Module 7: Home Planet
– the Earth
Activity 1:
The Earth as a Model Planet
Summary
In this Activity, we will investigate
(a) why astronomers study the Earth,
(b) the structure of the Earth, and
(c) volcanism on Earth.
(a) Why astronomers
study the Earth ...
This view of the Earth rising
over the Moon was taken on
the Apollo 8*mission on the
22 December 1968.
Apollo pictures like this one presented for the first time
direct visual imagery of Earth as one celestial body
among others in our Solar System, rather than as an
all-encompassing “world”.
* NASA Press release
Now that space missions
travel further than our
Moon, we have even
more graphic pictures of
our Earth as a planet here photographed with
the Moon, at a distance
of about 6 million km, by
the Galileo*spacecraft on
December 22 1992.
Click here to see a
simulation of the EarthMoon system
* NASA Press release
The Earth is a planet, and as such is studied by
astronomers as well geologists.
Modern astronomy and geology study “comparative
planetology” - comparing planets with each other to
find similarities, which in turn might suggest theories
to explain their formation and evolution.
Astronomers study the Earth because it is the planet
about which we know the most. Earth acts as a model
planet with which to compare the properties of other
planets.
In this Unit you will notice that we mostly quote vital
statistics of other planets in terms of Earth.
(For example, the mass of Mars is easier to conceptualise
if we say that it is approximately 11% that of Earth, rather
than that it has a mass of approximately
642,400,000,000,000,000,000,000 kg!)
If you think that Earth is too familiar a topic to be of
interest, wait till you see it from an astronomer’s point of
view - it may surprise you!
(b) The Structure of the Earth
The Earth is affected by
• geological influences from within
e.g. volcanic outflows
• biological influences on the surface
e.g. production of oxygen by plants
• astronomical influences from outside
e.g. tidal forces between the Earth & Moon
First we will investigate the Earth’s overall internal structure.
The average density of the Earth can be estimated by
measuring its gravitational attraction on satellites
(including our natural satellite, the Moon).
It turns out that the average density of the Earth is
about 5.5 times the density of water.
The density of rocks on the surface of the Earth is only
approximately half this value - therefore at least some of
the interior of the Earth must be very dense (otherwise
the average density would not be so high).
To determine the structure of the Earth, geologists study
the way earthquake waves travel through its interior
(“seismology”).
The large-scale model seismologists have come up with
for the Earth’s internal structure looks basically like this:
crust
mantle
liquid outer core
solid inner core
The inner core’s radius is approximately 20 % of that of
the whole Earth, with a density around 4.6 times that of
the Earth’s crust.
The inner core is hot (temperatures up to nearly 5000 C),
metallic, and rotates very slightly faster than the rest of the
Earth.
solid inner core
The outer core extends out to almost one-third of the
Earth’s radius, with density gradually decreasing until it
drops at its outer surface to approximately 1.6 times
that of the Earth’s crust.
Seismological evidence suggests that the outer core is
hot, liquid and metallic, but its exact composition is not
known for certain.
liquid outer core
The mantle extends out almost to the surface of the Earth.
Made up of solid silicate minerals, its density gradually
decreases until it drops at its outer surface to only
slightly more than that of the Earth’s crust.
mantle
The crust, plus the relatively
rigid outer part of the mantle
(roughly the top 150 km) are
collectively called the
lithosphere.
Below the lithosphere, the mantle is very hot (temperatures
from around 2,200 C down to 1,200 C). It is still solid rock,
as the high pressures acting on it due to its depth are
enough to keep it from becoming liquid even at these high
temperatures.
mantle
In this region - called the
asthenosphere - the mantle is
hot enough to be ‘ductile’,
i.e., to undergo a type of plastic
flow, so that it moves in
convective currents like those in
water heated on a stove - only much, much more slowly!
