Composition
of the Earth
Earth's interior is commonly subdivided on the basis of inferred composition
(from outer to inner): crust, upper mantle,lower mantle, outer core, inner core.
The oceanic crust is approximately 8-12 km thick and composed predominantly
of basalt. Importantly oceanic crust also contains thin layers of limestone,
serpentine and clay. Limestone is made of calcium and carbon dioxide. Serpentine
and clay contain water. Ocean crust has an average density of 3gm/cm3. The average
age of the oceanic crust is 65 million years. The oldest oceanic crust is only
about 250 million years.
The continental crust is found under the continents and includes the continental
shelf to the continental margin. Continental crust is approximately 40-75 km
thick and composed predominantly of granite with smaller amounts of limestone,
basalt, sandstone etc.
An important difference between oceanic crust and continental crust is the density.
Continental crust is less dense (average density 2.7 gm/cm3) than either the
mantle or the oceanic crust, therefore it "floats" on the mantle.
Average age of the continental crust is 3.8 billion years.
The Earth can be divide into layers that are defined by their mechanical behaviour
or physical properties. The outermost layer is termed the lithosphere. It covers
the entire surface of the earth and includes the crust and part of the upper
mantle. It is relatively cool, brittle and approximately 100 km thick. In contrast,
the asthenosphere (sphere without strength), is the high temperature plastic
layer within the mantle. It is thought to be about 700km thick.
How do we tell where the lithosphere stops and the asthenosphere starts? Well,
we can use the seismic waves generated by earthquakes to distinguish the layers.
There are two main types of seismic waves, 1.Compressional waves, also known
as primary or P waves. These travel fastest, at speeds between 1.5 and 8 km/sec
and can travel through both fluid and solid rock in the Earth's crust. P waves
are used to measure velocity (=density) changes in the Earth. 2. Shear waves,
also known as secondary or S waves. These travel more slowly, usually at 60%
to 70% of the speed of P waves. The rigid lithosphere is distinguished from
the more ductile asthenosphere by the drop in velocity of the P and S waves.
(Rarefaction- a decrease in density and pressure in a medium, caused by the
passage of a sound wave).
Plate tectonic theory explains the movement of the continents and the changing
shape of the ocean basins. The idea that the continents were joined, specifically
south America and Africa, was proposed in the 1500’s. Lord Kelvin postulated
that cooling and consequent contraction of the outer crust was the driving force
for mountain building and geological change.
A problem arose with cooling theories when it was realized, with the discovery
of radioactive decay, that the earth was not cooling.
Alfred Wegner put forward a continental drift hypothesis in 1912. He deduced
that all the continents had been joined together in a single land mass, which
began to disintegrate 300 million years ago. It took another 50 years before
his theory was excepted. The problem was, although the theory looked good, no-one
could explain what drove the continents to move. Arthur Holmes, a physicist
finally provided a possible mechanism in 1928. However, unfortunately Alfred
was not around to see the development of his hypothesis into the thoery of plate
tectonics as he froze to death during a field trip to Greenland in 1930.
The Earth releases its internal heat by convecting, or boiling much like a pot
of pudding on the stove. Hot asthenospheric mantle rises to the surface and
spreads laterally, transporting oceans and continents on a slow conveyor belt.
The lithospheric rock can’t stretch so it breaks into pieces, which we
call the plates.
Of course, an all singing all dancing reconstruction of plate tectonics for
the last 180 Ma looks like this...
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Source: Dietmar
Muller - Sydney University.
Solid state convection within the Earth's mantle determines one of the longest
time scales of our planet. The Earth's mantle, the 2900 km thick shell that
extends from the iron core, though solid, is deforming slowly by viscous creep
over long time periods. While gradual in human terms, the convection is impressive,
producing flow velocities of 1-10 cm/year.
This model of mantle convection comes from http://www.earth.monash.edu.au/~greg/Conv.html
The heat that drives the convection comes from two sources:
1. Radioactive decay - involves the loss of particles from the nucleus
of an isotope (the parent) to form an isotope of a new element (the daughter).
The radioactive decay of naturally occurring chemical elements, most notably
uranium, thorium, and potassium, releases energy in the form of heat, which
slowly migrates toward the Earth's surface.
2. Residual heat - gravitational energy left over from the formation
of the Earth 4.6 billion years ago.
How and why the escape of interior heat becomes concentrated in certain regions
to form convection cells is not understood. Until the 1990s, prevailing explanations
about what drives plate tectonics have emphasized seafloor spreading as the
primary mechanism. Cold, denser material convects downward and hotter, lighter
material rises because of gravity; this movement of material is an essential
part of convection.
In addition to the convective forces, some geologists argue that the intrusion
of magma into the spreading ridge provides an additional force (called "ridge
push") to propel and maintain plate movement. Thus, subduction processes
are considered to be secondary, a logical but largely passive consequence of
seafloor spreading. In recent years however, the tide has turned. Most scientists
now favour the notion that forces associated with subduction are more important
than seafloor spreading. The gravity-controlled sinking of a cold, denser oceanic
slab into the subduction zone (called "slab pull"), dragging the rest
of the plate along with it, is now considered to be the driving force of plate
tectonics.
