Puna Ridge: underwater segment of
Kilauea's east rift zone

The great bulk of each Hawaiian volcano lies beneath the sea, and Kilauea is no exception. Kilauea's east rift zone does not simply end where the ocean begins—more than half of the 130-km-long rift zone lies under the ocean, reaching a depth of 5.4 km (Kilauea's summit is 1.277 km above sea level). Known as the Puna Ridge, this submarine part of the east rift zone extends northeast 75 km beyond Cape Kumukahi. The average gradient of the subaerial portion of the east rift zone is 23 m/km. For the submarine Puna Ridge the gradient is much steeper, 50m/km to a depth of about 2,800 m; beyond this depth the gradient is even steeper, 100m/km. The eastern tip of Puna Ridge partly covers a much older rift zone from Kohala Volcano, known as the Hilo Ridge, and is surrounded to the north and south by extensive submarine lava flows. Discovered in 1986, these flows were erupted from the deepest segment of the Puna Ridge.
Shaded-relief map of Puna Ridge, Kilauea Volcano, Hawai`i
Shaded-relief map of Puna Ridge, Kilauea Volcano, Hawai`i

Map prepared by J. Robinson; data courtesy of Japan Marine Science and Technology Center, USGS, Monterey Bay Aquarium and Research Institute, and Scripps Institution of Oceanography

What we know about the geologic story of Puna Ridge is the result of several scientific research cruises beginning in the early 1960s and continuing to the present. These cruises involved a combination of bathymetric SeaBeam mapping, sonar imaging, echo-sound profiling, photographs and video from deep-tow remotely-operated vehicles, dredging of rock samples, magnetic surveys, and submersible dives. The descriptions and images below are from only a few of these cruises.

Historical activity

Scientists have yet to observe or document an eruption from the Puna Ridge, but historical records suggest that two eruptions have occurred recently. Hawaiian oral history points to an underwater eruption in the late 18th century, though evidence for such an eruption has not been found. In 1884 a one-day underwater eruption was observed at a depth of only 20 m. Some scientists have speculated that a voluminous submarine eruption occurred in 1924 when magma suddenly withdrew from Kilauea's summit caldera at Halemaumau Crater and intruded into the Puna Ridge or perhaps erupted from the ridge. There is no evidence yet to support an eruption, however. Severe faulting and widening of the rift zone took place near Kapoho in April 1924 and could have accommodated all of the magma lost from the summit.

Surface features of Puna Ridge

The earliest photographs and samples of the lower Puna Ridge, taken in the early 1960s, demonstrate that the axis of the rift zone consists mainly of young pillowed lava flows similar in chemical composition to flows erupted on the land portion of Kilauea. The presence of fresh lava pillows, a general lack of sediments between the pillows and lobes, and the absence of organisms on the surface of the flows suggested the surface of the ridge was covered with young lava flows. With more detailed maps and photographs of subsequent research cruises, scientists have identified a great variety of volcanic features, including fissures, pit craters, lava tube skylights, cones, pillow ridges, and broad lava terraces. Some features are similar to those found on land, but others are very different. For the latter, scientists have offered different hypotheses to explain how they formed. A few images of the submarine features are shown below.

Lava morphologies on flow fields

Areas with little or no relief or without distinctive structures are called flow fields. Scientists have characterized the lava flows covering these areas as consisting of (1) sheet flows with smooth surfaces and linear flow features (photo, left); (2) lobate flows with interconnected lobes of lava similar to pahoehoe lobes on land (photo, middle); and (3) pillow flows that are either bulbous, elongate, or tubular (photo, right). Photographs from along the ridge show that these flow morphologies are found from sea level to the sea floor.
Photo of sheet flow on Puna Ridge, Kilauea Volcano, Hawai`i
Sheet flow
(image width is 4 m)
Photo of lobate flow on Puna Ridge, Kilauea Volcano, Hawai`i
Lobate flow
(image width is 4 m)
Photo of pillow flow on Puna Ridge, Kilauea Volcano, Hawai`i
Pillow flow
(image width is 4 m)
Photographs courtesy of D. Smith, Woods Hole Oceanographic Institution; from Smith et. al.

