Kids from around the world draw volcanoes in remarkably similar ways: a steep-sided symmetrical cone with one or more lava flows pouring from a summit crater and a glowing black or gray cloud rising into the air. For Hawaiian volcanoes, such a sketch roughly illustrates a lava fountain that typically occurs from a small spatter or cinder cone. But when it comes to illustrating the shape of an entire volcano on the Island of Hawai`i, especially Kilauea, such a drawing is far too simplistic. Kilauea does indeed have a large summit crater called a caldera, which formed by collapse of the ground, but the volcano is far from symmetrical. Except for the south flank, the sides of Kilauea are not very steep. From most places on the volcano, you can't even see the summit.
Instead the overall shape of Kilauea is dominated by a broad and very long ridgethe east rift zonethat extends eastward 130 km from the caldera to the ocean floor at a depth of more than 5 km! This enormous low-profile ridge (see image) was built by countless eruptions of lava that originated from many different vents along the crest of the ridge, not at the summit.
Kilauea actually has two rift zonesthe east rift zone and the much smaller southwest rift zone. From the caldera to the eastern tip of the island at Cape Kumukahi, the east rift zone is 55 km long, and it continues beneath the sea for another 75 km. The southwest rift zone does not form a distinct ridge like the east rift zone and is only about 35 km long. The enormous size of the east rift zone compared to the southwest rift zone is clear evidence that much more lava has erupted from the east rift zone.
Topical presentations below:
Nearly the entire crest of the east rift zone is covered by lava erupted within the past 400 years. Only a few small, isolated patches of lava flows 750-2,500 years old are still exposed at the surface. Most of the young lava flows erupted from the rift zone spread southward toward the sea. In contrast, the north side of the rift zone is primarily covered by lava flows erupted from the summit of Kilauea about 550-600 years ago; fed from the `Aila`au vent east of present day Kilauea Iki, these flows spread to the northeast coast of Kilauea.
The crest of the east rift zone is 2-4 km wide, and vents for a single eruptive sequence typically occur discontinuously along lengthy segments of the rift zone, some as long as 30 km!
In the past 250 years, fissure eruptions have occurred along the entire east rift zone?no one part of the rift zone has been spared by this recent activity. Lava flows covered significant areas along the lower east rift zone during five eruptions in about A.D. 1750, 17901, 1840, 1955, and 1960; these flows cover about 68 km2 of land. The Nanawale Estates subdivision is built on the 1840 flow, and much of the Sea View community near Kehena is built atop the 1955 flow. Between 200 and 400 years ago, about 50 percent of the lower east rift zone (137 km2) was paved with lava.
In the middle east rift zone, the Pu`u `O`o-Kupaianaha eruption has covered more than 100 km2 of land, including most of the Royal Gardens subdivision and the communities of Kalapana and Kapa`ahu. In the upper east rift zone, the Mauna Ulu eruption (1969-1971, 1972-1974) covered nearly 100 km2 of land. Since 1955, nearly 30 percent of the area encompassing the east rift zone and the slope south of the rift zone has been covered by lava flows.
that erupts from Kilauea first rises through a deep magma conduit and
enters a magma-storage reservoir beneath the summit area. This
reservoir is between about 1-1.5 km and 6 km below the surface; the
elevation of Kilauea's summit
is 1,277 m, so the top of the reservoir is perhaps between
about 300 m above sea level and 200 m below sea level. Eruptions and
shallow intrusions at the summit are fed directly by magma from
Eruptions from the east and southwest rift zones are supplied by magma that has moved from the summit reservoir into and along the rift zones through magma conduits. The overall size, shape, and extent of the conduits beneath the east rift zone have been described by many scientists based on data collected by modern volcano-monitoring techniques during eruptions and intrusions along the east rift zone. Some differences exist in their descriptions, but there is an overall general agreement of a deep and a shallow component to the magma conduits beneath the east rift zone.
Magma moves toward the surface of the east rift zone as long, narrow bladelike
intrusions called dikes. The
dikes typically range in width from less than 1 m to only a few meters.
