# Volcano Watch — Kīlauea's south shore is...steep

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Kīlauea's south slope presents a spectacular drive by motorcar because the highway descends steeply to reach the coastal plain.

Kīlauea's south slope presents a spectacular drive by motorcar because the highway descends steeply to reach the coastal plain. These hillside slopes range from 20 to 33 degrees (40 to 67 percent grade), although the National Park roadway maintains a gentle six percent grade to enable vehicle traffic. Less apparent is the comparably steep slope that continues into the submarine setting.

If we could continue driving, we would find that the first 300 m (1,000 feet) downslope below sea level is almost as steep as the steepest slopes seen above the shore--ranging from 50 to 60 percent. The slope becomes only slightly less steep as it continues its plunge to the floor of the first perched basin, which lies at 3 km depth (9,800 ft). There is little in our above-water experience that prepares us to comprehend such steep slopes in so short a distance. The east face of the Sierra Nevada (California) would be analogously steep over a comparable elevation range. On our Big Island, the slope plunges away again beyond the 3-km lip, to reach the abyssal depths of 5.5 km.

The bathymetric contours (lines of equal depth) are chiefly parallel to coastline. Minor variations develop where lava enters the sea. The lava builds a small promontory or bench. The shape of the bench is mimicked by the bathymetric contours as they bulge outward.

The plunge of the slope remains equally steep, however. These lava flows coat the submarine slope but do little to lessen the steep gradient. Indeed, as the shoreline progrades slightly with each new lava flow, the net effect is to oversteepen the upper part of the slope even more. Fragmental debris deposited from nearshore explosions or that spalls from the lava flows adds a thick dusting to the submarine slope.

In a recent bathymetric survey we examined the shallow 300 m of the offshore slope adjacent to the bench. Our survey followed soon after a collapse had whittled part of the bench. A submarine canyon had formed, heading downslope from the area of bench collapse.

Today the current eruption is recovering from a lengthy September pause. The pause interrupted the supply of lava to the coast. Lava flows are forging new paths down the pali and onto the coastal plain. In the weeks ahead some of these flows likely will reach the coast and become the conduits for a new tube system. And from there, a major part of the drama of island building will once again take place beneath the waves on the steep southern slope of the Big Island.

### Volcano Activity Update

A 20-hour pause in lava flow activity of Kīlauea Volcano interrupted the nearly continuous output of lava during the past week. Lava stopped flowing outside of Puu Oo from 2:00 p.m. on Sunday, November 7 to shortly after 10:00 a.m. the next morning. The surface flow reported last week on the coastal plain got to within 1 km (0.6 mi) of the sea coast before it stopped. Lava is again flowing from Puu Oo and reoccupies the tube to the top of Pulama pali. Several perched lava ponds are located along the tube, and a new flow has reached the coastal plain. As of November 11, the distal end of the flow is within 800 m (0.5 mi) of the sea coast west of the old Kamoamoa campground.

Residents of Pahala and Naalehu reported feeling an earthquake at 10:49 p.m. on Monday, November 8. The magnitude-3.4 earthquake was located 3 km (1.8 mi) southeast of Pahala at a depth of 6.8 km (4.1 mi).

November 14, 1999 marks the 40th anniversary of the start of the memorable Kīlauea Iki eruption. Long-time volcano watchers will all agree that the Kīlauea Iki eruption was the most spectacular display of lava activity that they have seen. To relive this historic eruption, the National Park Service's "After Dark in the Park" program on Tuesday, November 16 at 7:00 p.m. will feature Dr. Donald Richter. Dr. Donald Richter was the HVO staff geologist at the time of the Kīlauea Iki eruption and later served as HVO Scientist-in-Charge. Dr. Richter will present a slide program of the eruption and narrate the award-winning film, "Kīlauea Erupts." Be sure to come early, for the Kīlauea Visitor Center auditorium will certainly be filled.