Earthquakes are a normal and natural part of live on our wonderful world. Earth is truly a living planet. An earthquake occurs when a rock underground, supporting the rocks above, breaks. This breaking causes vibrations to travel out from the point of the break in all directions. Many earthquakes occur each day and have no effect upon human lives, or the landscape. The slow movement of the Earth's lithospheric plates causes many earthquakes, both large and small, to occur.
Earthquakes DO NOT occur at depths greater than about 450 miles, since at that depth,the plasticity of the mantle allows the rocks to flow, or bend, rather than break. About 90% of all earthquakes happen near the edge of a lithospheric plate, or along an established FAULT, or weak area of rock, underground.
Learning about some of the specialized vocabulary will help you to understand more about earthquakes and how,why and where they occur. In class, you will learn how to locate where an earthquake happened by studying seismic waves to locate the epicenter of an earthquake.
Fault: A fault is the area underground where the rock actually breaks. The resulting shift of rock causes the seismic waves. A fault may also be a larger area underground where rocks have been weakened by past breaks. The weakened area of rock that is a fault is the most likely area for an earthquake to begin.
SEISMIC WAVES: Seismic waves are the vibrations caused then the rocks break underground. These vibrations travel through the Earth at various speeds. They come in three varieties:
----------P-Waves (Primary Waves) are the fastest moving seismic wave. These waves are compression waves, moving through the Earth by compressing or squeezing the rock as it travels. P-waves move in a way similar to how an earthworm moves.
----------S-Waves (Secondary Waves) are the second-fastest moving seismic wave. They travel in a side-to-side, "S" shaped movement. They move the same way that some snakes move.
----------L-Waves (Surface Waves) are the slowest moving, yet the most destructive type of seismic wave. They move in an up and down "S" shape, like a wave on the ocean. These waves probably cause the land to move the most, and that's why they cause the most damage.
FOCUS: The focus is the point along the fault where the rock actually broke, and therefore, the actual location of the seismic waves' origin.
EPICENTER: The epicenter is the point on Earth's surface directly above the focus. It is also the area on the surface MOST affected by the seismic waves. Generally speaking, the most damage from an earthquake will be seen at the epicenter.
SEISMOLOGIST: A scientist who studies seismic activity within the Earth.
SEISMOGRAPH: A scientific instrument which measures and records seismic waves as they happen.
SEISMOGRAM: A written record of the seismic waves. The equivalent of a cash register receipt. (which is a written record of what you just bought)
RICHTER SCALE: A chart which measures the magnitude or strength of an earthquake. It measures the actual amount of energy released during the 'quake.
MERCALLI SCALE: A chart which measures an earthquake's intensity, or the amount of damage caused by an earthquake. Earthquakes may cause a lot of damage, even thought they don't release all that much energy. (if the 'quake hits an unprepared, or heavily populated area, for example)
INTENSITY: This word means how much damage is caused by an earthquake. Intensity is measured on the Mercalli Scale.
MAGNITUDE: This word means how much energy was released by the earthquake. Magnitude is measured on the Richter Scale.
This is a photograph of two volcanoes located in Colima, Mexico. They are located about an hour's drive from the village of Manzanillo. The peak in the foreground has been recently very active, spewing plumes of smoke, ash and lava.
Here's a close up of the caldera. Tourists need to get special permission to climb to the peak. Such an activity would not be allowed these days due to the activity of the volcano.
A volcano is a specialized type of mountain. A volcano is not always active and erupting. Most volcanoes are active for a time, and then quiet or dormant for long periods of time between eruptions. A volcano is formed when hot rocks and gas underground find their way to the Earth's surface. The magma, now called lava, cools and hardens to form igneous rock. After repeated eruptions, the lava builds up and the volcano, or volcanic mountain, grows in size. Most volcanoes will be found near the edge of a lithospheric plate, as is the Colima volcano. As oceanic crust is forced down and beneath the continental crust, it is heated and melted. The resulting magma often finds its way to the surface through existing cracks in the crust. Occasionally a volcano does not form near the edge of a plate. These volcanoes are called HOTSPOT VOLCANOES, and they actually form in the middle of a lithospheric plate. Our mantle contains many areas known as "hotspots". In these areas magma from the mantle rises closer to the crust and actually burns a hole through the overlying lithospheric plate. It's in this way that these volcanoes form. Since the lithospheric plates continue to "drift" across the hotspot, a series or chain of volcanoes will form. Our Hawaiian island chain is part of a much large chain of volcanoes on the Pacific floor that have been formed in this way.
All volcanic mountains have several features in common:
VENT- A vent is simply a hole in the volcano from which volcanic material like lava and ash is ejected. The vent is connected to the lava chamber. Vents are also called "Fumeroles".
CALDERA- The caldera is a crater, generally near the top of a volcano. The top of a volcano generally collapses into the magma chamber below, forming the caldera. Volcanic vents are also located in this crater, and ejected material is often seen filling and then flowing down from, the caldera during an eruption.