A single dike may extend more than 5-10 km in length and 2-3 km in height. In the
cutaway diagram (right), a hypothetical dike is shown beneath part of the
east rift zone. During the history of the east rift zone, thousands of dikes have
intruded into the axis of the zone, and many have reached the surface to trigger
Between May 1963 and January 1983 (the onset of the Pu`u `O`o-Kupaianaha eruption), 53 intrusions took place in Kilauea's east and southwest rift zones. Of those, 60 percent took place without an accompanying eruption.
Fissure eruption above dike
|Wherever the intruding dike reaches the surface, one or more fissures erupt lava fountains. The discontinuous fissure eruption in the photograph (left) marks the top of a dike that reached the surface along Kilauea's east rift zone at the beginning of the Pu`u `O`o-Kupaianaha eruption in 1983. (Click on image for more details.)|
A typical dike intrusion into the east rift zone is characterized by a swarm of small earthquakes (<M 3) occurring every 15-30 seconds and lasting for a few hours to a day or two. The earthquakes occur beneath a segment of the rift zone where rock is breaking due to the growth of a crack or fracture. Many scientists believe that magma moves through the growing crack. During a shallow intrusion, earthquakes typically are concentrated 2-4 km below the surface, and they may occur beneath a segment of the rift zone that is more than 5-10 km long.
Based on many sequences of such earthquake swarms, scientists estimate that an advancing crack moves along the rift zone at speeds between a few hundred and a few thousand meters per hour. For example, prior to the onset of the Pu`u `O`o eruption on 3 January 1983, the timing and location of earthquakes suggest that the tip of a growing crack moved along the rift zone at 600-700 m/hr for more than 24 hours!
Earthquake swarms indicate vertical speeds of an advancing crack between 0.1 and 5 km/hr. Before a magma-filled crack (dike) reaches the surface, however, it generally slows to a fraction of its initial speed during the last kilometer of its travel. Scientists have observed that the delay time between the first earthquakes directly below an eventual vent and the moment of eruption is generally 2-3 hours but can reach 10-12 hours or more.
In addition to earthquakes, intrusions of magma into the top 0-3 km of the east rift zone are accompanied by deformation of the summit area and the ground above the intruded part of the rift zone. For example, see Remarkable Tilt Pattern at the summit of Kilauea.
Each shallow intrusion and eruption along the east rift zone involves a dike moving into the axis of the zone. Consequently the volcano must widen internally to make room for the new magma. For example, when magma moved from beneath Pu`u `O`o to erupt in Napau Crater on 30 January 1997, continuously recording GPS receivers showed that the east rift zone widened at least 36 cm (see details). Another intrusion on 4 August 1969 during the eruption of Mauna Ulu caused a short segment of the rift zone to widen 2.7 m! Clearly, these examples show that the volcano grows internally by the repeated injection of dikes into the rift zone. Kilauea also grows externally by the addition of new lava flows to the surface, layer after layer.
When the east rift zone widens as a consequence of dike intrusions and eruptions, scientists have discovered that most of the growth or widening occurs south of the rift zone in the direction of the ocean. Apparently, the north side of the east rift zone cannot spread northward easily because of the adjacent enormous mass of Mauna Loa volcano (click on image for entire figure).
The repeated injection of magma into the east rift zone, and internal growth (or spreading) of the volcano toward the south, have created a dike complex that grew progressively southward (right in sketch). This has caused the axis of the rift zone to migrate toward the ocean with respect to the more stable flank of Mauna Loa and to the summit area of Kilauea. The sharp bend of the east rift zone between Hi`iaka Crater and Napau Crater is probably the result of this southward migration of the axis of the rift zone while the summit area remains relatively fixed. See entire figure and more detailed explanation.
Studies of many inflation and deflation cycles at Kilauea's summit area, and of corresponding deformation and earthquake activity along the rift zones, have clearly shown that not all magma that moves from the summit reservoir into the rift zones reaches the surface to cause an eruption. A significant volume of magma remains below ground in the rift zones. For example, one study of the magma supply rate to Kilauea's summit reservoir and rift zones between July 1956 and August 1983 concluded that only about 35 percent of the total volume of magma entering the summit reservoir actually erupted from either the summit area or from one of the rift zones. The rest remained stored below ground in small pockets of magmascientists have estimated that ten percent remained in the southwest rift zone and 55 percent moved into the east rift zone during this short time period.
