by Kari Hetcher and Scott Hughes
Reading: Chapter 9 in The New Solar System, 4th edition,
review chapters 12 and 13
This is a pretty extensive module because it represents a review of material
covered in an introductory geology course. Use this module to refresh your
knowledge of how the Earth works. A lot of information was left out here,
but most of it will come up again in other modules.
Module 4 Assignments:
First, answer the study questions identified in
Tasks 1 through 4 below.
Then, for Task 5, write a short paragraph that outlines
the basic concepts of global warming.
NOTE: While working through this module, you can view much
larger, better quality images if you click on the smaller images. If you
are using a dial-up browser, some may be slow to load, but worth the wait.
A Planetary Perspective
Imagine an interstellar space traveler finding her way to our
solar system. She approaches a medium sized, mid-life star, and counts eight
planets (or is that nine?) rotating around it, plus a large number of moons
rotating around the larger planets. The planetary bodies are of varying colors
and sizes, but one strikes her as different in some way. It is a rather small
rocky planet, the third one out from that star. It looks blue, green, and
white from space, is continually changing, and its dark side twinkles with
thousands of faint pinpoints of light. How unique and beautiful that planet
is! The traveler has found the original home of Earthlings.
This incredible scenario of a space traveler conjures mental
images of our own fascination with far-away mystical places. Indeed, the story
may be fiction, yet we envision that it could be true someday -- to have our
home planet rediscovered as a land of beauty, a unique dynamic world where
internal and surficial processes are diverse, like so many other planetary
bodies in the universe!
View two images of our home, planet Earth: (NOTE: One image
is a photograph from space provided by NASA; the other one is a computer image
modeled from global topography provided by NOAA.)
What geologic features can you immediately recognize from
For updated information on NASA programs and activities, visit
NASA News and Features.
Internal and External (Surficial) Planetary Processes
The two pictures below illustrate internal AND external Earth
processes. On the left is South Sister volcano in Oregon, which may erupt
again someday in the not-too-distant future. The right picture is Mount St.
Helens volcano in Washington viewed south into the well-known amphitheater
crater created during an explosive eruption in 1980. Both volcanoes
are composite cones in the Cascade Range, a roughly north-south string of
volcanoes parallel to the west coast of North America. These volcanoes
belong to a continental magmatic arc, which is the surface manifestation of
a subduction zone caused by converging oceanic and continental lithospheric
A lot can be learned about planetary geology from images such
as these. Learn to recognize processes, and relate them to mechanisms that
are responsible. What is different about the evolutionary stages of
these two volcanoes? What can be inferred by their morphologic features?
What can be inferred about their relative surface ages? Even the clouds
tell you something about how these mountains evolve through time.
- Examples of Internal Processes: subduction, magma genesis,
magma ascent, heat flow, dome growth, explosive volcanism.
- Examples of Surficial Processes: eolian deposition(wind-carried
ash and other tephra deposits), glaciation, stream erosion, chemical and
Much of what we know about the Earth as part of a planetary
system has been discovered only in the past few centuries. We've come a long
way from imagining the Earth as the center of the universe to understanding
its meager position in the solar system. The pictures above of two Cascades
volcanoes reminds us that we live on a dynamic planet. It is very much alive
with internal and external activity. These mountains are composite volcanoes
that rise above a platform of older volcanic rocks. They are part of a magmatic
arc that is the surface manifestation of internal processes occurring many
kilometers deep where crust and mantle interact with each other and higher
temperatures and pressures prevail.
Click on the image to the right to see or download a rather large image of
the Pacific Ocean topography from the NOAA National Geophysical Data Center.
There is no way that everything we know about the workings of our planet
can be covered in this course, but think about the following topics and how
they fit into our understanding of the formation of the Earth:
- Differentiation into crust-mantle-core layers
- Plate Tectonics
- Origin of the Atmosphere
- Dynamic processes that shape the planet today,
such as fluvial, eolian, volcanic, and glacial activities.
We will use examples from Earth in subsequent modules to evaluate these
processes on other planetary bodies.
Birth Of The Earth
Most of what we know about the Earth's interior is derived from studies
of rocks at the surface. Internal variations in compositional and physical
properties are inferred from studies of other planets, seismic waves,
Composition refers to mineralogy, i.e. the types and abundances
of minerals (which are used to classify rock types) and rock chemistry,
i.e. the abundances of major-, minor-, and trace-elements, as well as certain
compounds, such as H2O and CO2).
Mineral and chemical compositions change from one region in the Earth to another.