MAGMA- Magma is the term for molten rock and dissolved gases located underground.
LAVA- When the magma reaches Earth's surface some of the dissolved gases escape to the atmosphere because there is less pressure on the surface than there is underground. Lava is the term for the molten rock located on Earth's surface.
-----There are two basic types of lava:
----------AA-pronounced (ah-ah) lava is thick and chunky. Its temperature is slightly lower and so its texture is somewhat thicker. When aa lava solidifies, it forms a rough and jagged mass. It is said that it takes its name from the sound that a barefoot person makes when walking on it---------or so my friend Maui claims, and he lived in Hawaii, so he oughta know!
----------Pahoe-Hoe- (pronounced pah-ho-E-ho-E) This lava is thinner and more runny than aa lava, mostly due to its higher temperature. When pahoe-hoe lava solidifies, it forms smooth, rippled rock. It often appears like the coils of a rope. The word "pahoe-hoe" means "rope-like".
There are several types of volcanic mountains that may form.
CINDER CONE VOLCANOES: These are small, steep-sided mountains formed from aa lava. Cinder cone volcanoes are violent, explosive and unpredictable.
SHIELD VOLCANOES: These are large, broad-based volcanoes formed from pahoe-hoe lava. The thinner lava spreads out to cover a large area when it reaches the surface. Shield volcanoes form the well-known Hawaiian island chain. The islands are just the peaks of very large volcanoes. The island of Hawaii is actually the world's tallest mountain, rising 11,033 meters (and counting) from the ocean floor.
COMPOSITE VOLCANOES: These are combination volcanoes. They are formed from alternating flows of aa and pahoe-hoe, cinders, fragments and volcanic ash. Mt. St. Helens is an example of a composite volcano.
DOME VOLCANOES: These are small, steep sided volcanoes formed from aa lava. They are dome shaped due to the thickness of the lava from which they are constructed. A dome shaped mass of lava is often seen in the caldera.
Section 5 Dangerous Beauty
The following is an excerpt from "A Short History of Nearly Everything" by Bill Bryson This is a wonderful book, and I would heartily recommend it as required reading to any serious student of this Wonderful World.
In the 1960's, while studying the volcanic history of Yellowstone National Park, Bob Christiansen of the United States Geological Survey became puzzled about something that, oddly, had not troubled anyone before: he couldn't find the park's volcano. It had been known for a long time that Yellowstone was volcanic in nature--that's what accounted for all its geysers and other steamy features--and the one thing about volcanoes is that they are generally pretty conspicuous. But Christiansen couldn't find the Yellowstone volcano anywhere. In particular what he couldn't find was a structure known as a caldera.
Most of us, when we think of volcanoes, think of the classic cone shape of Fuji or Kilimanjaro, which are created when erupting magma accumulates in a symmetrical mound. These can form remarkably quickly. In 1943, at Paricutin in Mexico, a farmer was startled to see smoke rising from a patch on his land. In one week he was the bemused owner of a cone five hundred feet high. Within two years it had topped out at almost fourteen hundred feet and was more than a half mile across. Altogether there are some ten thousand of these intrusively visible volcanoes on Earth, all but a few hundred of them extinct. But there is a second, less celebrated type of volcano that doesn't involved mountain building. These are volcanoes so explosive that they burst open in a single mighty rupture, leaving behind a vast subsided pit, the caldera (from a Latin word for cauldron). Yellowstone was obviously of this second type, but Christiansen couldn't find the caldera anywhere.
By coincidence just at this time NASA decided to test some new high altitude cameras by taking photographs of Yellowstone, copies of which some thoughtful official passed on to the park authorities on the assumption that they might make a nice blow-up for one of the visitors' centers. As soon as Christiansen saw the photos he realized why he had failed to spot the caldera: virtually the whole park--2.2 million acres--was caldera. The explosion had left a crater more than forty miles across--much too huge to be perceived from anywhere at ground level. At some time in the past Yellowstone must have blown up with a violence far beyond the scale of anything known to humans.
Yellowstone, it turns out, is a supervolcano. It sits on top of an enormous hot spot, a reservoir of molten rock that rises from at least 125 miles down in the Earth. The heat from the hot spot is what powers all of Yellowstone's vents, geysers, hot springs, and popping mud pots. Beneath the surface is a magma chamber that is about forty-five miles across--roughly the same dimensions as the park--and about eight miles thick at its thickest point. Imagine a pile of TNT about the size of Rhode Island and reaching eighty miles into the sky, to about the height of the highest cirrus clouds, and you have some idea of what visitors to Yellowstone are shuffling around on top of. The pressure that such a pool of magma exerts on the crust above has lifted Yellowstone and about three hundred miles of surrounding territory about 1,700 feet higher than they would otherwise be. If it blew, the cataclysm is pretty well beyond imagining. According to Professor Bill McGuire of University College London, "you wouldn't be able to get within a thousand kilometers of it" while it was erupting. The consequences that followed would be even worse.
