Lecture 12 - Hawaiian Volcanoes

Outline

I. Hawaiian Islands as "Hot Spot" Trace

II. Hawaii Volcanoes National Park

III. Haleakala National Park

I. Hawaiian Islands as "Hot Spot" Trace

So far we have talked about stratovolcanoes that occur associated with subduction zones (also known as convergent plate boundaries).Submarine volcanoes are also very common in the middle of the oceans where new seafloor is being created at divergent plate boundaries. Sometimes these volcanoes extend above the ocean like in Iceland. The Hawaiian Islands do not occur at a plate boundary but are in the middle of the Pacific Plate. They are believed to have been created by a "hot spot", a plume of hot material rising through the mantle and partially melting peridotite to form basaltic magma that builds these islands. The main evidence for the hot spot origin of the Hawaiian Islands comes from knowledge of the motion of the Pacific Plate and the ages of the rocks making up the Hawaiian Islands. From accumulated geophysical evidence such as the relative motion across the San Andreas Fault, we know that presently the Pacific Plate is moving in a northwesterly direction with respect to North America. From radiometric dating of volcanic rocks on the different Hawaiian Islands, we also know that the islands get younger as you move from northwest to southeast. Putting this together suggests that there is a stationary source of heat melting mantle material and producing magma that builds the Hawaiian Islands and that the plate moving over this source produces a line of volcanoes getting younger in the opposite direction of the plate motion. Think of a candle held stationary beneath a flat piece of wood. If we hold the wood above the candle long enough, we will burn a hole in the wood. Once this hole develops, if we translate the wood to the right we will burn a second hole, and if we keep translating the wood slowly to the right we will burn many holes in the wood. The holes farthest to the left side of the wood (opposite to the direction of translation) will be the newest holes and the ones on the right side of the wood will be the oldest holes. The Hawaiian Islands are analogous to the holes burned in the wood.

II. Hawaii Volcanoes National Park

Of the main Hawaiian Islands (Kauai, Oahu, Maui and Hawaii) only the island of Hawaii is presently volcanically active. Maui is the location of Haleakala National Park and that volcano last erupted in the late 1700's. The island of Hawaii consists of five shield volcanoes (broad gentle slopes produced by successive flows of primarily basaltic lava) Mauna Loa and Kilauea, which are both active, Mauna Kea and Kohala, which have been inactive in historic times, and Haulalai, which last erupted in 1801. The active volcanoes of Mauna Loa and Kilauea are within Hawaii Volcanoes National Park.

The editor of a local newspaper, L.A. Thurston was important in preserving the lands around the volcanoes as park land. He was also important in establishing Hawaii Volcano Observatory which conducts continuous monitoring of the Hawaiian volcanoes. He together with a geologist from MIT chose Kilauea as the site of the observatory because of the constancy of its eruptions and the fact that you could get pretty close to the eruptions (they are not very violent like the Cascade eruptions). The observatory is within the park on the rim of Kilauea crater.

Active Volcanoes

Mauna Loa- For the past 150 years Mauna Loa has erupted an average of once every 7 years. It is the world's largest active volcano! It stands at 13,680 feet above sea level; however, there is almost18,000 feet of volcano below the ocean. Mauna Loa has a caldera at its summit that formed from collapse of the crater vent. In addition to caldera or summit eruptions, Mauna Loa also has rift eruptions along its northeast and southwest rift zones. Rift eruptions are where lava erupts along long linear cracks on the flanks of the volcano and flows down the mountainside. Mauna Loa's southwest rift reaches the ocean and at times lava is erupted into the ocean. Submarine eruptions produce structures called pillow lava because of the rounded structure of the lava that develops from very fast cooling of the outer portion of the lava by ocean water and the continued squeezing through of new magma. Most lava is the fluid smooth type called pahoehoe but some more viscous aa lava is also erupted from Hawaiian volcanoes. The pahoehoe flows often contain lava tubes and tunnels where the outside of the flow cools and hardens insulating the inside where hot lava flows without cooling. Tree molds are also common in areas of pahoehoe flows where the lava flows around trees, the heat kills the tree and the lava mold of thetree remains.

Major recent eruptions of Mauna Loa (in terms of volume of lava erupted) occurred in 1859 and 1950 and included both summit and rift eruptions.

Kilauea- Rising from the southeast flank of Mauna Loa is the smaller but presently more active Kilauea volcano. It's summit caldera called Halemaumau was also created by crater collapse. This caldera has often been filled with a lava lake. Kilauea experiences both summit and flank eruptions along its southwest and east rift zones. Flank eruptions build up cones and/or craters along a linear trend. Most recently, summit eruptions have been rare and most of the activity has concentrated along the east rift zone. Recent activity includes: 1959 Kilauea Iki summit eruption produced large lava fountaining, 1971 Mauna Iki southwest rift zone activity1969-1974-Mauna Ulu east rift activity, 1983-present- Pu'u' O'o east rift activity- longest and largest eruption in Kilauea's history, flows reach ocean and are creating new land, communities on east shore have been destroyed as well as a Park Visitor Center.

III. Haleakala National Park

Represents an older stage in the volcanic cycle than Mauna Loa or Kilauea. Shield building was followed by erosion and than a final(?)stage of volcanism. Originally part of Hawaii National Park, Haleakala National Park was designated a separate park in 1960. It was expanded to include the Kipahulu Valley in 1969. The island of Maui presently consists of 2 volcanoes, Haleakala and West Maui. Originally the islands of Molokai, Lanai, and Kahoolawe were connected to Maui and the island consisted of five volcanoes. Since the end of the last great ice age, sea level has risen and created the separate islands. The dominant topographic feature of Haleakala is the so-called crater, a 7 mile long, 3 mile wide and .5 mile deep depression. In fact, it is not a crater at all but the much smaller original crater was significantly enhanced by stream erosion toproduce the present topographic feature.

Geologic History:

1. About 2 million years ago six separate basalt shield volcanoes grew on the ocean floor and coalesced into a volcanic complex.
2. Volcanism subsided and stream erosion and mass wasting reduced the height of Haleakala by more than 2000 feet and created the large topographic depression known as Haleakala crater. Chemical weathering processes such as oxidation of iron bearing minerals has turned much of the black lava multicolored.
3. Renewed volcanism in the last few thousand years formed cinder cones such as Puu O Maui on the floor of the eroded volcano. The last documented activity was in 1790.

Other Features:

Vegetation is sparse at higher altitudes due to low precipitation and the porous nature of the volcanic rock that cannot retain the moisture. Yucca-like silverswords are commonly seen plants. In 1920,these plants were almost extinct but have since come back after protection of the Park.

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