Lecture
Lecture |
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| Comparative Planetology: Earth
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You see the Earth as a bright blue and white Christmas tree ornament in the black sky. It's so small and so fragile - you realize that on that small spot is everything that means everything to you; all of history and art and death and birth and love.----Russell Schweikart, astronaut
Define each term and give a terrestrial example or explanation
Describe how the active Sun affects the Earth and how the interaction with the Earth's magnetic field produces aurorae
Outline the major features seen on Earth resulting from tectonics
Volcanism--not just volcanoes--explain. Also, explain the difference between a shield and a strato volcano.
Relate volcanic features seen on Earth to similar ones seen on the Moon.
List the basic properties of the Earth's atmosphere and state your opinion of whether or not we are having a negative impact on it.
VocabularyDifferentiation Interior Structure Magnetic Field Volcanism Tectonics Cratering Erosion Atmosphere |
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Differentiation and Interior Structure
What does differentiation mean? What are we taking about when we say that a planet is differentiated? Take a look at the glossary in the appendix of your text, and put the definition into your own words. Then, take a look at the following "worlds" and state which one(s) is completely differentiated, which one(s) is partially differentiated, and which one(s) is not differentiated.
The Earth is differentiated. To become differentiated, the world must have been at a high enough temperature for a long enough period of time that the material separated itself into regions going from the least dense at the surface to the greates density at the center. In general, this means the world had to be at least 1000 km in diameter.
How do we know what the interior structure of the Earth is like anyway? Our deepest wells or mining shafts have gone down merely a few kilometers. The science of oscillations is quite complex and sophisticated. Compression (pressure, primary, P) waves and distortion (shear, secondary, S) waves travel through the interior of the Earth in different ways. P waves travel faster than S waves and can propogate through liquids and thus through the core of the Earth. S waves get deflected at the core. See Savage Earth Earthquake Animation for a very good visual explanation. We have used these same principles to "investigate" the interior of the Moon and even the interior of the Sun (sun quakes!).
Magnetic Fields and Magnetosphere
The Earth has a fairly strong magnetic field (as long as you don't compare it to Jupiter or the Sun). This field is tilted slightly with respect to the spin axis by about 21 degrees. To have a magnetic field, a world must have a liquid region that conducts electricity and must have an energy source that makes that conducting region move. Recall: a changing electric field produces a magnetic field, and a changing magnetic field produces an electric field.
The magnetic field protects the Earth from the Sun's harmful solar winds, coronal mass ejections from the Sun that spew high energy charged particles at the solar system. Take a look at Fig. 7.4 of your text and see how the Earth's magnetic field deflects these charged particles.
Not all charged particles from the Sun get by the Earth. Some end up spiraling down the magnetic field lines of the North and South Poles and creating what we call aurorae or the Northern and Southern Lights. For complete information on what aurorae are, why they look the way they do, what causes them, etc., check out: Aurorae from NASA'S exploratorium. This is where I gathered most of my information for the lecture, and so it is important that you go through the self-guided tour provided there.
Here are just 2 of a number of images from a lively aurorae show on November 24, 2001. The images are copyrighted by the photographers noted.
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Why the different colors? Oxygen atoms when excited produce the strong yellow-to-green light; nitrogen molecules when excited produce a red light (although the colors produced may depend on the height in the atmosphere). These particular colors correspond to specific wavelengths being produced, wavelengths that are seen only in an extremely low-density gas.
The text covers plate tectonics in a fair amount of detail. Be sure to read that section carefully. Basically note that 1) the mantle of the Earth is continuously on the move; 2) the plates are driven by the slow convection of the Earth's mantle; 3) rift zones are where new crust is being produced, subduction zones are where it is destroyed; 4) in short, the Earth is still in the process of cooling down from its formation. Here in Seattle, as we all hopefully know, we live right over a subduction zone where the Juan de Fuca plate is plunging beneath the North America plate. See the US Geological Survey web page: Juan de Fuca Volcanics for more information.
