Textbook theory behind volcanoes may be wrong
She has brought me into the valley west of Hillsborough on land owned by the San About halfway down the hill, the continuous slope flattens out a little bit, only to Cracks in the earth have been filled; burrowing animals have turned the soil The much cooler and less dense plates of the earth's surface float on these . The name of an underground mountain range in central Texas. Fault line. A crack in the Earth's surface where two plates meet. Upgrade to remove ads. Only $1/. Yet violent earthquakes related to plate tectonics have caused One such area is the circum-Pacific Ring of Fire, where the Pacific Plate meets many surrounding plates. . With a thrust fault, whose plane is inclined to the Earth's surface, cracked by frequent earthquake activity in the recent geologic past.
While the top of the mantle is a sort of fluid sludge, the uppermost layer is rigid rock, broken up into plates that float on the magma-bearing layers.
Magma from the mantle beneath the plates bursts through the plate to create volcanoes.
As the plates drift across the hot spots, a chain of volcanoes forms—such as the island chains of Hawaii and Samoa. To look for the hypothetical plumes, researchers analyze global seismic activity. Everything from big quakes to tiny tremors sends seismic waves echoing through Earth's interior.
The type of material that the waves pass through influences the properties of those waves, such as their speeds. By measuring those waves using hundreds of seismic stations installed on the surface, near places such as Hawaii, Iceland, and Yellowstone National Park, researchers can deduce whether there are narrow mantle plumes or whether volcanoes are simply created from magma that's absorbed in the sponge-like shallower mantle.
No one has been able to detect the predicted narrow plumes, although the evidence has not been conclusive.
The jets could have simply been too thin to be seen, Anderson says. Very broad features beneath the surface have been interpreted as plumes or super-plumes, but, still, they're far too wide to be considered narrow jets.
But now, thanks in part to more seismic stations spaced closer together and improved theory, analysis of the planet's seismology is good enough to confirm that there are no narrow mantle plumes, Anderson and Natland say. Instead, data reveal that there are large, slow, upward-moving chunks of mantle a thousand kilometers wide.
In the mantle-plume theory, Anderson explains, the heat that is transferred upward via jets is balanced by the slower downward motion of cooled, broad, uniform chunks of mantle. The behavior is similar to that of a lava lamp, in which blobs of wax are heated from below and then rise before cooling and falling.
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But a fundamental problem with this picture is that lava lamps require electricity, he says, and that is an outside energy source that an isolated planet like Earth does not have. The new measurements suggest that what is really happening is just the opposite: Instead of narrow jets, there are broad upwellings, which are balanced by narrow channels of sinking material called slabs.
Textbook theory behind volcanoes may be wrong
What is driving this motion is not heat from the core, but cooling at Earth's surface. In fact, Anderson says, the behavior is the regular mantle convection first proposed more than a century ago by Lord Kelvin. When material in the planet's crust cools, it sinks, displacing material deeper in the mantle and forcing it upward. On a geologic scale, in which time is measured in millions of years, such extremely slow flow rates add up to form convection currents in the mantle, similar to the ones you can see in a pot of soup heated on a stove.
Some think the currents carry the plates above them along; others say that the plates are pulled by their own weight, as they sink downward into the mantle in subduction zones. Whatever the driving force, the plates with their jagged boundaries and corners ram into each other or push each other out of the way. Such violent plate interactions have played a major role in the geologic history of the San Francisco Bay Area.
The story began million years ago in the Early Cretaceous epoch. Another spreading center, similar to that in the Atlantic, existed in the Pacific Ocean far to the southwest of what is now the west coast of North America.
California, as we know it today, did not exist then. It was actually part of the deep ocean. At this spreading center in the Pacific, molten rock oozed out of the mantle. As a consequence, what is now known as the Pacific Plate moved toward the northwest. As in most tectonic collision zones where a continental and an oceanic plate ram into each other, the latter is forced underneath the former. Its remnants—the Juan de Fuca Plate to the north and the Cocos Plate to the south—continue to be subducted under the North American plate.
What term is used to describe cracks in the earth's surface where two tectonic plates meet
For the diving plate, subduction is a rather traumatic process. The deeper it sinks into the mantle, the more it melts. In addition, as it passes under the sharp leading edge of the massive and therefore rather stable continental plate, pieces of the top layer of rocks are skimmed off the subducting plate, much as a craftsman shaves off the surface of a rough piece of lumber with a plane.
In the case of the Farallon Plate, the shaved-off rocks began to accumulate at the leading edge of the continent and eventually formed a large component of the coast ranges of Central California. Elements of these shaved-off deep sea sediments and volcanics, known collectively as the Franciscan Complex, are exposed in many places around the Bay Area, including in the Marin Headlands just north of the Golden Gate. However, about 29 million years ago, the nature of the plate collision along the western edge of North America began to change.
