water gravity wave to oscillate is its disequilibrium with the flat
surface of equilibrium. The crests of the wave are being pulled down to the
surface while the troughs are being pushed up by slight differential pressure
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For Rayleigh waves traveling across solid surfaces, gravity forces do come into
lay but it is the elastic forces which dominate
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The wave represents an elastic distortion of the surface which is restored by the
elastic strengths of the material
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Gravity waves on water- the surface in a water wave are in the forward direction
if travel and in the reverse direction in troughs
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Rayleugh waves- the surface of the crest is moving backwards, and the waves
moves under it and the surface of the troughs moves forward (they are opposite)
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Love waves- produce often horizontal motions of the surface
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Love waves are really s-waves- which have become trapped or guided by the
layered structure of the earth near the surface
Magnitude scales for earthquakes
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The size relates directly to the amplitude of the wave
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The Richter scale- for every increase in M(s) by one point, the amplitude must
increase by 10x
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As earthquakes become larger, though they spread their energy over even broader
range of frequencies. The largest earthquakes can cause the earth to ring like a
bell with a gravest period of 54 minutes
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The source energy actually increases by 63x for ach Richter magnitude step
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1989- Chicoutimi earthquake- the largest to have occurred in Quebec in the past
25 years- in Montreal
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Both P-waves and S-waves show velocities which are inversely proportional to
the square-root of local material density
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Rigidity or shear modulus: rocks are solid and clearly resistant to shearing
stresses. Rigidity is properly a measure of hardness. Liquids have no resistance to
shearing tresses, which are sufficiently slowly imposed. The extremely high
viscosity of the mantle brings the mantle to respond with the nature of a solid to
seismic shear waves, which pass through it.
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For diamond, the hardest of all minerals 100 GPA
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Base of the lithosphere the atmosphere, the rigidity seems to decrease
marginally. Even here, in the most “fluid” region of the mantle, the rigidity is
almost as great as that of diamond
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The core mantle boundary into the iron outer core the rigidity drops to 0~
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The outer core is such a low viscosity fluid that even short period seismic shear
waves see it to posses no rigidity at all. It is because shear waves cannot travel
through a fluid that we know that outer core is fluid
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The inner core is solid. We know that seismic shear waves, those that impose only
shearing stresses in traveling through a sold material, do have velocity in the inner
core. That proves its solidity
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Incompressibility is a measure of a material resistance to changing its volume
under pressure
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It has just been discovered that the soft metal, osmium, has an even higher
incompressibility than diamond
Other Bodies in the solar system and beyond
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Seismology is the best of all tools for discovering the condition of planetary
interiors- and even the interior properties of the sun and stars
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ON July 29 1969, The Apollo 11 mission placed magnetometers and
seismographs on the moon
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These were the first seismogtaphs installed on a body beyond earth. It gave us a
fairly good picture of the Moon’s interior mechanical properties
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They shut down in 1978
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NASA”s Viking I and II missions to Mars handed seismographs systems on its
surface in 1976
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Viking’s II’s only recorded one possible marsquake
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The Soviet Union’s Venera missions of Venus managed to land 2 seismogrpahs
on the surface. No activity was recorded
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Explosions of the sun set off trans of “seismic” waves through the sun.
Helioseismology is a well developed extension of terrestrial seismology
Mineralogy and geological history of the terrestrial planets
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The chemical elements can combine into molecules and minerals. We normally
regard molecules as having stoichiometrically precise relative composition of
element
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Many minerals do not follow a precise stoichiometry. Rather, minerals may be
regarded as structural edifices into which certain atoms can be easily secured
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The cation is an atom or group of atoms, which tends to want to yield one or more
electrons and so becomes positively charged. The anion is an atom ire group of
atoms, which tends to take on one or more electrons and thus becoming
negatively charged. The negatively charged anions are thus attracted to and
complement cations
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Minerals typically have a range of compositions, which is allowed for by such
substitutions. Halite is one of the simplest minerals
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The minerals which compose rocks in the earth’s crust and mantle are more
complex
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They are typically silicates, oxides, and sulfide complemented with various
metallic cations
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The silicates and oxides form lithophile minerals which means they like to be
rock
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Certain metals form siderophile alloys
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Most important of minerals in the solar system is olivine
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Sometimes we note the generic range as (Mg, Fe) 2SiO4 allowing for any
possible mixed composition of magnesium and iron
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The magnesium rich form of olivine, forsterite, freezes at very much higher
temperature than does iron rich form foyalite
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The history of the cooling of an olivine rich magma is recorded in its solid
mineral phase
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Xenoliths- brought to the surface along with erupting magmas, of olivine from
Hawaiian volcano
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By studying details of rocks derived from freezing of granitic magmas,
mineralogists can infer much about the conditions which help during the
solidification
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Quartz- is a very important simply composed mineral, which is a common
constituent of rocks. It is in pure form a colorless crystal, although it can take on a
color
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Amethysts are quartz and often grading colorless to deep purple
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The color is consequence of traces of metallic elements in the quartz
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Quart is a characteristic mineral in continental crystal rocks like granite
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Granite is an igneous, meaning that it formed in cooling and freezing of magma
rock type which comprises a mix of mineral such as quartz, feldpar …
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It is typically relatively large graned because it cooled slowly under an insulating
layer of rock into what is called a pluton
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Cooling rapidly on the surface of the earth as an extruded lava, produces a
different rock type such as andesite, or rhyolite
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Basalts form of magmas having a composition like that of the earth’s mantle,
below its continental crust
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The ocean basins are basaltic
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Basaltic rocks are denser than granatic rocks such as andesite. Their high densities
account for their low-lying in ocean basins although they can flow (Hawaiian and
Iceland)
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They have little or no quartz mineralization as crystals even though their overall
element composition is not importantly different from granites.
