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Alaska geology

03.10.2012  |  35× přečteno      vytisknout článek 

Alaska geology

The ever-changing character of Alaska is evident by the various geologic forces that we are reminded of everyday. Earthquake, volcanoes, mountains, and glaciers are ever-present reminders of Alaska´s dynamic and complex geologic composition. When one considers the formation of the land that we know of as Alaska, its creation has occurred relatively recently within the context of geologic time. Geologic time began 4.5 billion years ago with the formation of the earth and continues to present day.

Alaska has been pieced together in complex ways over millions of years. At this time, geologists divide Alaska into two parts of somewhat different origin. These two parts are North American Alaska and Accreted Alaska.

North American Alaska largely consists of those parts formed in North America, although they may be somewhat displaced from their original locations.

Accreted Alaska contains many parts that could be exotic to Alaska. Basically speaking, the North Slope, Brooks Range, and Yukon-Tanana Upland are North American Alaska; southern Alaska is Accreted Alaska. In some areas, the boundary between these two general types is controversial.

As many as fifty pieces (the precise number/s a controversial debate among geologists), or terranes, have been added onto each other in waves. In fact, the entire Pacific Coast of North America, from Baja California to the Alaska Peninsula, are terranes that have been grafted together. As the North America continental plate and the Pacific Plate push and slide against one another, Alaska has been on the receiving end of the terranes that have been pushed northward. These terrains were created by a variety of geologic processes, and have been pieced together during the last 220 million years.

Geologists are still working on trying to understand Alaska´s complex geology. The entire sequence of events that created Alaska cannot be documented. Some rocks of southwestern Alaska are as old as two billion years, but researchers are not sure if these rocks were formed in North America. In the interior of Alaska, there are rocks 500 million to one billion years old that were formed in ancestral North America. In the 1960s, researchers at the University of Alaska-Fairbanks found evidence that the Alaska Peninsula and regions in southwestern Alaska were formed much farther south from their present location. By using paleomagnetic signatures (when rocks form, magnetic particles in the rock align themselves with the earth´s magnetic field and their alignment can be measured to determine the latitude that the rock was formed at), geologists have determined that some terranes in Alaska were formed near the equator. The Alaska Peninsula and the Wrangell Mountains were formed in equatorial regions. In some other terranes, the complex nature of their formation prevents the use of paleomagnetic signatures, and thus determining the latitude of their origin.

Plate Tectonics

The earth´s crust is made up of a dozen or so large fragments called "plates". Most of these plates are more than a thousand miles across and more than 40 miles thick. The plates move steadily, but slowly, past each other at rates currently up to 4 inches per year. Continental plates are less dense and more buoyant than the oceanic plates. There are three ways that plates can move relative to each other:

1. The plates move towards each other and one (the denser one) dives underneath the other, which is called subduction. This is the situation in southern Alaska and along the Aleutian Islands where the Pacific plate dives beneath the North American plate.

2. The plates slide by each other. This is the geologic setting offshore of southeastern Alaska, where the North American plate and the Pacific plate slide past each other on the Fairweather-Queen Charlotte fault. Fault lines occur between plates as well as within a plate (e.g., the Denali fault in Denali NP).

3. The plates move away from each other. This is called a spreading zone and it occurs mostly in deep oceans.

The collision of tectonic plates also results in the folding, faulting, and thickening of rocks. Rocks of the continental plate are pushed upwards to form mountains. Throughout Alaska, the evidence of plate collisions is abundant by the presence of mountains. Seismic activity within mountain ranges may indicate ongoing collisions or post-collisional adjustments. On some occasions, mountains are also built a part of a subducted oceanic plate scrapes off and is grafted onto the continental plate. The Chugach Mountains are an example of a terrain that was grafted onto the existing continental plate.

Earthquakes

Earthquakes occur either due to subduction of one plate underneath another or due to two plates sliding by each other. The immense plates move at a steady rate, but at their edges the sliding motion is neither smooth nor constant. Plates tend to get stuck in places and when they are not slipping by each other they are "locked" together. Eventually, enough strain will build up, the locked section will break the two plates will slide past each other, and we experience an earthquake. The whole process is much like pulling a concrete block with a bungee cord. At first you pull on the bungee cord and it stretches out and the block does not move. Eventually, the pull on the cord is strong enough to get the block moving. It slides forward with a jerk and then stops. If you keep pulling the cycle repeats itself, just like the earthquake cycle.