Physical Properties = density, rheology (response to
stresses), and states of matter (solid, liquid, gas). Physical properties
also change from one region to another, although the changes do not always
coincide with compositional changes.
Most geologists and geochemists believe that the bulk composition of the
Earth is approximately chondritic. Planetary bodies in the Solar System
accreted from, and are composed of, primordial Solar Nebula material represented
by carbonaceous chondrites, which are meteorites comprised of early condensed
solar grains and dust. Using isotopic dating (radiometric) techniques, geoscientists
have determined the time since meteorites formed in the cooling solar nebula.
These techniques yield an age of the Earth around 4.6 billion years old (4.6
x 109 years). In geochronological terms, this would be 4.6 Ga.
During the formation of the Solar System, the inner terrestrial planets
formed in the hotter region close to the Sun, while giant gaseous Jovian
planets formed in a cooler part of the solar nebula. Refer to Module
Two for details. Earths average distance from the Sun (1 AU) is
about 150 million kilometers (93 million miles), a distance favorable for
terrestrial planet formation as well as for condensation of water-bearing
Accretion of chondritic material into the early planet Earth was followed
by internal heating which caused differentiation. Light silicate
minerals, such as feldspars and quartz, separated from the heavier metallic
minerals and rose to the surface of the planet, eventually forming the continental
crust and mantle. The denser materials sank to the center of
the planet to form the iron-rich core. This scenario is much like that
of the other inner planets which we believe are similarly differentiated.
Answer the questions at the end of this section after visiting several Earth
websites and reading about the interior of the Earth in these sites, your
textbook and other available resources. You may want to locate an introductory
geology textbook if you are not familiar with some of the terms and concepts
in this module.
Many of the older geology professors today did not study plate tectonics
in their college years because the idea was not yet accepted widely. By the
same token, some of the processes we use to explain planetary features may
be obsolete in a few years. However, this does not prevent us from exploring
all possibilities, keeping our minds open, and learning what comprises our
planetary system. To this end we use our home planet as a means of understanding
planetary processes, albeit many are not easily recognized on other planetary
Our knowledge has grown considerably in the past three decades, and
it is continually growing and changing. Much of what we know about Earth came
from our travels to Earth orbit, and to the Moon during the Apollo missions.
The photo of the Tibetan Plateau on Earth, taken from space, illustrates how
geology as a planetary science suddenly emerged with space travel.
In 1915, Alfred Wegner, a German meteorologist and geophysicist published
the book "Origin of Continents and Oceans" in which he proposed
the idea of Continental Drift. His evidence included (1) the puzzle-like fit
of South America and Africa, (2) evidence of former glaciers in the hot land
of Australia, (3) similar fossils found on different continents, and, (4)
similar rock structures and mountain ranges on different parts of the world.
Wegner proposed that a supercontinent called Pangaea (all land) once contained
all the continents we now know, and that they began to split apart about 200
million years ago.
This idea was not well received. Who could imagine the continents floating
along the planet? What could possibly cause this drift? In the past, people
had wondered why there were mountains and huge oceans. Some folks thought
that the Earth was once molten, so when it cooled it contracted and shrank
leaving continents separated by gigantic chasms. The outer skin of the Earth
responded like a piece of dried fruit, folding up and down to accommodate
this shrinking. When you think about it, this was a pretty imaginative idea!
Wegner also had to contend with strong opposition from religious leaders and
the belief that the world was created in 4004 B.C., a date derived by Charles
Usher from Biblical accounts of family trees.
Not until the 1950's and 60's did the theory of Plate Tectonics start to
become widely believed and accepted. Detailed mapping of the ocean floor showed
raised relief ridges that we know to be where oceanic crust is undergoing
seafloor spreading. Work by Hess and Vine on magnetic 'stripes' of the ocean
floor added credence to this idea by illustrating how magnetic polar reversals
are recorded in basalts. Many concepts in geology now fit within the paradigm
of plate tectonics. Thus, we have much evidence to show that continents moved
around the planet, slowly but surely, slipping along three main types of plate
Plate Boundaries Review Figures 8 and 9, p. 118 in the textbook.
This figure, from the NASA Goddard Space Flight Center, shows plate boundaries
in light blue, measured earthquakes in yellow dots, and recently active volcanoes
in red triangles. Study it well in order to make associations of various tectonic
regions with the features present in them.
- Study the age relations of oceanic crust and the Mid Atlantic Ridge and
the Pacific Ocean floor of the Pacific from the National Geophysical Data
Center of NOAA (National Oceanic and Atmospheric Administration).
from Space is an online geomorphology book created by NASA.
