August 31, 2006

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Man is a powerful agent of landscape change

The Reef Runway at Honolulu International Airport.

If given a choice, most of us might prefer to hire the Kilauea Lava Company when faced with building new land beyond the island's coastline. The supply of lava rock seems endless, labor costs are trifling, and work takes place around the clock. In the 23 years since the current eruption began, Kilauea's coastal area has expanded by 228 hectares (ha) (563 acres), with never a permit required nor legal challenge mustered.

For better or for worse, mankind has proven a worthy competitor of the lava machine, at least for short periods of time. We often ignore the many small changes to coastline shape that result from our civilization, but their incremental extent across the island chain exceeds the shoreline expansion by lava flows of the past few decades.

Some statistics are in order. Owing to volcanism in the past 23 years, Hawai`i Island's coastline has grown by the aforementioned 228 ha of new land. The Mauna Ulu eruptions in the early 1970s also added land, of which about 19 ha (47 acres) remain strong against the force of coastal erosion. Thus an impressive 247 ha (610 acres) of new land have been generated by the Kilauea Lava Company in only two or three generations.

For comparison, the Reef Runway at Honolulu International Airport covers 319 ha (788 acres), infilling the once-shallow waters of Pearl Harbor. In a single man-borne project, O`ahu gained more new ground than has resulted from two major lengthy eruptions at Kilauea.

Statewide, the total area of manmade infill amounts to 2,693 ha (6,655 acres). Most is concentrated in the Pearl Harbor area, where 2,356 ha (5,822 acres) of what was once shallow bay, salt marsh, and estuary are now dry land, covered extensively with asphalt or buildings, baking in the tropical sun. Kane`ohe construction adds another 130 ha (321 acres). The remaining 207 ha (512 acres) are scattered chiefly on Kaua`i, Maui, and Hawai`i islands, where harbor improvements at Lihue, Kahului, and Kawaihae, respectively, form the major component of seaward expansion by infilling of the wet places.

This partly tongue-in-cheek analysis unfairly downplays the total area of land covered by lava flows, as opposed to the loss of only shallow bays. For example, lava flows have paved a whopping 11,700 ha (28,911 acres) of onshore ground during the past 23 years. Our wharves, runways, houses, and highways have caused monumental changes to the surface of the earth.

Statistics cited here arise from comparing modern maps with those showing island geology and shorelines in the 1920s, '30s, '40s, and '50s. In closing, one other notable area of nearshore land expansion-nearly 400 ha (988 acres) on Moloka`i-warrants comment. In the area west of Kaunakakai, a substantial expanse of tidal flats and open ocean shown on the 1922 topographic base map has been filled by mud eroded from upland sites.

The change has been insidious, a creeping expansion easily overlooked in the short view. Its progress was already well established by 1935, when Harold Stearns first started his geologic mapping of Moloka`i. He described the burial of the shoreward part of the reef that fringes the island's south coast by red mud carried seaward as a result of overgrazing in the previous 150 years. The erosion is a natural process, of course, but the increased sediment supply stems from man's stewardship.

Activity update

This past week, activity levels at the summit of Kilauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day that are large enough to locate). Widening of the summit caldera, indicating inflation, has resumed after pausing earlier in April. We continue to monitor unusual inflation south of Halema`uma`u Crater.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. There have been several gas pistoning events within the Drainhole vent in Pu`u `O`o crater. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean at East Lae`apuki. Lava also flows through a branch of this tube to the east, called the Campout flow, to the ocean at East Ka`ili`ili. Both locations where lava is entering the ocean are within Hawai`i Volcanoes National Park.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The National Park has reopened the surrounding area, however. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were two earthquakes beneath O`ahu and Hawai`i Island reported felt within the past week. A magnitude-1.9 earthquake occurred at 3:36 a.m. H.s.t. on Monday, August 28, and was located 16 km (10 miles) northwest of Na`alehu at a depth of 7 km (4 miles). An unusual magnitude-3.6 earthquake occurred at 8:10 p.m. the same day and was located 48 km (30 miles) northeast of Kailua, O`ahu, at a depth of 30 km (19 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (two earthquakes were located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.

Visit our web site (hvo.wr.usgs.gov) for daily Kilauea eruption updates and nearly real-time Hawai`i earthquake information.