The size of an earthquake is commonly stated in terms of magnitude. There are several ways that earthquake magnitude is expressed. The most famous was devised in 1934 by Charles F. Richter. On the Richter scale, each whole number step represents a thirty-fold increase in the size of seismic waves measured on a seismograph, a machine that measures how much the ground moves in an earthquake.

Alaska is the most seismically active state in the United States, with an average of 1,000 earthquakes per year that are 3.5 or more on the Richter scale. The most seismically active part of the state is the Aleutian Island chain. Seismic related to the Aleutian Islands extends into the Gulf of Alaska and north to near Denali (Mt. McKinley).

Along the edges of the Pacific Plate, tremendous is created and release. This area is called the "Ring of Fire," due to the seismic and volcanic activity that occurs along the edges of the plate. Seventy-five percent of the world´s earthquakes occur along the Ring of Fire, and eleven percent of the world´s earthquakes occur in Alaska. Alaska experienced three of the Top 10 worldwide earthquakes during the 20th century.

Between 1899 and 1995, ten earthquakes occurred within Alaska that equaled or exceeded magnitude of 8.0 on the Richter scale. More than 75 earthquakes occurred during that time frame that were magnitudes of 7.0 or greater on the Richter scale, with the most recent occurring in the Gulf of Alaska on March 6, 1988, that registered 7.6 on the Richter scale.

1964 Earthquake

On Good Friday, March 27, 1964, at 5:36 PM, the largest recorded earthquake in North America occurred in Alaska. At the time of the quake, it was rated between 8.4 and 8.6 on the Richter scale; however, its equivalent moment magnitude has been revised upward to 9.2 (new ways of measuring and ranking very large earthquakes have been developed by seismologists). The energy released by the Good Friday earthquake equals that of 73,000 Hiroshima-size atomic bombs. The epicenter of the earthquake was located beneath Miners Lake in northern Prince William Sound (about 70 miles east of Anchorage). The vibrational waves - which were felt 700 miles away - shook south-central Alaska for approximately 7 minutes. The 1964 earthquake moved the earth farther, both horizontally and vertically, than any other earthquake ever recorded except the 1960 Chilean earthquake. There were 12,000 after-shocks of 3.5 or greater during the 2.5 months after the original quake.

As a result of the earthquake, land west of Prince William Sound subsided. (22,000 square miles of land dropped as much as 5.4 feet), while land east of the epicenter was uplifted (12,000 square miles rose as much as 7.5 feet). The earthquake and tsunami (seismic ocean waves) killed 131 people, 115 of them Alaskans. Sixteen of the deaths occurred as a result of a tsunami that hit the coast of Oregon and California. The numerous tsunamis accounted for 119 of the 131 deaths that occurred. Many portions of Anchorage were hard-hit by the earthquake, as much of the soil that the city is built upon is subject to liquefaction. During an earthquake, some of the soil types act like a liquid as a result of the vibrations, and the liquefaction-affected soil cannot support structures. The soils are also prone to landslides, and many massive slides occurred in and around Anchorage during the earthquake.

Volcanoes

When an oceanic plate subducts underneath a continental plate, the oceanic plate dives down into the earth´s magma. The deeper it dives, the more it heats up until it melts. The molten rock is lighter than the surrounding magma and will move upwards. When it hits the earth´s crust it either forms a pocket in the hard rock of the crust cooling off slowly (this is called a pluton) or it breaks through the earth´s surface and spills out as a volcanic eruption. A volcano constitutes a vent, a pipe, a crater, and a cone. The vent is an opening at the earth´s surface. The pipe is a passageway in the volcano in which the magma rises through to the surface during an eruption. The crater is a bowl-shaped depression at the top of a volcano where volcanic materials like ashes and lava and other pyroclastic (mixture of hot gas and ash) material are released. Solidified lava and ashes form the cone. Layers of ashes and lava build the steep sided cone higher and higher.

Along the Ring of Fire, the zone where the Pacific plate subducts beneath other plates, volcanoes are a common occurrence; in fact, over half of the world´s active volcanoes are located here.

Alaska has more than 10% of the world´s identified volcanoes. Most of them are located in a volcanic belt that extends from Mount Spurr, about 80 miles west of Anchorage, to beyond Buldir Island in the western Aleutians. Alaska´s volcanic belt on the chain of volcanoes contains about 80 major centers with one or more volcanoes that have erupted in geologically recent times, and activity has been recorded at about 40 of these volcanoes since 1700. Pavlov Volcano on the Alaska Peninsula, Alaska´s most active volcano, has erupted about 40 times since 1790.

