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I. The Moon
An Astronomical Natural Disaster
IMAGE 1: The Moon
From: NASA JPL
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The Moon is our closest neighbor and, when compared to other planet/moon systems we know of, is the largest moon relative to the planet it orbits.
The Moon's gravity is equally responsible for the tides of our oceans (see Day 6 Background Reading for more information) as is the Sun. The Sun is larger, but farther away and the math works out such that they both affect the Earth approximately equally.
The surface of the moon has light and dark areas corresponding to mountainous areas and low-lying areas known as "maria" or seas. These maria are not dark because they are low-lying, but because of their composition. Maria are basalt, a dark extrusive igneous rock, or lava. These maria filled with basalt after being impacted by solar system debris some 3 billion years ago.
The maria are more dense than the mountainous regions, which are composed of mostly feldspars, the greyish part of granite rocks.
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As it turns out, the maria happen to be concentrated on one side of the Moon's surface - the part that faces the Earth. The reason why we don't see the "dark side of the Moon", which isn't dark, is because the more dense maria are more strongly attracted by Earth's gravity than the less dense mountainous areas. So, the Moon's orbit and rotation are locked together with the Earth's.
How is the Moon a "Natural Disaster"?
In 1969, Apollo 11 spacecraft brought Man to the surface of the Moon. Approximately 850 pounds of samples from the Moon's surface were brought back to Earth for analysis. Radiometric dating techniques, chemical composition, and other anslysis of these samples helped confirm the Collision Ejecta Hypothesis for how the Moon was formed.
There have been three long-lived theories explaining the formation of the Moon. One theory held that the Earth captured the Moon as it passed by - the Moon was just an asteroid or planetoid drifting by. Another theory held that the Earth and Moon formed together from the same primordial materials. Another theory held that the Moon was flung from the Earth's crust (Pacific Ocean) as theEarth rotated far faster than it does today.
All of these explainations have problems: The Moon formed about 4 billion years before present and the Pacific Ocean as such didn't exist at that time. The Moon doesn't contain the iron and other heavy elements as does the Earth, so it is unlikely that it formed from the same primordial materials. The Moon is round, like the Earth, which is very different from the typical shape of asteroids or comets.
The Collision Ejecta Hypothesis states that a mars-sized object struck the Earth at a glancing agle causing the destruction of the object, the melting of Earth, and the resulting debris, or ejecta, to disperse into orbit around the planet. Over time, some of this material fell back to Earth, but the rest coalesced into the Moon. What we have learned about the Moon fits the Collision Ejecta Hypothesis:
- The Moon's composition matches Earth's crust and upper-most layer of the mantle.
- The Moon doesn't contain the heavy elements found in Earth's lower mantle and core
- The Moon is round in shape
- The Moon orbits in the ecliptic plane, the same plane the Earth orbits the Sun,
which is where debris ejected from a collision would tend to coalesce
- The Moon's slow movement away from the Earth is consistent with motions
associated with a collision taking place at an oblique angle (a glancing blow).
If we humans were around when the Moon was created, it would have been a disaster.
II. Today's Natural Disasters
Astronomical
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We know that the Earth was hit by a large object that formed our Moon. Could this happen again? Yes! About 50 thousand years ago, a meteor about 40 meters in size came into contact with Earth's atmosphere where is began to break apart. The core of this iron body struck the surface of the Earth at between 10 and 20 km per second. The result is the crater pictured in IMAGE 2 (right). Meteor Crater is approximately 170 meters deep and 1.2 km in diameter - a rather large crater considering it was formed by a 40 meter object!
65 million years ago, a much larger object impacted the Earth. The size of that object is estimated to be between 6 and 12 miles wide and created a crater on the order of 115 miles in diameter. The remnant of that crater is located in Mexico, across the Yucatan Peninsula, near the village of Chicxulub.
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IMAGE 2 - Meteor Crater
From Woods Hole / USGS Web Site
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This impact resulted in the extinction of 70 percent of all life on Earth and can be located in the geological record at a feature known as the K-T boundary (Cretaceous-Tertiary). This boundary is an iridium-rich layer found around the Earth and marks a transition in the fossil record where many species can no longer be found above this layer (more recent time).
Meteorologic
Severe weather, such as tornadoes, hurricanes, ice storms, and heat waves, are natural phenomina resulting from the uneven heating of Earth's surface. Wind, air moving from areas of high pressure to areas of low pressure, helps transfer heat from the lower latitudes toward the higher latitudes. Low pressure areas result from sunlight warming the Earth, which warms the air at the same location, which, in turn, expands and also tends to take-up more moisture (greater evaporation comes with warmer temperatures). The expanded air is less dense (by definition). Moist air is also less dense than is dry air because the molecular weight of H2O is less than that of typical air (Nitrogen & Oxygen). So, warm moist air weighs less and is therefore associated with an area of low pressure. The energies associated with severe storms is a function of solar radiation and evaporation.
