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Earth scientists believe that most earthquakes are caused by slow movements inside the Earth that push against the Earth's brittle, relatively thin outer layer, causing the rocks to break suddenly. This outer layer is fragmented into a number of pieces, called plates. Most earthquakes occur at the boundaries of these plates. In Washington State, the small Juan de Fuca plate off the coast of Washington, Oregon, and northern California is slowly moving eastward beneath a much larger plate that includes both the North American continent the land beneath part of the Atlantic Ocean. Plate motions in the Pacific Northwest result in shallow earthquakes widely distributed over Washington and deep earthquakes in the western parts of Washington and Oregon. The movement of the Juan de Fuca plate beneath the North America plate is in many respects similar to the movements of plates in South America, Mexico, Japan, and Alaska, where the world's largest earthquakes occur. Hawaiian Hot Spot > The plate tectonics theory is a starting point for understanding the forces within the Earth that cause earthquakes. Plates are thick slabs of rock that make up the outermost 100 kilometers or so of the Earth. Geologists use the term "tectonics" to describe deformation of the Earth's crust, the forces producing such deformation, and the geologic and structural features that result. Earthquakes occur only in the outer, brittle portions of these plates, where temperatures in the rock are relatively low. Deep in the Earth's interior, convection of the rocks, caused by temperature variations in the Earth, induces stresses that result in movement of the overlying plates. The rates of plate movements range from about 2 to 12 centimeters per year and can now be measured by precise surveying techniques. The stresses from convection can also deform the brittle portions of overlying plates, thereby storing tremendous energy within the plates. If the accumulating stress exceeds the strength of the rocks comprising these brittle zones, the rocks can break suddenly, releasing the stored elastic energy as an earthquake.
Hawaiian
"Hot Spot" Lava
Flows > From: Tilling, Heliker, and Wright, 1987, Eruptions of Hawaiian Volcanoes: Past, Present, and Future: Department of the Interior/U.S.Geological Survey Publication The great majority of the world's earthquakes and active volcanoes occur near the boundaries of the Earth's shifting plates. Why then are the Hawaiian volcanoes located near the middle of the Pacific Plate, more than 2,000 miles from the nearest plate boundary? In 1963, J.Tuzo Wilson, a Canadian geophysicist, provided an ingenious explanation within the framework of plate tectonics by proposing the "Hot Spot" hypothesis. Wilson's hypothesis has come to be accepted widely, because it agrees well with much of the scientific data on the Pacific Ocean in general, and the Hawaiian Islands in particular. According to Wilson, the distinctive linear shape of the Hawaiian-Emperor Chain reflects the progressive movement of the Pacific Plate over a deep immobile hot spot. This hot spot partly melts the region just below the overriding Pacific Plate, producing small, isolated blobs of magma. Less dense than the surrounding solid rock, the magma rises buoyantly through structurally weak zones and ultimately erupts as lava onto the ocean floor to form volcanoes. Over a span of about 70 million years, the combined processes of magma formation, eruption, and continuous movement of the Pacific Plate over the stationary hot spot have left the trail of volcanoes across the ocean floor that we now call the Hawaiian-Emperor Chain. Scientists interpret the sharp bend in the chain, about 2,200 miles northwest of the Big Island, as indicating a change in the direction of plate motion that occurred about 43 million years ago, as suggested by the ages of the volcanoes bracketing the bend. Part of the Big Island, the southeasternmost and youngest island, presently overlies the hot spot and still taps the magma source to feed its two currently active volcanoes, Kilauea and Mauna Loa. The active submarine volcano, Loihi, off the Big Island's south coast, may mark the beginning of the zone of magma formation at the southeastern edge of the hot spot. The other Hawaiian islands have moved northwestward beyond the hot spot, were successively cut off from the sustaining magma source, and are no longer volcanically active. The progressive northwesterly drift of the islands from their point of origin over the hot spot is well shown by the ages of the principal lava flows on the various Hawaiian Islands from northwest (oldest) to southeast (youngest), given in millions of years: Kauai, 5.6 to 3.8; Oahu, 3.4 to 2.2; Molokai, 1.8 to 1.3; Maui, 1.3 to 0.8; and Hawaii, less than 0.7 and still growing. Even on the Big Island alone, the relative ages of its five volcanoes are compatible with the hot-spot theory. Kohala, at the northwestern corner of the island, is the oldest, having ceased eruptive activity about 60,000 years ago. The second oldest is Mauna Kea, which last erupted about 3,000 years ago; next is Hualalai, which has had only one historic eruption (1800-1801), and lastly, both Mauna Loa and Kilauea have been vigorously and repeatedly active in historic times. Because it is growing on the southeastern flank of Mauna Loa, Kilauea is believed to be younger than its huge neighbor. The size of the Hawaiian hot spot is not know precisely, but it presumably is large enough to encompass the currently active volcanoes of Mauna