Many of the following examples can be looked up here using the index to
lookup specific places.
- Divergent Boundary: An area where two lithospheric plates
are separating, allowing magma to rise and form new oceanic crust. An example
of a divergent boundary, or spreading center, is the Mid Atlantic Ridge.
Here, the ocean floor is moving away from an area of upwelling mantle, producing
a warm, less dense ridge that appears to be an underwater mountain range.
Upwelling of the mantle and rise of overlying lithosphere is caused by large
slowly-moving convection currents in the mantle. Divergent boundaries are
found in all levels of maturity, from a juvenile East African Rift
to the Red Sea to full ocean basins around the world. Visit Understanding
Plate Motions, a chapter from This
Dynamic Earth, an online book from the United States Geological Survey,
for an excellent overview of plate motions.
- Convergent Boundary: At a convergent boundary, two plates
are being pushed or pulled together. There are two sub-types of convergence
which lead to either a subduction zone and volcanism or a crustal orogeny
(mountain building event).
- At a subduction zone, cool and dense ocean crust comes
in contact with less dense continental crust. The ocean crust is heavier,
so when the two plates exert force in opposite directions, the ocean crust
begins to subduct, or sink below the continental crust. This type of boundary
forms the Cascades of North America and the Andes of South America.
- The Nazca Plate is subducting beneath the South American Plate to form
the high volcanic mountains of the Andes.
- If both plates are continental crust, neither will give way and sink beneath
the other, so they will form huge mountains as the continent crust deforms.
This type of convergence occurred on the East Coast of North America during
the formation of the Appalachian Mountains during the Ordovician and Devonian
time periods. A current example of continent-continent convergence
is the Himalayan Mountains.
- More Himalayan pictures can be found at Geomorphology
from Space. The subcontinent of India is riding the Indian-Australian
Plate which is colliding with the Eurasian Plate,
pushing up the highest mountain range in the world like a hallway rug pushed
up against the wall.
- Transform Boundary: A plate margin where neither plate
is subducted or pushed up; they simply slide past each other. Another name
for this type of boundary is a strike-slip zone. The most famous example
of this type is the San
Andreas Fault in California. There are two types of strike-slip, or
transform, faults: left lateral and right lateral. You can
tell which one you are looking at by imagining yourself standing on one
side of the fault looking across to the other side. Whichever direction
the other side is moving is the type of strike-slip fault.
TASK 1: Answer the Study Questions
- What are the major layers of Earth's interior? What
evidence is used to make this interpretation? [Earth
Figure (same as Figure 4, Chapter 9, in your textbook)]
- Of what significance is the mantle to Earth's
evolution? Write your answer in terms of bulk mineralogy, bulk chemical
composition, layering, proportion of Earth's mass, temperature gradient,
and pressure gradient.
- What is the lithosphere? What is the aesthenosphere?
What is the difference between these two?
- How does oceanic lithosphere differ from continental
- What causes true polar wander and apparent polar
- What kind(s) of faults would you expect to see
at a convergent plate boundary? A divergent boundary? A transform
boundary? Would there be only one kind of fault? Why?
- Which type of fault is the San Andreas, right
or left lateral?
- What is the age range of the ocean floor?
- Give definitions for each of the following components
of the Earth's interior. (see Table 1, p. 114 in your textbook): continental
crust, oceanic crust, upper mantle, transition mantle, lower mantle, outer
core, inner core.
Earth: The Story of Plate Tectonics from the United States Geological
Survey,an online book by Jacquelyne Kious and Robert I. Tilling
Introduction to The ABC's of Plate Tectonic by Donald
Tectonics: Chemical and physical layers of the Earth, historical development
of the theory, and descriptions of the location and types of plate boundaries
from Volcano World at the University
of North Dakota.
There are two main types of igneous rocks on the Earth: intrusive and
extrusive (see Module
3, Planetary Evolution and Processes). Intrusive rocks are those that form
as magma slowly cools inside the crust, producing large visible mineral crystals
of quartz, feldspars, micas, or amphiboles, among others. Examples of igneous
intrusive rocks are granites, diorites, and gabbros, named depending on their
chemical and, especially, mineralogical compositions. Extrusive rocks are produced
by volcanism; either the relatively quiet eruption of fluid magma forming lava
flows, or by explosive volcanism that produces ash and cinder. Volcanic eruptions
have played a significant part in the shaping of the Earth and have produced
many dramatic features both here and on other planets. Visit University
of North Dakota's Volcano World
site for everything you ever wanted to know about volcanoes, and what they tell
us about earth processes.