During the history of the east rift zone, magma in most of these shallow pockets probably cools and crystallizes in place to form a network of dense solid dikes. Magma stored in other pockets may eventually erupt or be intersected by subsequent intrusions and then erupt later with the new, hotter magma. In the past few decades, scientists have documented several eruptions on the rift zone, including the onset of the Pu`u `O`o eruption in 1983, that began by expelling old magma that had already partially crystallized. When compared to magma that has erupted directly from the summit reservoir, these mixed magmas are cooler and contain more crystals of plagioclase and pyroxene and fewer of olivine.
Geologic mapping of the lower east rift zone has identified many lava flows that must have erupted from such old pockets of magma. This discovery suggests that there are areas beneath the lower east rift zone that are favorable to storage of intruded magma for long periods of time. Another such body is inferred to exist beneath the Makaopuhi-Kane Nui o Hamo area on the upper east rift zone.
Magma often enters the rift zone without generating earthquakes between the summit reservoir and the area of rift intrusion. Scientists know that an intrusion is occurring by (1) abrupt subsidence and related shallow earthquakes at the summit, which indicate that magma is moving out of the summit reservoir; (2) uplift of the ground and accompanying earthquakes along a segment of the rift zone, which indicates that a shallow intrusion is occurring beneath this part of the rift zone; and (3) volcanic tremor, which is continuous low-frequency shaking of the ground that accompanies the underground movement of magma.
How can magma move without breaking rock and generating earthquakes?
magma-filled passageway exists within the east rift zone. Clearly, magma can move more easily and
"quietly" through a passageway with a molten core than if it has to
force its way through solid rock. Many scientists have
conjectured that one or perhaps many coalescing dikes have indeed formed a
body of molten magma that serves as conduit beneath the rift zone.
Such a molten core is likely centered 3-4 km below the surface and extends as far as 30-40 km from the summit caldera. This subhorizontal passageway is probably no longer dikelike, however, if it ever was. Instead, it has probably widened by the melting and excavation of wall and roof rocks. The actual shape of this presumed molten core is not known.
A molten core within the east rift zone was first suggested by scientists in 1960. A dike intrusion and surface eruption in the lower east rift zone in 1960 was triggered and fed by magma from the summit reservoir more than 40 km away (see eruption details). The eruption was not preceded by a swarm of earthquakes that moved downrift as if at the leading edge of a dike; instead, virtually all earthquakes occurred only in the eruption area and at the summit. Similar patterns have been observed several times since 1960. This is the main evidence for the molten core, but the details regarding its size, shape, and depth are still unknown.
Scientists have recently proposed that the shallow magma system described above may actually extend from the area of the molten core downward to the base of the volcano at 9 to 10 km below the surface (see figure above). Measurements of surface changes on Kilauea since a M 7.2 earthquake on November 29, 1975, show a pattern of deformation that is much more widespread than those deformation patterns caused by either (1) recurrent inflation and deflation of the summit magma reservoir; or (2) episodic intrusions of dikes into the rift zones. Inflation of a magma system deeper than the molten core can account for the widespread deformation of Kilauea. The deformation pattern can, however, be explained in other ways, such as offshore faulting and folding of the seafloor. The cause of the deformation is currently the topic of intense research by HVO scientists.
Like the molten core, the shape, dimension, and structure of the proposed deep magma system is not known. The proposed dike-like magma body probably consists of numerous pockets of magma separated by partially molten and solidified material.
1 This age is uncertain. Conventional radiocarbon ages of several flows yield ages of less than 200 years before present (A.D. 1950), and Hawaiian accounts told to the Rev. William Ellis in 1823 tell of an eruption in the area in about 1790.
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Hazlett, R. W., 1993, Geological field guide Kilauea Volcano: Hawai`i Natural History Association, 123 p.
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