![]() ![]() Using the information given on the map, it is a simple matter to calculatethe rate of movement (velocity) of the Pacific Plate. What does thismean with regard to the movement of the Pacific Plate since the beginningof the Cenozoic Era about 65 million years ago? What compass direction is the Pacific Platecurrently moving? Pick the closest compass direction from N, NE,E, SE, S, SW, W, NW.ġ.2 For approximately how longhas the Pacific Plate been moving in this current direction?ġ.3 The Emperor Seamounts arealigned along a different, more northerly, trend. The map gives the ages of selected islands andseamounts, and their distances from the Island of Hawaii.ġ.1 Notice that the islands andseamounts are aligned along what are referred to as the Hawaiian Ridgeand Emperor Seamounts. Here is a map showing the locationsof shield volcanoes and what used to be shield volcanoes (before they wereeroded), produced over the Hawaiian hot spot, which is situated beneaththe southern part of the Island of Hawaii.Īll that remains of most of the former shield volcanoes are completelysubmerged seamounts. ![]() Let's look atthe consequences of this process. Basically,no sooner does a shield volcano form than it is carried away by the movementof the plate on which the shield volcano is formed. Well, that doesn'tsound too blinding, but over the course of 100 million years this amountsto 2 billion cm of movement, or 20,000 km (12,400 miles). By comparison, tectonic platescan move at blinding speeds of up to 20 cm/year. ![]() Above such structures, the asthenosphereis unusually hot and pushed close to the surface, creating the ideal situationfor the production of enormous amounts of basaltic magma.įor practical purposes, these mantle plumes remain fixed in place inthe mesosphere (mantle below the asthenosphere) for long periods of geologictime up to 100's of millions of years. The terms, mantle plume and hot spot,are used synonymously to describe a localized, upwardly convecting blob-likemass of hot mesospheric mantle. As already notedin the introduction to this exercise, the source of the magma for thisenormous outpouring of basaltic lava is the asthenosphere above a mantleplume (hot spot). There is aneven bigger shield volcano, Olympus Mons, on Mars. The Island of Hawaiirepresents the largest volcanic structure on the Earth, and perhaps thesecond largest volcanic structure in the solar system. This photograph was taken from the south flank of MaunaKea, which is another one of the shield volcanoes that make up the island. Note the gently sloping flanks of of the shield. Mauna Loa (above) is one of the shield volcanoes that make up the Islandof Hawaii. (Courtesy of USGS, Hawaiian Volcano Observatory, photo by D. Click hereor on image for bigger picture. North slope of Mauna Loa, as seen from Mauna Kea. This upward movement is why the caldera is called resurgent, which means “risen Shield Volcanoes, The Island of Hawaii Over time, the refilling of the magma chamber pushes up the caldera floor. These broad, vast calderas result when very large magma chambers empty quite forcefully, causing a series of pyroclastic flows. The Kilauea caldera on Kilauea, one of the volcanoes that make up Hawai’i, is one example.Īnother type of caldera is a resurgent caldera. Over time, this creates a series of nested depressions. Lava flows from shield volcanoes more slowly and often at regular intervals. This often occurs with shield volcanoes, which are typically flatter and more gradually sloped. One of these eruptions was so large it formed an island in Crater Lake named Wizard Island.Ĭalderas such as Crater Lake and those in Yellowstone National Park result from dramatic eruptions, but slower eruptions can also create calderas. For several thousand years after this eruption, smaller volcanic eruptions continued inside the caldera. This caldera formed about 7,000 years ago when a stratovolcano, Mt. A famous example is Crater Lake, in Oregon. Some calderas form a lake as the bowl-shaped depression fills with water. Calderas vary in size from one to 100 kilometers (0.62 to 62 miles) in diameter. As a result, the sides and top of the volcano collapse inward. When the magma chamber empties, the support that the magma had provided inside the chamber disappears. During a volcanic eruption, magma present in the magma chamber underneath the volcano is expelled, often forcefully. A caldera is a large depression formed when a volcano erupts and collapses. ![]()
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