Rock near the surface of the Earth is so cold and at such low pressures that
it cannot flow like mantle rock. So how does the heat get through this rigid
layer, the lithosphere? Two ways 1. Conduction - at the top of the asthenosphere
the hot rock flows along the bottom of the lithosphere transferring the heat
to the cold rocks by conduction. 2. Another way to get heat through the lithosphere
is to melt some of the mantle rock and let it flow through cracks in the lithosphere,
ie volcanism. Most of the heat from the mantle is released primarily through
volcanism associated with the passive upwelling of lava at the oceanic ridges
as well as volcanism associated with subduction.
The continental jigsaw.
The continents can be fitted back together. The best fit uses the 2000m depth
contour
Distribution of mountains and rock sequences
There are many examples of continuous rock types and mountain ranges across
the continents. For example the Appalachian mountains in North America and a
similar range in Europe connect up when continents are reconstructed. Also distinctive
rock types and patterns of folds on Africa and South America match up well for
a reconstructed Pangea.
Distribution of sedimentary rocks reflecting paleoclimate
Sedimentary rocks reflect the climate (and hence latitude) where they form.
For example, ancient coral reefs and coal swamps form in warm, humid, low-latitudes.
Glacial deposits should form at cold, high latitudes. Wegener found evidence
for glaciated regions widely scattered in the Southern Hemisphere, and for coal
deposits. His reassembly of the continents explains the paleoclimatic record
in these sedimentary rocks.
Distribution of fossils
A number of identical fossil organisms are found on widely separated continents.
For example:
Cynognathus -- land reptile (couldn't swim at all), is found in both in South
America and Africa
Mesosaurus -- fresh water reptile (couldn't swim far and not in salt water),
is found in South America and southern Africa
Lystrosaurus - fat land reptile (would sink faster than a lead balloon), is
found in Africa, India, Antarctica
Glossopteris - fern with heavy seeds (couldn't be blown across the ocean) is
found widely distributed across continents.
The distribution of those organisms only makes sense for a reconstructed Pangaea.
Source:USGS
Apparent polar wander
Many
rocks contain records of earth’s magnetic field at the time they were formed.
Such rocks act as "fossil compasses" that indicate the direction and
distance to the magnetic poles. At face value the rock record would indicate
that the magnetic poles have shifted all over the face of the earth. In reality
the poles do move slightly (true polar wander). However most of the effect is
apparent polar wander (APW) due to movement of the continents relative to the
poles. Continents that were once joined together have parallel APW paths.
Sea floor paleomagnetism
The deep sea drilling program identified narrow strips of oceanic crust where
the magnetic properties differed from those of currently forming crust. These
strips are magnetic anomalies that represent periods when the polarization of
the earth’s magnetic field was reversed. The polarity changes on one side
of an oceanic ridge are identical to the sequence of reversals on the other
side. Dating the reversal points on either side of the ridge reveals that rocks
become older with increased distance from the ridge axis ie. new oceanic crust
forms at the ridges and moves away on each side of the ridge.
Image source: USGS
"The magnetic north and south poles may be about to swap places, effectively
turning the world upside down. Since the last great flip in the time of homo
erectus, the poles have been relatively stable.
But, if they had invented a compass, our tool-making ancestors would have noticed
something odd happening to the Earth’s poles 780,000 years ago. Slowly
but surely, the world was turning on its head. The magnetic north pole for our
African ancestors, was bang in the middle of Antarctica. A modern compass would
have shown the northernmost point of the continent as the Cape of Good Hope,
while far in the south lay the Mediterranean and Europe. But over the centuries
the poles began to move. A compass needle on the prehistoric Earth would have
begun to swing erratically from point to point until, after a few thousand years,
it settles on a new direction.
The new North Pole was now at the bottom of he ancient world, in the centre of the ice cap we call the Arctic. During the changeover, which lasts up to 5000 years, the magnetic field is weakened, reducing protection against solar winds. Last week, French and Danish scientists announced they has spotted an anomaly in the field with origins in the spiralling columns of liquid metal that flow in the core of the planet.
Dr Gauthier Hulot, of the Institut de Physique du Globe de Paris, and colleagues at the Danish Space Research Institute compared the recent magnetic field with similar readings made in 1980 by the American Magsat satellite. The results, published in Nature magazine, confirmed that the earth’s magnetic filed is getting weaker. And, at this rate the field will have vanished in 2000 years. Dr Hulot and his team also found a large area of "reversed magnetic flux" - where the magnetic field runs counter clockwise to the rest of the world’s field – below South Africa and the Southern Ocean. Under this area, the columns of moving liquid iron in the core may be rotating a little differently than they are in the rest of he core, weakening the magnetic field.
The team also found an area where the field is stronger on the opposite side of the world, under the Pacific. This asymmetry could be part of the chaotic mechanism that periodically reverses the poles.