By comparison, the crust of the Earth is only about 35 km
thick under the continents (and approximately 5 km thick
under the oceans), but together with the atmosphere it
supports all the Earth’s known life forms, including us.
crust
The convective currents in the ductile part of the Earth’s
mantle (the asthenosphere) are driven by the
considerable temperature difference between the hot
core (approaching 5000 C) and the relatively cool crust.
The Earth’s crust is made up
of a number of separate
continental & oceanic plates,
all supported by the mantle.
The convective currents
in the asthenosphere very
slowly drag along regions
of the lithosphere, including
the thin crust. These regions are
called tectonic plates.
The crust is thinnest under the oceans, where it tends
to be made up of heavy, plastic oceanic basalt
(solidified lava).
ocean
crust
mantle
Where the convection currents under the oceans
sink down, they drag down regions of crust,
forming deep chasms called mid-ocean trenches.
Where convection currents in the asthenosphere “well up”
under the ocean, with its relatively thin crust, the pressure
can drop enough for the ductile mantle material to ‘melt’,
turning into molten rock called magma.
ocean
crust
mantle
As the magma pushes upwards, it lifts up the crust to
form mid-ocean ridges, such as the mid-Atlantic ridge.
This upwelling of lava pushes the oceanic plates apart,
causing continental drift - at a few centimetres per year.
When continental plates collide, they produce
folded mountain chains.
The Himalayan
Mountains show
intricate folding
patterns resulting
from the collision
of the Indian & Asian
continental plates.
When a continental plate collides with an oceanic plate,
the heavy plastic oceanic basalt tends to slide under
the light, brittle continental granite.
continental granite
oceanic basalt
mantle
The rising crust crumples up into coastal mountain ranges.
In the process, the basalt is likely to heat up and melt into
magma and heat trapped gases and water, forming outflows
called lava and releases of gases as volcanic activity volcanism - and associated earthquakes.
The Andes mountains in South America are the result
of the Pacific Ocean floor slipping under the continental
plate.
(c) Volcanism on Earth
• Earthquakes
Where two tectonic plates collide, the two sections of the
lithosphere meet each other at what is called a fault. As the
two plates continue to drift, stress builds up at these faults,
threatening to tear them apart temporarily.
When this happens, the surrounding ground shifts suddenly
to relieve the stress, and an earthquake occurs. Many
faults have histories of repeated ruptures over thousands or
millions of years.
As oceanic plates try to ‘slip under’ continental plates,
they can form one sort of fault (called technically a
variety of ‘dip-slip fault’), represented schematically
below:
The arrows indicate the directions of the stresses which
build up across the fault.
Other sorts of faults also occur:
More varieties of ‘dip-slip faults’:
and ‘strike-slip faults’:
The famous San Andreas fault (which has many
neighbouring faults) in California, USA, is a classic example
of a strike-slip fault, brought about as the North American
plate gradually drifts south-east, while the Pacific plate
gradually drifts north-west.
In the spectacular quake on this fault which devastated
parts of San Francisco in 1906, the resulting relative
displacement of one side of the fault to the other was
up to roughly 6 metres (21 feet).
Pacific plate
(moving north-west)
North American plate
(moving south-east)
Here, looking southeast, the San Andreas
fault runs down the
centre of the photo,
under the San Andreas
lake and next to
Interstate 280 in San
Mateo county, Northern
California.
This picture illustrates a
section of the fault where
it temporarily changes
direction, setting up a
region of compression.
Relative motion
between tectonic
plates, detected by GPS
(Global Positioning
System) measurements,
increases the strain on
the fault until it ruptures,
causing series of
earthquakes.
This map shows the major
faults in southern
California
(for full details, see
http://www.scecdc.scec.org/faultmap.html)
The San Andreas fault
is indicated in red:
To see ‘virtual reality’
panoramas of earthquake
faults in the Californian
area, visit the Internet site
http://virtualguidebooks.com/ThematicLists/EarthquakeFaults.html
California’s spectacular fault system, lying as it does under
high density urban areas, rightly gains a great deal of
publicity, but earthquakes are a world-wide phenomenon.
