Hawaiian Islands. Lesson 13

Transcription

1 Hawaiian Islands Lesson 13 T he Hawaiian Islands are a chain of volcanic islands inside the Andesite Line that have formed over a stationary hot spot in the middle of the Pacific Plate. Hundreds of thousands of basalt flows accumulated to build the island s shield volcanoes. The Hawaiian shields are the largest mountains on Earth. Mauna Loa is the largest in volume and Mauna Kea is the highest measuring from the base to the mountain s summit. Mauna Kea is approximately 10,213 meters tall and Mount Everest in the Himalaya Mountains is approximately 8,828 meters tall. The Hawaiian shields dwarf stratovolcanoes on the continents. Shield volcanoes characteristically have a broad summit with a caldera formed by collapse. Flank eruptions produce calderas when magma drains out of shallow chambers beneath the summit. Craters and pit craters are smaller than calderas and form as collapse features. Rift zones radiate from summit calderas extending down the flanks of the volcanoes. Neighboring volcanoes influence the orientation of the rift zones. Rift zones are long ridges with open fissures, pit craters, cinder cones, and small volcanic shields. They extend down a volcano s flanks into the sea. Rift zones control the shape and size of shield volcanoes. Rift zones are weaknesses in the volcano. Faults in the rift zones are magma passages allowing molten rock to flow down the volcano s side and enlarge it. There are usually three rift zones on shield volcanoes 120 degrees apart. Two of the three fissures are major rift zones. Basaltic lava produced by shield volcanoes contains ferromagnesian minerals that originated in the mantle. These lava flows form more than 99% of the volcano s mass above sea level. High lava fountains and rivers of molten rock gushed out of Kilauea during the Puu Oo eruptions. Photo courtesy HVO/USGS Native Hawaiians first used the terms pahoehoe and aa to describe lava flows that occurred on their islands. Pahoehoe flows are characterized by smooth, billowy or ropy surfaces. Aa flows are rough and jagged. Many pahoehoe flows form rivers of molten rock or lava tubes before flowing into the ocean. After the two terms were introduced into scientific literature in the late 19 th century scientists worldwide began to use the terms to describe similar flows. Lava streams, lava lakes, lava cascades and lava falls are volcanic features created by rift and caldera eruptions. Lava jetting out of the ground during a flank eruption creates a river of molten

2 rock flowing into the ocean. Levees form as the edges of the flow cools containing the flowing lava like riverbanks in a stream. Lava streams that fall over the rim of a caldera form lava cascades or lava falls. Lava cools first on the outside of a lava flow during flank eruptions. Lava tubes form as the top surface and sides of a flow hardens containing the molten rock inside. Some historic lava flows traveled over 48 km before empting into the ocean. Geologists often take lava samples through skylights, openings in the roof of a lava tube. Fire hose entries are places where lava tubes extend to the ocean and lava pours from the lava tube directly into the sea. Some tubes are up to 16 km long and many meters across. The Volcanoes National Park Thurston Lava Tube is large enough for visitors to walk through. Ancient Hawaiians used other tubes for shelter and burial caves. A small short-lived lava fountain built this elongate cone of spatter around a vent located on the flank of a much larger spatter and cinder cone. Photo by T.N. Mattox HVO/USGS During the 1975 eruption of Mauna Loa, lava cascaded into the Lua Hou Crater forming a lava lake. Lava that cascades into preexisting craters forms an inactive lava lake. Inactive lava lakes form a hard crust on the surface of the lake within a few days. The lava beneath the crust takes years to cool and solidify. Lava erupting inside a caldera or crater produces active lava lakes. Active lava lakes crust over but the surface breaks up in response to lava movement beneath the crust. Scientists in 1971 studied tectonic plate movement by studying the crustal movement of magma in the Mauna Ulu crater. They looked for similarities with tectonic plate movement as the hardened crust on the lake broke apart and moved about due to hot magma bubbling up beneath the lakes surface. The slabs of crust were only a few centimeters thick and ranged in size from a meter to several meters across. The circulating lava caused the crustal slabs to move apart, slip

