THE NATURAL ENVIRONMENT

Geography 101

     

 

ToC

MOUNTAINS

Crust

Folding

Earthquakes

Volcanoes

Hawai'i

 

Hawaiian Volcanoes

 
  1. Which volcanoes are active in Hawai'i?
  2. What are rift zones?
  3. What is a lava hazard map and what areas are the most hazardous?
  4. Which volcanoes are dormant in Hawai'i? Which are extinct?
  5. What is the rejuvenated stage of volcanism and what are some examples of it?
 
BOX 1

As noted in earlier sections, the Hawaiian Islands were created by volcanism from plumes of magma rising over a mantle hot spot (see Chapter 9 -> Hot Spots). Hot spots produce mafic magma that hardens at the surface into basalt; hence the vast majority of the Islands' rocky bulk is built of this material.

Hot spot volcanism created all of the Islands and each is composed of one or more volcanic shields as shown in the map.

location of hawaiian volcanoes

Active Volcanoes

landsat image of big islandVolcanoes are considered active when they are either in the process of erupting or showing signs of an impending eruption. Three Hawaiian volcanoes fit this description. Mauna Loa, the world's largest mountain, last erupted in 1984, sending lava to within seven kilometers (four miles) of Hilo town. Kilauea, the world's most active volcano, has erupted continuously since 1983. Loihi, an underwater volcano 110 kilometers (70 miles) south of the Big Island, has erupted sporadically since 1996.

lava enters the oceanThe satellite image of the Big Island highlights the areas of recent lava flows and bare lava fields in black and dark blue. The latest eruptions of Kilauea have occurred east of the central Halemaumau crater. The easternmost flow draining into the ocean shown in the image originated several miles inland at the Pu'u O'o vent, a source that has since become inactive.

All of these active volcanoes appear to be connected directly to the underlying hot spot magma plume and evidence suggests that eruptions from one of the mountains may affect activity in the others. Mauna Loa, for example, has been fairly quiet since the onset of Kilauea's latest series of eruptions.

kilauea southwest rift zoneThe active Big Island volcanoes are among the most studied on Earth. In 1912, the Hawaii Volcano Observatory was established on the rim of Kiluaea caldera as one of the earliest science stations of its kind on Earth. Interest in understanding the inner workings of volcanoes at the time was high due to a catastrophic 1902 eruption that killed over 30,000 people on the island of Martinique in the Caribbean. In an interesting parallel to Hawaiian legend, the name of that volcano is Pelée.

lava hazard map of the big islandMuch has been learned over the past century. The two active mountains, for example, are split by rift zones, which are shown as purple stripes on the hazard map. These huge cracks essentially separate the northern and southern flanks of the mountains and are frequent sites of eruptive activity. The current eruption of Kilauea, for example, has taken place largely in the East rift zone with occasional activity in the central caldera.

lava flow through royal gardens subdivision (zone 2)Because Kilauea and Mauna Loa are active, lava flows often create a hazard, posing a threat to human life and property. This has prompted the US Geologic Survey to create a lava hazard map. The map objectively shows the chances of land being covered with lava over various periods of time. At highest risk are areas directly atop the rift zones and directly downslope from them (1 and 2 on the map). Zone 3 areas are partially protected by topography. This map is somewhat controversial, not in its scientific basis, but because it has been used to set insurance rates on the Big Island.

Dormant and Extinct Volcanoes

haleakala lava flow of 1790Volcanoes that have erupted in historic time, but currently show no evidence that new activity is imminent, are referred to as dormant. (Note: some geologists would consider dormant volcanoes to be active.) Two Hawaiian mountains fit this description. Hualalai, on the Big Island last erupted in 1800 and 1801 sending two long flows of lava down its flanks that ultimately reached the ocean. Haleakala, on Maui, last erupted around 1790. The flow from this event entered the ocean on the south shore of Maui in the La Perouse Bay area.

The other Hawaiian volcanoes are considered extinct, meaning that they have not erupted in historic times and are not expected to erupt anytime in the near future. This does not mean that they have been entirely quiet, however.

A rejuvenation phase of volcanism can still produce eruptions millions of years after the Islands have moved away from the hot spot. Blobs of magma, cut off from their mantle source, continue to slowly rise through the lithosphere by buoyancy. When they reach the surface they produce small eruptions and tuff cones. These small cones exist on most of the major Islands. On O'ahu, Diamond Head, Punchbowl, and Manana (Rabbit) Island are examples. These apparently formed within the past 100,000 years or so. Lehua Island (below) near Ni'ihau is another example of rejuvenated volcanism on the flanks of extinct volcanoes in Hawai'i. Eruptions like the one that formed Lehua are said to be phreatomagmatic, meaning infused with water, which can make them explosive when the water flashes to steam.

lehua island

     
   

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