Terraces


A few cones and terraces on Puna Ridge
click for larger image
Along the crest and flanks of Puna Ridge, scientists have mapped many prominent near-circular flat-topped structures as wide as several kilometers and as tall as a few hundred meters. These broad features, called terraces, have relatively flat or slightly domed surfaces. Some of the submarine terraces have one or more summit craters.

Because of their large volume (to 1 km3), terraces are thought to result from long-lived eruptions that extrude lava at low to moderate rates. Scientists have proposed that they grow either by (1) the upward and outward inflation of the network of tubes that develop within lava flows; or (2) as large, ponded lava lakes behind levees that stop the advance of lava. The cold submarine temperatures are also considered important for the the formation of these features.

Cones

Pillow flows cover side of a small cone (downslope is to the right)

Flanks of a pillow cone
(image width is 10 m)
Circular-shaped cones as large as 2.5 km in diameter and 140 m tall occur along the crest of Puna Ridge (see shaded-relief map, above). The cones have flat tops and craters; relatively deep craters suggest that lava drained from the cones after they formed, similar to subaerial cones. Scientists have also observed small cones <100 m in diameter with nearly vertical sides covered with elongate, tubular pillows and rubble (photograph, left). These small steep-sided cones are interpreted to be rootless vents that form on the surface of a flow or above a lava tube. On land, such features are known as hornitos.

Photograph courtesy of D. Smith, Woods Hole Oceanographic Institution; from Smith et. al.

Skylights


Skylight of lava tube (arrow)
(image width is 20 m)

Top Photograph courtesy of D. Smith, Woods Hole Oceanographic Institution; from Smith et. al.

The top skylights (arrow marks largest one) are located on the margin of a crater 150-200 m in diameter at the top of a lava terrace at a depth of 1,100 m. The bottom skylight is located at a depth of about 3,500 m. The skylights mark the paths of lava tubes.

Bottom Photograph courtesy of F. Trusdell, USGS Hawaiian Volcano Observatory.


Skylight of lava tube
(image width is about 10 m)

Pillow ridges or pillow walls


Artist sketch of a pillow ridge

Copyrighted image: the full paper was published in the Geological Society of America Bulletin, 1978, April, p. 615 (see Fornari et. al. in references).

Tall narrow ridges as long as a few kilometers and composed primarily of lava pillows are called pillow ridges. When the sides of the ridges are very steep, they are referred to as pillow walls. The ridges observed thus far are between 40 and 200 m wide at their bases and as high as 30 m; the ridges narrow to only a few meters wide at their crest.

Scientists have interpreted pillow ridges to be primary vent features built directly over long submarine fissures. The actual fissures are no long visible because they are buried beneath the accumulation of lava pillows and tubes. The equivalent subaerial features are spatter ramparts.

 

| Back to main east rift zone features | upper east rift zone | middle east rift zone | lower east rift zone |

References

Clague, D. A., Hon, K. A., Anderson, J. L., Chadwick, W. W., Fox, C. G., 1994, Bathymetry of Puna Ridge, Kilauea Volcano, Hawaii: U.S. Geological Survey Miscellaneous Field Studies Map MF-2237, 1994.

Fornari, D. J., Malahoff, A., Heezen, B. C., 1978, Volcanic structure of the crest of the Puna Ridge, Hawaii: Geophysical implications of submarine volcanic terrain: Geological Society of America Bulletin, v. 89, p. 605-616.

Holcomb, R. T., Moore, J. G., Lipman, P. W., Belderson, R. H., 1988, Voluminous submarine lava flows from Hawaiian volcanoes: Geology, v. 16, p. 400-404.

Lonsdale, P., 1989, A geomorphological reconnaissance of the submarine part of the East Rift Zone of Kilauea Volcano, Hawaii, Bulletin of Volcanology, 51, p. 123-144.

Smith, D.K., Kong, Laura S.L., Johnson, Kevin T.M., Reynolds, J.R., 2002, Volcanic morphology of the submarine Puna Ridge, Kilauea Volcano: in Takahashi, E., Lipman, P.W., Garcia, M.O., Naka, J., and Aramaki, S. (eds.) Hawaiian Volcanoes: deep underwater perspectives: Washington D.C., American Geophysical Union Monograph 128, p. 125-142.