Large igneous provinces on Earth are made of thick sequences of flood basalts
such as the Columbia River Basalt of Washington, Idaho, and Oregon. Another
example is the Deccan
Traps of India. They represent voluminous fissure eruptions of non-explosive
fluid basaltic magma. Both resulted from heating and stretching of the lithosphere
related to mantle plumes. These are similar to basaltic lava plains that make
up the ancients surfaces of Mercury and the Moon. The closest Earth analog for
lava plains-style volcanism is the Snake River Plain of Idaho.
Convergent plate boundaries tend to produce a very different type of volcanism.
The magma is slightly more silicic than basalt magma, and more viscous, which
causes more explosive volcanism. Generally, their compositions are andesitic
or dacitic, which is intermediate between basaltic and rhyolitic. Composite
or strato volcanoes, such as Mt.
St. Helens in the Cascade Range result from this type of volcanism. They
are usually found in magmatic arcs that run parallel to subduction zones.
Other features related to subduction zone volcanism include cinder
cones, pyroclastic flows, collapsed
calderas, and lahars.
TASK 2: Volcano study questions
Name at least three important types of volcanic features
found on other planets.
Why are there volcanoes on Mars that are so much larger
than those on the Earth?
What are Large Igneous Provinces? Name two of
them. Why are Large Igneous Provinces important?
Name the major types of volcanoes and give examples
and locations of each type.
Volcano World - Everything you want
to know about all kinds of volcanoes, and the structures they create.
From the University of North Dakota.
Mount St. Helens: K-8 stories - http://volcano.und.nodak.edu/vwdocs/kids/stories/MtStHelens.html
K-12 activities- http://volcano.und.nodak.edu/vwdocs/vwlessons/rocks_activities.html
Deadly Eruptions- http://volcano.und.nodak.edu/vwdocs/vw.hyperexchange/deadly_volcs.html
Cinder Cone Activities- http://volcano.und.nodak.edu/vwdocs/vwlessons/landforms/land_activities.html
Volcano Observatories and Organizations
How Volcanoes Work
(Vic Camp, UCSD)
Hawaii Volcano Observatory
Alaska Volcano Observatory
Cascades Volcano Observatory
Long Valley Caldera Observatory
Yellowstone Volcano Observatory
World Organization of
Volcano Observatories (WOVO) (UND site)
New Millennium Observatory
(NeMO -- off the coast of Oregon)
U.S. Geological Survey Volcanoes Hazards
International Association of Volcanology
and Chemistry of the Earth's Interior (IAVCEI)
American Geophysical Union -- Volcanology, Geochemistry
and Petrology Section
Nordic Volcanological Institute, Iceland
Smithsonian Global Volcanism Network
University of Washington - Volcano
Michigan Technological University
Fluvial features are those created by running water, including rivers,
streams, and floods. The water either acts to erode the rocks and soil of the
surface, or it deposits its load of sediment. Fluvial erosion forms various
landforms on the Earth, as well as on Mars,
including river channels and intricate valley systems. Stream erosion may act
by the removal of soil, downcutting of stream channels, or by headward erosion
of the valley, the shape of which depends on the gradient of the stream channel,
the amount of discharge, and the stream load. Visit the Web
Links on Rivers and Other Fluvial Processes to get an overview of these
Go to the Earth from Space website
and investigate some famous rivers, such as the Nile, the Mississippi, and the
Amazon. This NASA site has views from space of cities, rivers, deserts,
mountains, and many other features.
We will not spend very much time on fluvial processes. A good understanding
of them would require an entire course in geomorphology that focuses on rivers
and streams. Althought they are very important on Earth, theya are not so important
on other planets, except Mars.
Any planet that has an atmosphere will experience ongoing eolian activity,
caused by the movement of gases (air) across the surface and the concomitant
transport of fine-grained dust, silt and sand.
Between 10 and 30 North and South of the Equator lie some of the most spectacular
deserts on Earth. The Moenkopi
Plateau is being extensively studied by the USGS. The process of wind transport
and deposition of sand have produced unique and interesting landforms, some
of which can also be seen on Mars. Go to Wind
Processes and Landforms for discussion, video and more about eolian systems.
Read about all these landforms and understand them: Chapter
8 from Geomorphology from Space should help in visualizing eolian landforms:
- deflation basins
- lag deposits
- desert pavement
- dunes (longitudinal, star, transverse, and barchan)
TASK 3: Eolian Questions
- What role has loess played in the agriculture of the
Idaho Snake River Plain?