The largest eruption in the world in the 20th century occurred in 1912 at Novarupta on the Alaska Peninsula. An estimated 15 cubic kilometers of magma was explosively erupted during 60 hours beginning on June 6 - about 30 times the volume erupted by Mount St. Helens in 1980. The expulsion of such a large volume of magma excavated a funnel-shaped vent 1.2 miles wide and triggered the collapse of Mount Katmai volcano 6 miles away to form a summit caldera (depression) 200 feet deep and about 2 miles across. Extrusion of the lava dome, called Novarupta, near the center of the 1912 vent marked the end of the eruption. (If you read up on the climbing history of Denali, you will notice that the Parker/Browne expedition attempting to reach the summit in 1912 was turned away by a storm, which forced them off the mountain. After they got off, a big earthquake hit the region that changed the face of Denali forever and would have killed the climbing party had they still been on the mountain. This earthquake was caused by the Novarupta eruption.) Little was known about the spectacular effects of this great eruption until 1916, when a scientific expedition sponsored by National Geographic Society visited the area. To their amazement, they found a broad valley northwest of Novarupta marked by a flat plane of loose, "sandy" ash material from which thousands of jet streams were hissing. The eruption produced pyroclastic flows that swept about 14 miles down the upper Ukak River valley. The thickness of the resulting pumice and ash deposits is not known but may be as great as 600 feet. In 1916, the deposits were still hot enough to boil water and form countless steaming fumaroles; hence the expedition named this part of the Ukak River the "Valley of Ten Thousand Smokes."

Today, scientists are particularly concerned about volcanoes whose eruptions can affect the Cook Inlet region, where about 60% of Alaska´s population lives. Redoubt volcano (on a clear day is easily visible on your drive along Turnagain Arm, or in Anchorage), erupted for the fourth time in the 20th century on December 14, 1989. Following several days of strong explosive activity, a series of lava domes grew in Redoubt´s summit crater during the next four months. Most of the domes were destroyed by explosions or collapsed down the volcano´s north flank. Ash produced by the eruptions affected air traffic en route to Anchorage. Many domestic carriers suspended service to Alaska following major explosive events, and several international carriers temporarily rerouted flights around Alaska. On December 15, a jetliner en route to Japan encountered an ash cloud while descending into Anchorage. The plane quickly lost power in all four engines and dropped 12,000 feet in altitude before the pilots were able to restart the engines. The aircraft landed safely in Anchorage but it sustained more than $80 million in damage.

Lahars (mudflows formed by the mixing of volcanic particles and water) generated during the Redoubt eruption threatened an oil-storage facility located on the banks of the Drift River on the west side of Cook Inlet. Oil is pumped from more than a dozen wells in Cook Inlet to the facility and then loaded onto tankers, which docked just offshore. A lahar on January 2nd flooded part of the facility with neatly 3 feet of water, forcing a shutdown until workers could restore power. This and subsequent lahars prompted Cook Inlet Pipeline Company to temporarily halt oil production from some oil wells and reduce the amount of oil stored at the facility between tanker loadings.

Glaciers

In Alaska, the effects of glaciation, both from present valley glaciers and from the recent Ice Age, dominate the landscape. There are an estimated 100,000 glaciers in Alaska covering about 5% of the state, or 29,000 square miles! About three-quarters of all fresh water in Alaska is stored as glacial ice.

Most of Alaska´s glaciers do not occur in the north where temperatures are colder and the air is drier, but along the coast where snowfall is greatest. The Chugach and St. Elias Mountains rise abruptly from the Gulf of Alaska and contain the highest concentration of glaciers in the state. Mt. Marcus Baker, the highest mountain in the Chugach Range, reaches a height of 13,176 feet just a few miles from the sea. These high mountains intercept moisture-laden air from the gulf, resulting in snowfalls of over 100 feet annually. That´s not 100 inches, but 100 feet, of snowfall each winter - the height of a 10-story building! (Mt. Marcus Baker is located north of Prince William Sound along College Fjord.)

The largest glacier within Alaska is considered to be the Bering Glacier complex, which is about 2,250 square miles in size, and includes the Bagley Icefield. The Malaspina Glacier is the largest piedmont lob glacier in Alaska, and it is located along the coast of Wrangell-St. Elias National Park. At 850 square miles, the Malaspina Glacier is larger than the state of Rhode Island.

More on:http://www.alaskadenalitours.com/alaska_natural_history/alaska_geology.html

Geological map on: http://pubs.usgs.gov/ha/ha730/ch_n/N-AKtext1.html

ZDROJ: www.alaskadenalitours.com


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