[Click Image to Enlarge]
FIGURE 1 - Coriolis Effect
From City Univery New York Web Site
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As air moves away from the equator, it will begin to rotate. This rotation is imparted by the rotation of the Earth - the Coriolis Effect.
Imagine you are at the center of a merry-go-round and you are trying to throw a ball to someone sitting on the outside edge. When the merry-go-round is not rotating, the thrown ball will follow a straight line directly to its target. However, once the marry-go-round begins to rotate, you would have to anticipate the future location of your target and aim your throw to that location. If you didn't do this, the thrown ball would land behind your intended target.
If you are trying to throw the ball from the perimiter of the merry-go-round towards the center, you would have to anticipate the opposite effect. An unadjusted throw would fall ahead of the intended target.
In fact, the faster the rotation of the merry-go-round, the farther the intended target will move in the direction of rotation, and the more the path of the ball will appear to curve.
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The curving motion imparted to the ball would also be imparted to moving air - the air would begin to curve. The location from where the wind starts to move from, and in what direction it moves to, determines the direction of rotation. Clockwise rotation is said to be anti-cyclonic and counter-clockwise is cyclonic. Strong cyclonic rotation is what one finds in hurricanes and tornadoes.
Snow, ice, freezing rain, etc., result from air temperatures dropping below the freezing point of water. This can happen at ground level, or at higher levels in the atmosphere. When a warm layer of air lies over a cold layer at ground level, frozen precipitation can melt as it passes through, and then either refreeze closer to the ground, or upon contact with a surface at ground level such as a roadway.
Geologic
Earthquakes and volcanoes are also examples of nature's natural events. Earthquakes result from the bumping and grinding of the Earth's tectonic plates. The surface of the Earth can be thought of as similar to that of a hard-boiled egg whose shell has been cracked prior to peeling (FIGURE 2 below). As the plates push into and past one another, pressure and stresses build up and are periodically released in the form of earthquakes. Earthquakes can be shallow or take place deeper underground where plates are being subducted.
FIGURE 2 - Earth's Plates
From Prof. Steve Gao, Kansas State University
Tectonic Plates are moving in the direction shown by the white arrows. The plates are being driven by slow moving convection currents within partially melted rock (the asthenosphere) just below the rigid lithosphere. Eathquakes result from plate motion, but other events also.
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As one plate, composed of the more dense basalt or gabbro (basalt's intrusive igneous equivalent) typical of oceanic crust, encounters a second plate composed of the less dense granitic or sedimentary rock typical of continental material, the heaver plate will subduct beneath the lighter plate (FIGURE 3 right).
As the more dense oceanic plate sinks, it becomes more dense relative to the hotter, less dense asthenosphere. Consequently, its rate of descent will increase, helping to pull the plate down. This is called slab-pull.
Meanwhile, at the mid-ocean ridge, new magma wells up to fill the cracks of the rifting-apart sea floor forming new oceanic crust as it cools. This less dense, buoyant material will push its way up through the rift, which helps the plate move away from the spreading center. This is called ridge-push. Together, slab-pull and ridge-push form something like a converyor belt that keeps the plates moving and the geography of the Earth constantly changing.
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[Click Image to Enlarge]
FIGURE 3 - Earthquakes & Volcanoes
From Prof. Steve Gao, Kansas State University
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As the subducting plate (FIGURE 3 above) sinks deeper into the Earth, some of the rock will melt as a consequence of having been mixed with other materials. Just as mixing halite and ice (two minerals) will lower the resulting mixture's melting point, water and other materials incorporated into the subducting plate will cause a portion of it to melt more readily. The resulting hot, less dense melt will tend to rise up through the relatively colder, denser rock above. If it makes it to the surface, we get a volcano. If a piece of ocean crust is displaced vertically during an earthquake, the ocean water above this portion of the crust will also be displaced vertically. Water, however, rapidly succumbs to gravity resulting in a wave formed by the falling water column. This is a seismic sea wave, better known as a tsunami.
Summary
The background information presented here covers the basics that will help you get more out of the field trip. We covered the origin of our Moon, as well as the Meteorologic, Astronomic, and Geologic origins of many of the natural disasters.
What Next?
The information presented above is meant to provide a context with which to better appreciate the IMAX and Planetarium programs, as well as the various exhibits, at the Boston Museum of Science.
Continue on to the
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Tarbuck, Edward J.,Lutgens, Frederick K., 2005,
Earth, An Introduction to Physical Geology, 8th Edition
Pearson Prentice Hall
Tarbuck, Edward J.,Lutgens, Frederick K., 2004,
The Atmosphere, 9th Edition
Pearson Prentice Hall
Kuhn, Karl F.,Koupelis, Theo, 2004,
In Quest of the Universe, 4th Edition
Jones and Bartlett Publishers
Hewitt, Kenneth, 1997,
Regions of Risk: A Geographical Introduction to Disasters
Addison Wesley
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