Superplumes of the type on which Yellowstone sits are rather like martini glasses--thin on the way up--spreading out as the near the surface to create vast bowls of unstable magma. Some of these bowls can be up to 1200 miles across. According to theories, they don't always erupt explosively but sometimes burst forth in a vast, continuous outpouring--a flood--of molten rock. Superplumes probably contributed to the demise of the dinosaurs, and may also be responsible for the rifts that cause continents to break up.
Such plumes are not all that rare. There are about thirty active ones on the Earth at the moment, and they are responsible for many of the world's best-known islands and island chains--Iceland, Hawaii, the Azores, Canaries and Galapagos archipelagoes, little Pitcairn in the middle of the South Pacific, and many others--but apart from Yellowstone they are all oceanic. No one has the faintest idea how or why Yellowstone's ended up beneath a continental plate. Only two things are certain: that the crust at Yellowstone is thin and that the world beneath it is hot. But whether the crust is thin because of the hotspot, or whether the hot spot is there because the crust is thin is a matter of heated (as it were) debate. The continental nature of the crust makes a huge difference to its eruptions. Where other supervolcanoes tend to bubble away steadily and in comparatively benign fashion, Yellowstone blows explosively. It doesn't happen often, but when it does you want to stand well back.
Since its first know eruption 16.5 million years ago, it has blown up about a hundred times, but the most recent three eruptions are the ones that get written about. The last eruption was a thousand times greater than that of Mount St. Helens; the one before that was 280 times bigger, and the one before that was so big that nobody knows exactly how big it was. It was at least twenty-five hundred times greater than St. Helens, but perhaps eight thousand times more monstrous.
We have absolutely nothing to compare it to. The biggest blast in recent times was that of Krakatau in Indonesia in August 1883, which made a bang that reverberated around the world for nine days, and made water slosh as far away as the English Channel. But if you imagine the volume of ejected material from Krakatau as being about the size of a golf ball, then the biggest of the Yellowstone blasts would be the size of a sphere you could just about hide behind. On this scale, Mount St. Helens would be no more than a pea.
The Yellowstone eruption of two million years ago put out enough ash to bury New York State to a depth of sixty-seven feet or California to a depth of twenty. This ash made fossil beds in eastern Nebraska. That blast occurred in what is now Idaho, but over millions of years, at a rate of about one inch a year, the Earth's crust has traveled over it, so that today it is directly under northwest Wyoming. In its wake it leaves the sort of rich volcanic plains that are ideal for growing potatoes, as Idaho's farmers long ago discovered. In other two million years, geologists like to joke, Yellowstone will be producing French fries for McDonald's, and the people of Billings, Montana, will be stepping around geysers.
The ash fall from the last Yellowstone eruption covered all or parts of nineteen western states (plus parts of Canada and Mexico)--nearly the whole of the United States west of the Mississippi. This, bear in mind, is the breadbasket of America, an area that produces roughly half the world's cereals. And ash, it is worth remembering, is not like a big snowfall that will melt in the spring. If you wanted to grow crops again, you would have to find some place to put all of the ash. It took thousands of workers eight months to clear the 1.8 billion tons of debris from the sixteen acres of the World Trade Center site in New York. Imagine what it would take to clear Kansas.
And that's not even to consider the climatic consequences. The last supervolcano eruption on Earth was at Toba, in northern Sumatra, seventy four thousand years ago. No one knows quite how big it was other than it was a whopper. Greenland ice cores show that the Toba blast was followed by at least six years of "volcanic winter" and goodness knows how many poor growing seasons after that. The event, it is thought, may have carried humans right to the brink of extinction, reducing the global population to no more than a few thousand individuals. There is some evidence to suggest that for the next twenty thousand years the total number of people on Earth was never more than a few thousand at any time. That is, needless to say, a long time to recover from a single volcanic blast.
All of this was hypothetically interesting until 1973, when an odd occurrence made it suddenly momentous: water in Yellowstone Lake, in the heart of the park, began to run over the banks at the lake's southern end, flooding a meadow, while at the opposite end of the lake the water mysteriously flowed away. Geologists did a hasty survey and discovered that a large area of the park had developed an ominous bulge. This was lifting up one end of the lake and causing water to run out at the other, as would happen if yo lifted one side of a child's wading pool. By 1984, the whole central region of the park--several dozen square miles--was more than three feet higher than it had been in 1924, when the park was last formally surveyed. Then in 1985, the whole of the central part of the park subsided by eight inches. It now seems to be swelling again.
The geologists realized that only one thing could cause this--a restless magma chamber. Yellowstone wasn't the site of an ancient supervolcano; it was the site of an active one. It was also at about this time that they were able to work out that the cycle of Yellowstone's eruptions averaged one massive blow every 600,000 years. The last one, interestingly enough, was 630,000 years ago. Yellowstone, it appears, is due.