Condensation of Minerals in the primordial solar nebula
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As the solar system was condensing into a disk with a proto- sun as the
gravitational centre, heat from the newly igniting proto-sun warmed the disk
which would spawn the terrestrial and other planets
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Near the proto-sun in the interior regions of the nebula, the temperature of the
cloud of chemical elements and dust increased
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In the interior, only high temperatures refractory minerals (pervoskite) condensed
into mineral at temperature of probably more than 1400L
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The inner regions of the nebula became, therefore relatively riche in such
minerals as other minerals could not find their chemical components were swept
further out into the solar nebula by the thermal radiation and proto- solar wind
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Lighter minerals (carbon dioxide, water) are most common is the distinct regions
of the now condensed solar system
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The terrestrial planets are largely composed of the regractory minerals, metals and
high temperature silicates
Composition of the terrestrial planets
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Geologists have studied the mineralogy of the earth. In 1969 man landed on the
mood and the Apollo 11 returned rocks from the Moon, allowing the study for the
first time of another body in the solar system
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Moon rocks were like earth rocks but not identical
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Example: moon basalts were less rich in SiO2 than basalts on earth
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The higher iron content in its crustal rocks, is consistent with a model of a moon
splashed up in a giant collision of the proto- earth with a Mars sized body
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The moon, being small, quickly froze with little internal heart generation except
from radioactive decay, did not manage to express an earth like mantle convection
in bringing about further differentiation
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Spectral surveying of Mercury by the Marnier 10 probe reveals that its surface is
apparently less iron rich than crustal rocks on earth. This suggests a more
complete and early differentiation of the planet.
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It possesses a magnetic field indicating that its iron core is in circulation
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This further suggests that an inner core might still be freezing out within Mercury,
releasing a latent heat of fusion which drives the fluid circulations generating the
magnetic field
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As that heat is transported through the Mercurian mantle to the surface, it is
possible that small scale convection becomes established allowing for further
differentiation of its mantle
The crater- density clock
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Rock samples from the moon obtained by Apollo 11- through radiosotope dating
to be between 3.7 and 3.9 billion years old
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Rocks no younger then 3.3 billion years old were returned
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The later expeditions were directed towards the highlands
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Granatic rocks returned by Apollo 16 from a highland region were found to be
much older at 4.44 billion years
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This is the probable age of solidification of the moon’s crust
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Subsequent flowing of basalts consequent to great impacts by large asteroids
filled the shadow maria basins with younger and more dense rock
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The highlands are seen to be very heavily cratered while the maria basins are
much less so
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A small crater which is seen to be within another larger crater had to have been
caused by an impact subsequent to that which caused the larger one
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By ordering creaters and the nby calibrating their “age” with the radioisotope
determinations of actual age for rocks from tem, we can calibrate a “crater density
clock”
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We se that the highlands are very old while the surface of Mare Imbrium the basin
within the largest obvious crater on the moon, is only about 1.8 billion years old
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This might be regarded as the youngest area on the moon’s surface
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The moon has been geologically inactive for the past 1.8 billion years
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What distinguishes the planets surfaces is the actual age since regions of their
surfaces solidified or were resurfaced by other processes such as erosion
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We discover that Mercury’s southern polar highland region is very old, more than
4 billion years while its Mare Caloris is well less than 3 billion years
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The magella orbiter mapped the surface of Venus using a synthetic aperture radar
(SAR) which could penetrate the planet’s thick clod cover
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Surface is actually very young at about 400 million to 700 million years
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A lack of relatively small craters on the surface
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This is due to the fact that Venus’s very dense atmosphere 90 times denser than
earth’s, protects its surface from being hit by small impactors
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The young age of Venus’s surface proves that it has been completely resurfaced
by flooding basalts in relative recent geological time
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Ocean Basin average age= 100 million years
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The ocean basins cover 71% of earth’s surface
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The cratering clock does not show the degree of cratering that a 2 billion year old
surface would show on the moon, for example. Why? The continents on earth are
under continuing attack by erosional process of water, glaciation, plant life and
wind
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The continental surfaces are being continually carved back by these processes and
as a consequence, the record of cratering, except for the very largest and deepest
craters is lost
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The line crater density- crater diameter for earth shows a sharp decrease for
craters smaller than about 8km in diameter
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This is due to the “shielding effect”
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Cratering line for earth is that the line does not become parallel to the age lines
even for craters as large as 100km in diameter
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This is largely effect of erosion of the continental craters, as on Earth, only
continental surfaces older than 200 million years
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Earth is the most geologically active of the terrestrial planets; geologically
activity erases the cratering record
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There is evidence that there is still erosion caused b water and wind on Mars
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The linear platinum age is about 2 billion years
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Mars gravity is so much less than earth’s that warm volatile water cannot be
gravitationally bound against slow evaporation in space
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It may well be that until 4 billion years ago, Mars had a substantial atmosphere,
until about 2 billion years ago, water covering much of the surface of the northern
hemisphere
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IN face, evidence of lakes and oceans has recently been found in high resolution
images obtained from Mars Surveyor satellite
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Histogram of the surfeit the terrestrial planets was placed
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Mercury and Moon have the oldest surfaces and earth and Venus the youngest
with Mars somewhere in between
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The highlands of all the terrestrial planets might be regarded as the long term high
standing granites like crystal masses and the lowlands, the denser basaltic masses