Where there are rift zones, there may just be deep oceanic vents, thermal vents spewing out water reaching 700 degrees Fahrenheit! Here, just a short distance from the steaming water, lies a region rich with life, life that lives without the energy from the Sun. We will be interested in these objects on Earth when we come to studying Europa, one of the moons of Jupiter. For now, get additional information on-line from PBS: Savage Earth and Creatures of the Thermal Vents.
Extension of the crust can make cracks and valleys, what we call faults (these were called "rilles" on the Moon). Compression of the crust can make mountains. The subduction of the Juan de Fuca Plate is creating the Cascade Range and drives the Cascade Volcanoes
The awesome names of these two types of lava are justification enough to give them a brief spot in these notes. Start with a visit to learn about the geology of Hawai'i For a large collection of images of these two forms of lava, go to Google Image Search and ask for pahoehoe + lava and aa + lava. Also good for background information and generally snooping around volcanic information: USGS website on volcanoes.
Shield volcanoes form when the lava has medium viscosity and thus does not flow far
from the vent. These volcanoes are not very steep, but they can be extremely tall.
The slopes are generally between 5 and 10 degrees from the horizontal. Examples of
shield volcanoes on Earth: the Hawaiian volcanoes, Mt. Etna. Lava may flow
nonexplosively from these volcanoes, although anything in its way gets destroyed.
Shield volcanoes are usually over a hot spot in the crust.
Mt. St. Helens and the rest of the Cascade Range volcanoes are stratovolcanoes. These volcanoes usually have steep sides and are built up from eruptions of lava. These volcanoes can erupt very explosively, especially if the magma is mixed with material such as water and sand that has been brought down through the subduction zone. These volcanoes are build up with sticky lava, viscous. The lava does not flow far before solidifying. Photo courtesy of the USGS.
Study first the geology of the Craters of the Moon National Monument in Idaho (See also Craters of the Moon Topography). You will recognize terms such as rift zone, hot spot, lava, basalt.
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| photo credit to: John C. Dohrenwend, Ph.D. P.O. Box 141 Teasdale, Utah 84773 (435)425-3118 |
Take a close look at the image shown above of a lava river on the Big Island of Hawai'i. Where is the lava coming from? How does the river "look"? |
Now take a look at this image of the Moon (courtesy of NASA)
that shows an oblique view of Schroeter's Valley and Aristarchus crater (the bright one to the left,
35 km in diameter) as taken by a camera aboard the Apollo 15 command/service module. Notice the
large sinuous rille -- nicknamed the cobra head. If you look closely, you will see what looks like
at least another half dozen sinuous rilles. How are these features similar to those found on Earth?
How are they different? Look above Aristarchus crater to the mare (Ocean of Storms) and notice the
different shades of the basalt. Relate what you note here to the difference seen in the lava flows
in the Craters of the Moon. (You should be able to identify a number of other features here as well:
bright rays, complex and simple craters, secondary craters; there may even be a wrinkle ridge or
two.
(Image courtesy of NASA)
Among the many startling views of the Earth the astronauts have, one of the most noticeable is just
how thin our life-sustaining atmosphere really is. The radius of the Earth is about 6400 km; the
height of the atmosphere, barely 1/1000 of that, and the region within which most life exists is
less than 1/3 of that. Since the start of the industrial revolution, humankind has been polluting
this thin shell of nitrogen and oxygen, contributing greenhouse gases in the process. The primary
greenhouse gas is carbon dioxide, followed by methane and water vapor. We will be studying Venus as
an excellent example of a runaway greenhouse process. Is this what is in store for Earth if we
don't control our emissions?
For an image on the distribution of methane in the Earth's atmosphere, check out
Methane Earth.
Cratering has been discussed as a completely separate topic.
What do you recall about erosion from your elementary, junior high, and high school years? What kinds of erosion does Earth have? Keep these in mind as we visit other worlds.
Earthquakes from NASA's Exploratorium
Volcanoes of the World from the USGS.
A poster from the USGS that gives the why, where, what on volcanoes, included some incredibly stupendous images of Mt. St. Helens blowing its top on 18 May 1980.
An absolutely volcano website with lots and lots of pictures.
Make sure that when you click on a 3-D image to enlarge it that you also read the side bar and click on "Features of Interest" for highlights of geologic features.
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