So, as the plate moved over the hot spot, the string of islands that make up the Hawaiian Island chain were formed. The archipelago is made up of islands, atolls, reefs, shallow banks, shoals, and seamounts stretching over 1, miles from the island of Hawaii in the southeast to Kure Atoll in the northwest.
How did the Hawaiian Islands form? The Hawaiian Islands were formed by volcanic activity. One could just as easily, or perhaps more easily, fit a slowly varying curve to the data. This curve would indicated a small but significant slowing of plate velocity over the past 70 million years, a result that seems reasonable as the heat driving plate motion is slowly lost to space. The plates show a remarkable and abrupt bend about 44 millions years before the present.
There has been much speculation regarding the cause for this bend. Geophysicists generally agree that the bend originated with an abrupt change in plate motion. Prior to 44 million years ago the plates were moving in a much more northerly direction. There is considerable less agreement, however, on just what caused this bend.
Many scientists believe it was the collision of India with the Eurasian subcontinent, and event that has raised the Himalayan Mountains, that did the dirty deed. Other feel that it was the beginning of spreading on the Antartic Ridge south of Australia that was the culprit.
Whatever the cause, it is clear that there was a massive reorganization of plate motion nearly 50 million years ago. Perhaps it is even more amazing that in the past 65 million years there has been only one such bend. Even more remarkable is the observation that the straight portions of the chain are straight.
As we shall see below, the configuration of the plate boundaries in the Pacific have changed dramatically during the lifetime of the Hawaiian hotspot. If, as many geophysicists believe, subduction drives tectonics, then how on earth can the straight parts be so straight and move at constant velocities for tens of millions of years?
The answer to these questions remains a mystery! One other remarkable consistancy remains to be discussed. The eruption rate for Hawaiian volcanoes has remained quite constant over most of the 65 million years of preserved activity. This is shown by the figure on the left, where cumulated volume is plotted against distance along the chain.
For each distance, the volume plotted is the total amount of lava erupted previosly by all older volcanoes. Over the last 65 million years, about 1 million cubic kilometers of lava has been produced. This is enough lava to fill a box kilometers on a side. The width of the Big Island is roughly kilometers to give you some idea of how much material this represents.
If all this lava were somehow removed from the vicinity of the Hawaiian "hot spot", there should be a great hole in the bottom of the ocean nearly km in depth. Since there is no such hole, there must be some mechanism to replace the material lost and erupted to the surface.
Essentially this proves that there must be some kind of convection or fluid motion in the rocks that make up the Earth's upper mantle. It should be noted that for the 10 million years following the bend, very little lava erupted. This is a bit of a bad situation for the previous inhabitants of the islands, since there is very little other dry land for thousands of kilometers. Indeed, almost all of the previous life must have been exterminated, so that the current flora and fauna must have arrived more recently.
Although we don't have a clue as to why this cessation occured immediately following the reorganization of plate motion, its existence would seem to provide some powerful constraints on the models proposed for the "hot spot" discussed in the next lecture.
At present the age of the sea floor beneath the Big Island is roughly 95 millions years old. This means, that it was created at a mid-oceanic ridge 95 million years ago before being rafted to the central Pacific Ocean. This is shown in the figure on the left, where the difference in age between the volcanos and the underlying seafloor is plotted with distance along the island chain.
Curiously, this age difference does not change as far back in time as the bend, demonstrating that the active volcanoes were being built on sea floor somewhat more than 90 million years old for the past 44 million years. After the bend, however, the picture changes.
From the bend north along the Emperor chain the age difference steadily decreases until it is less than 10 million years for the oldest known volcanos in the chain. This means that these volcanos were being erupted on sea floor that was much younger than is the case today.
If the trend is continued back to about 80 million years, it would appear that the "hot spot" was building volcanoes on ocean floor of the same age. How can this be?
The answer, clearly, is that roughly 80 million years ago the Hawaiian "hot spot" was collocated with an oceanic ridge, much as the Iceland "hot spot" is today. But which ridge was it near. Current ocean floor beneath Hawaii was constructed at the Mid-Pacific rise just off the west coast of the Americas. To answer these questions we must reconstruct the configuration of the Pacific ocean floor going back nearly million years, a daunting task indeed. The situation during the past few million years, an eyeblink for a geologist, is shown in the figure on the left.
The plate is moving in a northwesterly direction, leaving a trail of distinct volcanos on the sea floor. The H marks the current "hot spot" position. Similarly, the Yellowstone "hot spot" is shown by a large block labelled with the letter Y. The image on the right shows the situation about 43 million years ago, just after the platemotion reorganized into the present configuration. Note the Emperor chain trending toward the North.
There is also a small piece of ridge off the Pacific Northwest known as the Kula Ridge. This ridge has since been subducted beneath Alaska, but then it was actively making oceanic lithosphere. The next image on the left shows the situation about 56 million years ago. The oldest volcanos of the chain that still exist today were then only about 10 million years old.
The Kula Ridge can now be seen to be much closer to the "hot spot", and consequently the age of the sea floor at the "hot spot" is much younger as discussed previously. The trend continues with the image on the right. Again, the Kula ridge is quite close to the Hawaiian "hot spot", and the ocean floor beneath the growing volcanoes is very young. This image shows the configuration when the oldest volcanoes that still exist today were just being formed. Remember, this is also the time of the extinction of the dinosaurs.
By studying the conditions in the deep sea around Loihi, researchers can better understand what other worlds in the solar system may look like.
Some hot spots produce volcanoes. Also called lithospheric plate. Also called an eruption column. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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These tectonic plates rest upon the convecting mantle, which causes them to move. The movements of these plates can account for noticeable geologic events such as earthquakes, volcanic eruptions, and more subtle yet sublime events, like the building of mountains.
Teach your students about plate tectonics using these classroom resources. A hot spot is an area on Earth over a mantle plume or an area under the rocky outer layer of Earth, called the crust, where magma is hotter than surrounding magma.
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