This map
shows the
location of 10
recent (June
2000) large
earthquakes
in the world.
• Volcanoes
Many of Earth’s
volcanoes are
stratovolcanoes (‘layered
volcanoes’), raised up as
releases of magma rise
up through the crust.
A typical stratovolcano is
conical, built up from
layers of solidified lava
flows and ash and rock
fragments from eruptions.
Mt Fuji, Japan, photographed
from Earth orbit (NASA)
Shield volcanoes are
another variety of volcano
which we will meet again
when we study volcanoes
elsewhere in our Solar
System.
Shown in this NASA image
taken from Earth orbit, the
Hawaiian archipelago of
islands have been formed
by this form of vulcanism.
Geologists explore the 1800 - 1801 lava flow at Hualalai Volcano, Hawaii
The chain of Hawaiian islands, with their shield volcanoes,
have formed as their tectonic plate drifts slowly over ‘hot
spots’ in the underlying mantle - the volcanoes form like
“scorch marks left on a sheet of paper that is moved across
a candle flame”*.
Once the motion of the tectonic plates is allowed for, hot
spots in the Earth’s mantle are found to be stationary, or
very nearly so. They are very long-lasting: the Yellowstone
hot spot has been active for at least 15 million years.
Geologists model hot spots as being due to mantle plumes:
*S. Sieh & S. LeVay, “The Earth in Turmoil”, W.H. Freeman & Co, 1998
crust
mantle
Convective
currents in the
asthenosphere
A mantle plume is thought to occur when, once every few million
years or so, a mass of ductile rock rises up under the influence of
the convective currents in the asthenosphere, and breaks free
and expands sufficiently to melt, forming magma as it rises.
When the head of the plume reaches the Earth’s crust,
widespread volcanic eruptions occur, covering large areas of the
Earth’s surface with lava which cools to form flood-basalt
provinces, extending for up to several million square kilometres.
This Landsat image (taken
from Earth orbit) shows part
of the Deccan Traps, a
flood-basalt province which
once covered more than
one million square
kilometers with an average
thickness of 1 km.
This scene shows a part of
the surviving Deccan cover
along the west coast of the
peninsula section of India.
Watch our animation of a mantle plume event, and pay
special attention to the narrow ‘conduit’ (like the string of
a balloon) which trails after the plume.
crust
mantle
Convective
currents in the
asthenosphere
This narrow conduit (only a few kilometres across) may
continue to transport magma to the surface for hundreds of
millions of years, while the Earth’s tectonic plates move
gradually across it.
In the process, a chain of shield volcanoes is likely to be
produced, especially in ocean crust, which is relatively thin
and uniform - see, for example, the Hawaiian islands.
crust
mantle
Convective
currents in the
asthenosphere
In later Activities we will contrast this situation to that on
Venus and Mars, where shield volcanoes occur but there is
little if any evidence of tectonic plate movement.
By studying evidence
such as matches
between nowseparated shore lines,
and tracks left by hotspots on the Earth’s
crust, geologists
conclude that the
present-day
continents on Earth
originally (2 or 3
hundred million years
ago) made up a
‘super-continent’,
called Pangea.
This NASA globe
shows the presentday boundaries
between
some of Earth’s
tectonic plates, with
associated volcano
& earthquake
regions.
Andes mountains
When we study other
terrestrial planets in the
Solar System we will
find more evidence of
volcanoes, but
relatively little evidence
of tectonic plate
movement or
earthquakes.
The differences
between Earth and
other planets in this
respect can teach us
quite a deal about how
planets form and
evolve.
We haven’t discussed the outermost visible layer of the
Earth yet - its inner atmosphere. We’ll study it in the
next Activity.
Further out still (though not visible in optical images
from space) are the upper atmosphere and the van
Allen belts, regions containing charged particles from
the solar wind trapped by the magnetic field of the
Earth.