So is this the first sign of a reversal? One argument in favour is that the flip is long overdue. The last took place 780,000 years ago. Over the past few million years they have tended to arrive every 250,000 years on average. But there are longer gaps. Between 118 million and 83 million years ago there were no flips. If a flip is due, should we be worried? Dr Hulot says that one side affect could be an increase in cancers as more charges particles flow to Earth."
The Telegraph, London, April 2002
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Image Source: Wheeling Jesuit University
New
oceanic crust is created along divergent plate boundaries. Divergence stretches
and thins the lithosphere, thereby reducing the weight and pressure that the
lithosphere exerts on the underlying asthenosphere. The drop in pressure causes
partial melting in the asthenosphere. This magma rises upward into the gap between
the diverging plates, where it cools and solidifies into basalt. The process
repeats as the plates diverge such that new basalt (oceanic crust) is continually
added onto the trailing edges of both diverging plates, termed spreading ridges.
Spreading ridges extend around the world. They are 1,000 -1,500 km wide 3,000
m high. They are associated with lots of shallow earthquakes The shape of a
spreading ridge and the style and rate of volcanism is controlled by the rate
the plates move apart. Fast spreading ridges are characterized by smoother topography
and absence (although not always) of central rift valley. Slow spreading ridges
are characterized by rougher topography and a pronounced rift valley.
Perhaps the best known of the divergent boundaries is the Mid-Atlantic Ridge.
It is a submerged mountain range, which extends from the Arctic Ocean to beyond
the southern tip of Africa. The rate of spreading along the Mid-Atlantic Ridge
averages about 2.5 cm per year or 25 km in a million years. This rate may seem
slow by human standards, but because this process has been going on for millions
of years, it has resulted in thousands of kilometers of plate movement. Seafloor
spreading over the past 100 to 200 million years has caused the Atlantic Ocean
to grow from a tiny inlet of water between the continents of Europe, Africa,
and the Americas into the vast ocean that exists today.
Insert figure 7Map showing the Mid-Atlantic Ridge splitting Iceland and separating
the North American and Eurasian Plates. The map also shows Reykjavik, the capital
of Iceland, the Thingvellir area, and the locations of some of Iceland's activevolcanoes
(red triangles), including Krafla.
Image Source:USGS
Most
transform plate boundaries occur along the mid-ocean ridges, where they offset
the spreading centers. Spreading centers consist of separate segments which
can be anything from 10-80 km in length and are centered over a magma chamber.
The lithosphere is typically thicker between adjacent segments and maybe offset
at this boundary. The ridge offset can be a few meters to several kms. The offsets
are required because spreading occurs on the surface of a sphere.
The classic example of a transform plate boundary is the San Andreas fault in
California. The North American and Pacific Plates are moving past each other
at this boundary, which is the location of many earthquakes. These earthquakes
are caused by the accumulation and release of strain as the two plates slide
past each other.
The destruction (recycling) of crust takes place along convergent boundaries
where plates are moving toward each other, and sometimes one plate sinks (is
subducted) under another. The location where sinking of a plate occurs is called
a subduction zone.
The type of convergence that takes place between plates depends on the kind
of lithosphere involved. Convergence can occur between an oceanic and a continental
plate, or between two oceanic plates, or between two continental plates.
Oceanic-continental
convergence
The oceanic crust forms a descending slab beneath the continent because the
granite in the continental crust is too light to sink into the mantle. As the
slab, which contains the CO2 and H2O, descends, it releases these gases and
melts and produces volcanoes (eg. Mount St Helens).
Image
Source: USGS
Oceanic-oceanic convergence
As with oceanic-continental convergence, when two oceanic plates converge, one
is usually subducted under the other, and in the process a trench is formed.
The Marianas Trench (paralleling the Mariana Islands), for example, marks where
the fast-moving Pacific Plate hits the slower moving Philippine Plate. The Challenger
Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's
interior (nearly 11,000 m) than Mount Everest, the world's tallest mountain
rises above sea level (about 8,854 m).
Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.
Image Source:USGS
The
Himalayan mountain range dramatically demonstrates one of the most visible and
spectacular consequences of plate tectonics. When two continents meet head-on,
neither is subducted because the continental rocks are relatively light and,
like two colliding icebergs, resist downward motion. Instead, the crust tends
to buckle and be pushed upward or sideways.
The collision of India into Asia 50 million years ago caused the Eurasian Plate
to crumple up and override the Indian Plate. After the collision, the slow continuous
convergence of the two plates over millions of years pushed up the Himalayas
and the Tibetan Plateau to their present heights. Most of this growth occurred
during the past 10 million years. The Himalayas, towering as high as 8,854 m
above sea level, form the highest continental mountains in the world. Because
very little rock in these collisions is forced to great depths little melting
occurs and therefore few volcanoes form.
Image Source:USGS
ExercisesFeatures of Ocean BasinsThis exercise is simply designed to test you understanding of the terminology required to describe and explain Mid-Ocean Ridges, the oceanic crust and several common sea-floor features. Lets
go!
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