Loa, Kilauea, Loihi, and, possibly, also Hualalai and Haleakala. Some scientists have estimated the Hawaiian hot spot to be about 200 miles across, with much narrower vertical passageways that feed magma to the individual volcanoes. Three major types of plate boundaries are recognized. These are called spreading, convergent, or transform, depending on whether the plates move away from, toward, or laterally past one another, respectively. Subduction occurs where one plate converges toward another plate, moves beneath it, and plunges as much as several hundred kilometers into the Earth's interior. The Juan de Fuca plate off the coasts of Washington and Oregon is subducting beneath North America. Ninety percent of the world's earthquakes occur along plate boundaries where the rocks are usually weaker and yield more readily to stress than do the rocks within a plate. The remaining 10 percent occur in areas away from present plate boundaries -- like the great New Madrid, Missouri, earthquakes of 1811 and 1812, felt over at least 3.2 million square kilometers, which occurred in a region of southeast Missouri that continues to show seismic activity today. The Cascadia subduction zone off the coast of Washington, Oregon, and northern California is a convergent boundary between the large North America plate and the small Juan de Fuca plate to the west. The Juan de Fuca plate moves northeastward and then plunges (subducts) obliquely beneath the North America plate at a rate of 3 to 4 centimeters per year. In sum, the subduction of the Juan de Fuca plate beneath the North America plate is believed to directly or indirectly cause most of the earthquakes and young geologic features in Washington and Oregon.
What
are Lava Flows? Volcano
Observatory > For up to date eruption information click here. Lava flows are streams of molten rock that either pour from a vent quietly or explosively by lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. The speed at which lava moves across the ground depends on several factors, including the type of lava erupted (viscosity), the steepness of the ground, and the rate of lava production at the vent. Fluid basalt flows may extend tens of kilometers from their source, and the leading edges of basalt flows can travel as fast as 10 km/hour on steep slopes. The flow front of a basalt flow on a shallow slope typically advances less than 1 km/hour. Where basalt lava flows are confined within channels or lava tubes, however, velocities can reach over 30 km/hour. Viscous andesite flows move only a few kilometers per hour and rarely extend more than 8 km from their vents. Viscous dacite and rhyolite flows often form steep-sided mounds called lava domes right over an erupting vent. Lava domes often grow by the extrusion of many individual flows over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour. Lava flows in two distinct types, for which the Hawaiian names have become universal geological terms: a'a and pa'hoehoe. They're easily distinguished in appearance, but chemically they're the same. A'a is extremely rough and spiny and will quickly tear up your shoes if you do much hiking over it. Also, if you have the misfortune to fall down, you'll immediately know why they call it a'a. Pa'hoehoe, a billowy ropey lava that looks like burned pancake batter, can mold itself into fantastic shapes. Examples of both lavas are frequently encountered on various hikes throughout the Big Island. Other lava oddities that you may spot are peridots, green gem-like stones called "Pele's Diamonds," clear feldspar like white cotton candy called "Pele's hair," and gray lichens covering the older flows known as "Hawaiian snow."
Kilauea
Volcano: lava-flow hazards where lava enters the ocean.
About
the Hawaiian Volcano Observatory The Hawaiian Volcano Observatory (HVO) enjoys a world-wide reputation as a leader in the study of active volcanism. Due to their usually benign natures, Kilauea and Mauna Loa, the most active volcanoes on the Island of Hawai`i, can be studied up close in relative safety. While observations made by 19th-century missionaries and travelers constitute a large part of the early and colorful history of volcano watching in Hawai`i, HVO's origins are rooted in a desire to use scientific methodology to understand the nature of volcanic processes and to reduce their risks to society. What We Do HVO is part of the Volcano Hazards Program of the U.S. Geological Survey. Our staff conducts research on the volcanoes of Hawai`i and works with emergency-response officials to protect people and property from earthquakes and volcano-related hazards. We work to reduce the risks from these hazards by: Monitoring volcanoes and earthquakes to track their behavior before, during, and after eruptions and to determine the nature of their activity. Studying the eruption histories of Hawai`i's volcanoes in order to achieve a long-term perspective that can help to anticipate their future behavior and identify potentially hazardous areas. Communicating results of our studies with the public, emergency managers, educators, and students through the media, presentations and workshops, field trips, and the USGS Volunteer Program. Knowledge of past eruptions and earthquakes, and careful monitoring of ongoing activity, form the basis of our current hazard assessments in Hawai`i and our studies of volcanic and seismic processes. By keeping abreast of what is happening, and by knowing what has happened, we can coexist with Hawai`i's active volcanoes, living, working, and playing on them and minimizing their dangers to people and property.