By then, almost the entire Farallon Plate had been consumed under the North American continent, which slowly but surely began to override the Pacific spreading center, encountering a new tectonic partner—the Pacific Plate.
With that encounter, the nature of the plate boundary underwent a fundamental shift. The subduction caused by the almost head-on collision between the North American and Farallon plates stopped. As the North American Plate started to override the Pacific spreading center, the northwestward motion of the Pacific Plate began to take over the interaction between the two.
This type of interaction remains the strongest tectonic driving force in the Bay Area today.
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The slipping and sliding of rocks along the San Andreas Fault, however, is not a smooth and gentle process. Indeed, the Pacific Plate scrapes and grinds against the North American continent, sometimes grabbing pieces, ripping them off, and carrying them along. The Point Reyes peninsula has been carried northward by this motion for more than miles and as the process goes on, 10 million years or so from now, Los Angeles—which sits on the Pacific Plate—might indeed become a suburb of San Francisco.
Neither Lawson nor any of his colleagues knew about this movement when they investigated the great earthquake of Nevertheless, their report was so thorough and the scientific conclusions presented so far-reaching that their work is regarded as the birth of modern seismology in the United States. While looking at the data, Henry Fielding Reid, a professor of geology at Johns Hopkins University, concluded that the rocks along the San Andreas behave somewhat like an elastic rubber band.
Once the force has reached a threshold, the rubber band snaps. So do the rocks along the fault: Once the threshold is reached, they give way in an earthquake and rebound several feet within a few seconds, thereby releasing the load, or strain, that had accumulated within them for dozens or even hundreds of years.
The constant motion of the plates in opposite directions loads sections of the fault until the strain becomes so large that the friction holding the rocks is overcome, resulting in an earthquake. As long as the plates continue to move, earthquakes will happen. The San Andreas Fault has splintered into many strands, resulting in a broad contact or fault zone in the Bay Area, stretching from the east side of the Diablo Range to the coast.
One of its sister faults, the Hayward, runs along the west side of the East Bay hills. Farther to the east lies the Calaveras Fault; to the west we find the San Gregorio. These are all part of what is called the San Andreas Fault System. The different blocks between these faults have been—and are still being—shifted like pieces of a jigsaw puzzle.
Almost imperceptible in everyday life—except when an earthquake strikes—these forces have shaped and altered the landscape of the Bay Area.
During the last 2, years alone, the mere blink of an eye in geologic terms, the regions have moved horizontally relative to each other by almost 80 yards as the Pacific Plate slides past the North American.
In addition, the whole region is being squeezed by a slight shift in the relative motion between the local components of the North American and Pacific Plates, so that the relationship between them is no longer strictly strike-slip, but contains a measure of compression or collision as well, representing about 10 percent of the relative motion, according to recent measurements.
These very young mountains are being pushed ever higher, by dozens of inches over the last two millennia alone.Jugs and Rods - Critical Role RPG Episode 94
If it were not for the erosion caused by winter rains, their growth rate would be even greater. The horizontal movements of the various blocks along the San Andreas system occur at roughly the same speed at which a fingernail grows. Until very recently, it was quite a technical challenge to measure such extremely slow movement.
Repeated measurements at dozens of fixed sites in the Bay Area have led to a detailed map of the movement of Northern California. Instead, when subjected to tectonic forces, it gives a little, tries to absorb a little as well, breaks here and there. Therefore each of the various northwest-southeast-trending faults in the Bay Area takes up some of the movement. On the geologic scale, such movements along the faults are remarkably fast. The ongoing change makes the Bay Area—indeed all of Northern California—a very young landscape.
No mountain or hill in this region is more than a few million years old. Compared to the East Coast, to the vast stretches of prairie in the Midwest, or the sequence of rocks exposed in the Grand Canyon, the region around San Francisco is just a youngster in the geology of the United States.
In geologic terms, the young landscape here seems as dynamic as the high-tech industry of Silicon Valley, and the geologic features are as diverse as the multiethnic population, drawn to the region by its natural, cultural, and economic riches.
Living on Shaky Ground A timely wake-up call? This fact is too easily ignored, as the earth under Northern California has been suspiciously quiet since the great earthquake of —except for the Loma Prieta quake in October Earthquake hazards geologist Mary Lou Zoback and her colleagues at the USGS have studied the history of temblors in the Bay Area and found that during the 70 years before the region sustained 18 destructive quakes of magnitude 6.
Between the arrival of the Spanish padres in —marking the beginning of our historical record—andthe earth under Northern California was also rather quiet without any notable temblors.