Volcano Watch — Interactions between land, air shape island

Currently in Hawaii, we find ourselves in the middle of ho’oilo, or the wet season. While we all enjoy the dry season, known as kau, ho’oilo is, in part, responsible for an interesting process that helps shape the landscapes we see here.

The driving force behind ho’oilo and kau are the trade winds. The trade winds, which typically come from the northeast, are created by the North Pacific anticyclone (NPA). The strength and frequency of the trades is a product of the NPA’s location. During kau, the NPA is in a northerly position, and the trade winds are more prevalent and regular. When the NPA drifts south, the islands experience ho’oilo, marked by heavy winter storms and infrequent trades.

As air masses travel across the ocean, clouds form. When these clouds, which travel at low altitudes — typically 1,500-3,000 feet — come into contact with Hawaii Island, they are forced aloft by the mountains. This gain in altitude, known as orographic lift, creates conditions in which temperature drops and relative humidity rises. Passively reacting to the change in external conditions, clouds release moisture while climbing up the slopes of Kohala, Mauna Kea, and Mauna Loa, creating precipitation on the windward side of the Island and a rain shadow (dry area) on the leeward side. During trade wind conditions, a weather phenomenon known as trade wind inversion strongly limits precipitation to elevations below 6,500 feet.

The overall result is a large disparity in rainfall rates between the east and west sides of Hawaii Island, and a drastic difference in the landscapes of the wet and dry sides. For example, the northeast-facing region of Kohala, from the valleys of Waipio to Pololu, receives more than 160 inches of rain, while the area southwest of Kohala, near Kawaihae, receives less than 10 inches annually.

Also shaping the landscape are the geologic processes inherent in the formation of Hawaii’s volcanoes. Kohala, like Mauna Loa and Kilauea volcanoes, formed rift zones, with faults and dikes. The faulting on the northeast flank of Kohala is thought to be responsible for the massive Pololu landslide that occurred 250,000 to 300,000 years ago, leaving behind a large 1.9-mile-wide-by-11-mile-long “amphitheater” scar. The steep cliffs that meet the ocean in this area also owe their origins to this landslide.

Faulting along a rift zone creates grabens, or troughs. Grabens are formed in areas that were, at one point, under outside tensional force, which, when released, caused the area to subside. In the case of Kohala, northwest- to southeast-trending grabens can be found near the volcano’s ridge crest. These features, in effect, channeled rainwater toward the large scar of the Pololu landslide. Over time, the combined effect of the landslide, grabens, and water erosion resulted in the formation of large valleys like those at Pololu, Waimanu, and Waipio.

In many ways, the interaction of climate and weather with the landscape creates a bit of a chicken-and-egg scenario. Did rainfall weaken the structure over time? Or did the landslide lead to the formation of grabens and sufficiently weaken the rock, which enhanced the erosional effects of rainfall in creating the valleys? As is typically the case, the processes worked hand-in-hand, and their interactions led to the favorable conditions that resulted in the formation of large valleys and steep cliffs.

On the leeward (dry) side of Kohala, in clear contrast to its windward (wet) side, a gentler slope extends to the sea. The structural conditions that favor the formation of landslides and grabens are not found on the leeward side of the volcano. This, coupled with significantly less rainfall, results in a far less dramatic landscape.

In some cases, landscapes can be shaped by high rainfall rates and erosion, regardless of the volcanic structures. For instance, on Hawaii Island, streams carved out the impressive valleys at Maulua, Laupahoehoe, and Kaawalii gulches north of Hilo. Nothing comparable exists on the leeward side of the Island.

Next time you’re cruisin’ Hawaii Island, take time to appreciate the complex air-land interactions that create the climate and weather patterns responsible for some of Hawaii’s most impressive landscapes.

Kilauea activity update

A lava lake present within the Halema’uma’u Overlook vent during the past week resulted in night-time glow that was visible from the Jaggar Museum overlook. The lake, which is normally about 330-410 feet below the floor of Halema’uma’u Crater and visible by HVO’s Webcam, rose and fell slightly during the week in response to a series of large deflation-inflation cycles.

On Kilauea’s east rift zone, surface lava flows were slowly advancing down the pali over the past week. As of Thursday, flows were active in the middle part of Royal Gardens subdivision, at an elevation of about 900 feet. These flows are following the west margin of the Peace Day flow, which may bring them close to the last occupied structure in Royal Gardens. There are no active flows on the coastal plain, and there is no active ocean entry.

Five earthquakes beneath Hawaii Island were reported felt this past week. A magnitude-3.2 earthquake occurred at 11:18 p.m., HST, on Saturday, Feb. 18, and was located 62 miles northeast of Kaunakakai, Molokai, at a depth of 16 miles. A magnitude-3.2 earthquake occurred at 7:21 a.m. on Tuesday, Feb. 21, and was located 9 miles southeast of Hookena at a depth of 7 miles. Three earthquakes — a magnitude 3.0 at 3:57 a.m., a magnitude-2.5 at 6:35 a.m., and a magnitude-3.2 at 6:55 a.m. — occurred on Wednesday morning, Feb. 22, and were located 3 miles northwest of Kilauea summit at a depth between 2.5-2.7 miles.

Visit the HVO Web site (http://hvo.wr.usgs.gov) for detailed Kilauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.