Image Credits
NASA Photo AS08-14-2392: High-oblique view of Moon’s surface showing earth
rising above horizon
http://images.jsc.nasa.gov/images/pao/AS8/10074963.jpg
NASA Photo NUMBER p-41508c: Image of the Earth and Moon from Galileo
http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_earth_moon.jpg
NASA: View of Australia
http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_australia.jpg
NASA: Volcanoes & Earthquakes
http://www.hq.nasa.gov/office/ese/gallery/Originals/Volcanes&Quakes.jpg
NASA: The Western Himalayas (from the Shuttle Atlantis)
http://kidsat.jpl.nasa.gov/kidsat/exploration/explorations/ESC.00212656/index.html
NASA: Hawaiian Island Archipelago
http://images.jsc.nasa.gov/images/pao/STS26/10062983.jpg
NASA: Deccan Traps, Landsat image
http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_3/geo_images_V-23/PlateV-23.jpeg
Image Credits
San Andreas lake, Credit Joe Dellinger
http://sepwww.stanford.edu/oldsep/joe/fault_images/BayAreaSanAndreasFault.html
10 recent large world-wide earthquakes
http://www.ash.udel.edu/ash/exhibit/Earthquake/faults.html
Faults of Southern California, © John Marquis
http://www.scecdc.scec.org/faultmap.html
Relief map of San Andreas Fault in Southern California, Southern California
Earthquake Center Data Center
http://www.scec.gps.caltech.edu/masterfault1.html
NASA: Mt. Fuji from Earth orbit
http://images.jsc.nasa.gov/images/pao/STS2/10060527.jpg
NASA: Geologists exploring a lava flow on Hualalai Volcano, Hawaii
http://olias.arc.nasa.gov/publications/McGreevy.AFO.WWW/Hualalai/Hual.nvent.jpeg
Pangaea at 200 Ma, an image constructed by Paul Olsen
http://jmchone.web.wesleyan.edu/Pangaea.JPG
Now return to the Module home page, and
read more about the structure of the Earth in
the Textbook Readings.
Hit the Esc key (escape)
to return to the Module 7 Home Page
Press Releases:
NASA Photo AS08-14-2392: High-oblique view of Moon’s
surface showing earth rising above horizon
http://images.jsc.nasa.gov/images/pao/AS8/10074962.htm
File Name: 10074962.jpg Film Type: 70mm
Date Taken: 12/22/68
Description:
High-oblique view of the moon's surface showing the earth rising above the
lunar horizon, looking west-southwest, as photographed from the Apollo 8
spacecraft as it orbited the moon. The center of the picture is located at
about 105 degrees east longitude and 13 degrees south latitude. The lunar
surface probably has less pronounced color than indicated by this print.
Click here to return to the Activity!
Press Releases:
NASA Photo p-41508c: Image of the Earth and Moon from
Galileo
http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_earth_moon.jpg
GALILEO
December 22, 1992
P-41508
Eight days after its encounter with the Earth, the Galileo spacecraft was able
to look back and capture this remarkable view of the Moon in orbit about the
Earth, taken from a distance of about 6.2 million kilometers (3.9 million miles),
on December 16. The picture was constructed from images taken through the
violet, red, and 1.0-micron infrared filters. The Moon is in the foreground,
moving from left to right. The brightly-colored Earth contrasts strongly with
the Moon, which reflects only about one-third as much sunlight as Earth.
Contrast and color have been computer-enhanced for both objects to improve
visibility.
Antarctica is visible through clouds (bottom). The Moon's far side is seen; the
shadowy indentation in the dawn terminator is the south-Pole/Aitken Basin,
one of the largest and oldest lunar impact features, extensively studied from
Galileo during the first Earth flyby in December 1990.
The Galileo project, whose primary mission is the exploration of the Jupiter
system in 1995-97, is managed for NASA's Office of Space Science and
Applications by the Jet Propulsion Laboratory.
Click here to return to the Activity!