3 by or subduct beneath other slabs. The lake showed three types of plate boundaries: convergent, divergent and transform faults. The transform faults offset the spreading boundary slabs. Kilauea is the most active volcano on the planet. The volcano began the longest rift-zone eruption in historical time on Jan 3, Lava fountains formed in the rift zone 17 km below the summit caldera. Powerful eruptions threw molten rock 450 meters in the air building a cinder cone. A new vent in 1986 created a lava lake on top of a small shield volcano. The small shield continued to increase in size and was 60 meters tall and 1600 meters at the base by The eruption created 120 new hectares of land and destroyed 179 homes that were in the path of a lava flow. Mauna Loa is not as active as Kilauea and the eruptions on this volcano are shorter in duration. Each eruption produces a much larger volume of lava than Kilauea. Lava fountains began shooting into the air as fissures opened up across the caldera floor on March 25, A series of vents opened on the northeast rift zone and lava flowed down the slopes. The eruption lasted for three weeks and lava flows were within 6.5 km of buildings in Hilo when the eruption ceased. Mauna Loa is 375 times larger than Mount Hood. The sea floor beneath the volcano is deformed and looks like an inverted cone because of Mauna Loa s great weight. The portion of the mountain below the level of the seafloor is almost as large as the portion above. Eruptions on the big island of Hawaii are rarely dangerous and thousands of people flock to the area when a major eruption occurs. Visitors often park and walk two miles to watch an eruption. A party of warriors crossing the summit of Mauna Loa in 1790 died when a pyroclastic flow suddenly swept over them killing 80 people. Footsteps of the ill-fated party are still visible on a trail used by visitors. Lesson summary The Hawaiian Islands are a chain of volcanic islands formed over a stationary hot spot in the middle of the Pacific Plate. Scientists in 1971 studied the crustal movement of magma in the Mauna Ulu crater to look for similarities with tectonic plate movement. Lava streams, lava lakes, lava cascades and lava falls are volcanic features created by rift and caldera eruptions. The longest rift-zone eruption in historical times began on Kilauea in Mauna Loa is 375 times larger than Mount Hood and deforms the seafloor beneath the volcano.

4 Name Date Hawaiian Islands Quiz 13 Fill in the blanks using words from the Word Bank 1. and aa are the two main types of lava flows found on the islands. 2. Lava tubes form after the surface of pahoehoe flows over and harden. 3. The Hawaiian Islands are a of volcanic islands inside the Andesite line that have formed over a stationary hot spot. 4. Rift zones are long with open fissures, pit craters, cinder cones and small volcanic shields. 5. Hawaiian crossing the summit of Mauna Loa were struck by a pyroclastic flow in 1790 and eighty people were killed. 6. The islands have been built by the of hundred of thousands of basalt flows. 7. Lava flows form more than 99% of the volcano s above sea level. 8. Shield volcanoes characteristically have a broad with a caldera formed by collapse 9. There are usually rift zones on shield volcanoes that are about 120 degrees apart. 10. zones radiate from summit calderas extending down the flanks of shield volcanoes. Word Bank three mass pahoehoe chain rift accumulation ridges summit crust warriors

5 Fantasy Island Activity 13 Introduction Creating a fantasy island is an excellent way for you to review landforms. Many volcanic islands in the Pacific Ocean are unnamed. Pretend you are going on a voyage of discovery and find an uncharted island for this activity. Materials White drawing paper 12 x 18 inches Colored pencils Black ink pen for lettering Directions Step 1 Researching information 1. Look over areas of the Pacific Ocean where there are chains of islands. You have been on a voyage of discovery and have just found a new uncharted island. 2. You are going to draw the island and all the landforms found on the island. 3. Since you are the discoverer of the island, you must name it and all the physical features you find. 4. Be sure and include a volcano on your island. 5. Include at least twelve different landforms on your map.

6 Step 2 Mapping your island 1. Write the name of your island at the top of your map. 2. Make a compass to show the directions on your map. North is usually at the top of a map. 3. Sketch your island on the drawing paper. 4. Color your map with colored pencil. 5. Label each land feature with the black ink pen. Some land features you might include on your map. Archipelago A large group or chain of islands Bay Any inlet of an ocean, sea, or lake that extends into the land Beach The shore of an ocean or a lake that is covered with sand or pebbles Creek A small stream Coast Land along the ocean Cove A small sheltered indentation in the shoreline of a sea, lake or river Crater A bowl-shaped opening or depression at the top of a volcano Delta A deposit of sand, silt and pebbles that forms at the mouth of a river Dune A hill or ridge of sand heaped up by the wind Foothills A hilly area at the base of a mountain range Gorge A narrow passage between steep mountains or hills Hill A natural elevation of the Earth s surface that is smaller than a mountain Island A body of land completely surrounded by water Lake A body of water surrounded by land Meadow An area of level land where grass is grown and usually cut for hay Mesa A flat-topped hill with steeply sloping sides Mountain range A chain of connecting mountains Pass An opening through hills or mountains used as a route for highways or railroads Plateau A high flat area of land with steep sides Peninsula A piece of land nearly surrounded by water and attached to a larger area of land Strait A narrow strip of water between two bodies of land Valley An area of land between hills or mountains Volcano A mountain formed by lava and cinders that erupted through a crater