Early
Hawaiian History The
Makahiki > Settlers Original settlers of Polynesia migrated through South-East Asia and Indonesia across Melanesia, before settling the Polynesian islands from 1000 BC to 500 AD. Hawaii was one of the last island groups to be settled. Archaeological evidence indicates the first Polynesians arrived in Hawaii from the Marquesas between 500 and 700 AD. The first wave of Tahitians arrived in Hawaii in about 1000 AD conquering and subjugating the Marquesans, forcing them to build temples, irrigation ditches and fishponds. The menehune legends of a tribe of little people may well refer to the Marquesans for the word ‘menehune’ is very similar to the Tahitian word for ‘outcast’. The earliest Hawaiians had simple beliefs which were the result of being in tune with the spirits of nature. Offerings to their gods consisted mainly of praying and a sharing of their harvest. The Hawaiians had gods for all natural phenomena, consisting of the four main gods: Ku, Lono, Kane, and Kanaloa. Ku was the ancestor god and took charge of the male gods and Hina took charge of the females. They were responsible for heaven and earth, fishing, forests, and farming. Lono was the god of the rain, the harvest, fertility, and peace. Kane created the first man and was the god from which all Hawaiians descended. Kanaloa was the god of the underworld and ruler of the dead. Under these main four were many lesser gods. Temples erected in ancient Hawaii, called heiaus, were built in two basic styles using lava rock. One was a rectangular enclosure built directly on the ground, the other consisted of raised terraced platforms with rocks piled high. Many of these heiaus can still be found throughout the islands today and are considered sacred places. According to legend, the god Lono, after a spiff with a chief who was lusting after Kaikilani, his wife, killed her and set sail on a canoe with a tall mast hung with sails and promised to return one day on a “floating island”. The Hawaiians remembered Lono each year with a harvest festival, called the makahiki, which lasted from October to February. Even during wartime, fighting would be suspended for the festivities dedicated to Lono.
The
Polynesian Settlement of the Pacific The Polynesian migration to Hawai'i was part of one of the most remarkable achievements of humanity: the discovery and settlement of the remote, widely scattered islands of the central Pacific. The migration began before the birth of Christ. While Europeans were sailing close to the coastlines of continents before developing navigational instruments that would allow them to venture onto the open ocean, voyagers from Fiji, Tonga, and Samoa began to settle islands in an ocean area of over 10 million square miles. The settlement took a thousand years to complete and involved finding and fixing in mind the position of islands, sometimes less than a mile in diameter on which the highest landmark was a coconut tree. By the time European explorers entered the Pacific Ocean in the 16th century almost all the habitable islands had been settled for hundreds of years. The voyaging was all the more remarkable in that it was done in canoes built with tools of stone, bone, and coral. The canoes were navigated without instruments by expert seafarers who depended on their observations of the ocean and sky and traditional knowledge of the patterns of nature for clues to the direction and location of islands. The canoe hulls were dug out from tree trunks with adzes or made from planks sewn together with a cordage of coconut fiber twisted into strands and braided for strength. Cracks and seams were sealed with coconut fibers and sap from breadfruit or other trees. An outrigger was attached to a single hull for greater stability on the ocean; two hulls were lashed together with crossbeams and a deck added between the hulls to create double canoes capable of voyaging long distances. The canoes were paddled when there was no wind and sailed when there was; the sails were woven from coconut or pandanus leaves. These vessels were seaworthy enough to make voyages of over 2,000 miles along the longest sea roads of Polynesia, such as the one between Hawai'i and Tahiti. And though these double-hulled canoes had less carrying capacity than the broad-beamed ships of the European explorers, the Polynesian canoes were faster: one of Captain Cook's crew estimated a Tongan canoe could sail "three miles to our two." After a visit to the Society Islands in 1774, Andia y Varela described the canoes he saw: "It would give the most skillful [European] builder a shock to see craft having no more breadth of beam than three [arm] spans carrying a spread of sail so large as to befit one of ours with a beam of eight or ten spans, and which, though without means of lowering or furling the sail, make sport of the winds and waves during a gale, their safety depending wholly on two light poles a couple of varas or so long (about eight feet), which, being placed athwartships, the one forward and the other aft, are fitted to another spar of soft wood placed fore and aft wise in the manner of an outrigger. These canoes are as fine forward as the edge of a knife, so that they travel faster than the swiftest of our vessels; and they are marvellous, not only in this respect, but for their smartness in shifting from one tack to the other." (Corney, Vol. II, 282). The voyaging was by no means easy. There was always a danger of swamping or capsizing in heavy seas, of having sails ripped apart or masts and booms broken by fierce winds, of smashing the hulls against unseen rocks or reefs; and while there were grass or leaf shelters on the decks of voyaging canoes, the voyagers were often exposed to the wind, rain, and sun, with only capes of leaves or bark-cloth wrappings for protection. A stormy night at sea, even in the tropics, can be brutally chilling. If supplies ran short during a long voyage, and no fish or rainwater replenished them, then starvation became a possibility. As a tradition about a voyage from Hiva (the Marquesas) to Rarotonga puts it: "The voyage was so long; food and water ran out. One hundred of the paddlers died; forty men remained." A long voyage was not just a physical, but a mental challenge as well, particularly for a navigator without compass or chart. To navigate miles of open ocean required an extensive and intimate knowledge of the ocean and sky. Captain Cook noted that Polynesian navigators used the rising and setting points of celestial bodies for directions. Andia y Varela was told how Tahitians also used the winds and swells to hold a course: "There are many sailing-masters among the people, the term for whom is in their language fa'atere (Hawaiian: ho'okele). The fa'atere are competent to make long voyages like that from Otahiti to Oriayatea [Ra'iatea] (about 150 miles) and others farther afield. One of these sailing masters named Puhoro came to Lima on this occasion in t he frigate; and from him and others I was able to find out the method by which they navigate on the high seas. "They have no mariner's compass, but divide the horizon into sixteen parts, taking for the cardinal points those at which the sun rises and sets. "When setting out from port the helmsman partitions the horizon, counting from E, or the point where the sun rises; he knows the direction in which his destination bears. He observes, also, whether he has the wind aft, or on one or the other beam, or on t he quarter, or is close-hauled. He notes, further, whether there is a following sea, a head sea, a beam sea, or if the sea is on the bow or the quarter. He proceeds out of port with a knowledge of these [conditions], heads his vessel according to his calculation, and aided by the signs the sea and wind afford him, does his best to keep steadily on his course. "The task becomes more difficult if the day is cloudy, because the sailing-master has no mark to count from for dividing the horizon. Should the night be cloudy as well, the sailing-master regulates his course by the wind and swells; and, since the wind i s apt to vary in direction more than the swell does, he has his pennant, made of feathers and palmetto bark, by which to watch changes in the wind, and he trims his sails accordingly, always taking his cue for holding his course from the indications the sea affords. When the night is clear, he steers by the stars; and this is the easiest navigation for him because he knows the stars which rise and set over not only the islands he is familiar with, but also the harbors in the islands, so that he makes straight for the entrance by following the rhumb of the particular star that rises or sets over it. These sailing masters hit their destinations with as much precision as the most expert navigators of civilized nations could achieve" (Corney, Vol. II, 284-6) . To keep track of their position at sea during long sea voyages, the navigators used a system of dead reckoning and memorizing the distance and direction traveled until the destination was reached. Finding islands before they could actually be seen was also part of the art of navigation. Voyagers followed the flight of land-dwelling birds that fished at sea as these birds flew from the direction of islands in the morning or returned in the evenings. The navigators also watched for changes in swell patterns, cloud piled up over land, reflections on clouds from lagoons, and drifting land vegetation. When European explorers found the islands of Polynesia, the common ancestry of the Polynesians was evidently the inhabitants of widely separated islands looked alike, spoke alike, and had similar cultural practices. Their manufactured products such as fishhooks, trolling lures, adzes, and ornaments also revealed similarities. And they had the same basic stock of domesticated plants and animals. |
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