- Give an example of a location on Earth and one on
Mars for each of the landforms listed above.
from Jet Propulsion Laboratories might be helpful.
Continental ice sheets have advanced and retreated over the planet several
times. Several factors influence the cyles of ice ages, such as the tilt and
precession of Earths rotational axis. The most recent large advance
of the glaciers was approximately 20,000 years ago, which left behind many
glacial landforms across the United States and Canada. An example of a current
continental glacial system is the Antarctic ice sheet. Visit the National
Snow and Ice Data Center to learn more about Antarctica and other snow
and ice environments. Alpine glaciation can be seen at Grand
Plateau, Mt. Cook, New Zealand, from the Glacial
Geology at the University of Cincinnati homepage.
TASK 4: Glaciation study questions
Look up and define the
glacial features listed below. Start at the Glacier
Question Page from NSIDC, then visit the Illustrated
Glossary of Alpine Glacial Landforms from the University of Wisconsin-Stevens
- ice bergs
- moraines (terminal, medial, and lateral)
- U-shaped valley
- kettle lake
For more glacial geology, take some virtual field trips or visit these websites:
Glacial dams have been known to block large amounts of water behind them.
When the ice finally begins to melt, or the excess water builds up behind
the dam, causing a catastrophic glacial flood. Evidence of flooding of this
magnitude can be seen in the Channeled Scabland.topography of eastern Washington.
Steep walls, giant ripple marks, dry waterfalls, cataracts, and huge boulders
can be seen along the flood path. Another type of glacial flood is called
which often occurs in Iceland during volcanic eruptive periods.
Photos courtesy of Los Alamos National Laboratory
Visit the National Oceanic and Atmospheric
Administration for links to many atmosphere resources.
Visit NOAA/OSEI"s Image
of the Day to see fascinating photos of significant atmospheric events and
find links to many weather, oceanic, and atmospheric research sites. Visit the
OSEI homepage from NOAA/Operational
Significant Events Imagery
to see various images of significant weather and atmospheric events.
Remember that Earth is a dynamic, living planet.
Historical geology is the branch of geology that researches
how the Earth and all of its systems have changed through time. Early
Earth was not much like our home today. The atmosphere was dominantly
hydrogen (H) and helium (He) and the planet was being bombarded with asteroids
and meteorites. Hydrogen and helium are the most abundant gases in the universe,
but probably escaped from the atmosphere into space because they are also
the lightest elements.
As the first couple of million years passed after the birth of the Earth,
a second, more dense atmosphere began to develop. This atmosphere was formed
by the constant outpouring of gases from the Earth's interior through volcanic
eruptions, known as outgassing.
Volcanism released water vapor, CO2, and some Nitrogen (N2),
which led to the formation of the rivers and lakes, and oceans of the world.
Note how strikingly different the presence of water has made our planet compared
to others. Eventually, through chemical and biological processes, much of
the CO2 in the atmosphere became locked up in the carbonate rocks
of the planet, and our atmosphere became rich in N2.
The oxygen upon which we live became the second most abundant gas in our
atmosphere through a slow process of photodissociation. This happens when
the energetic rays from the sun split apart H2O molecules into
Hydrogen and Oxygen. Since the Hydrogen is so light, it probably escaped out
into space, leaving the Oxygen to accumulate in the juvenile atmosphere. After
oxygen began to accumulate, primitive plants could have evolved. Once plant
life appeared on the Earth, photosynthesis could occur which combines CO2
with H2O and sunlight to produce oxygen. This would have dramatically
increased the oxygen content in the atmosphere and allowed life to flourish
on our planet.
Earth History websites for you to view:
More Terms to Look Up and Understand - Many of these terms
you will already know from introductory geology courses.
TASK 5. Global warming
Recently, global warming has been a hot topic in the news. Your final task
for this module is to examine this controversy using web sites, books, journals,
papers on your own. Write one paragraph
that briefly outlines (1) evidence both supporting and refuting the global
warming hypothesis, and (2) why global warming is a major concern for scientists.
If you can, list a couple of real Earth examples of why we should (or should
not!) think about this problem.
There is much evidence for and against global warming that
is related to fluvial, eolian, glacial, and climatic processes.
Here are some global warming websites for you to visit.
This Is The End Of The Module
Take a deep breath! This is the biggest module, because Earth (our
home sweet home) is the planet we know best. When you have finished
the work and e-mailed it in, pat yourself on the back for a job well done.