August 14, 2003
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Summertime along the Wailuku River - cool water, cool geology
On hot summer days in Hilo, many people visit the refreshing waters of the Wailuku River. In addition to offering scenic views, lush vegetation, and beautiful waterfalls, the Wailuku River has the distinction of being Hawai'i's largest river, on average discharging 1 million cubic meters (275 million gallons) of water each day - enough to fill about 300 Olympic-sized swimming pools.
The Wailuku offers a number of features and characteristics to delight geology buffs as well. For starters, it marks the approximate boundary between the lava flows of Mauna Loa and Mauna Kea. At the popular Boiling Pots, below Pe`epe`e Falls, a keen observer can see lava "pillows" across the river from the overlook. These pillows, created when lava that was filling the river channel entered deep water, originated from Mauna Loa about 3,500 years ago. Humans were just starting to hunt using bows and arrows when this lava erupted; it is the youngest in the river.
Pillow lavas, which often form when basalt flows underwater, are so named because of their resemblance to puffy pillows or cushions. They form when the molten core of an underwater lava flow oozes or bursts out, and is quickly chilled by the surrounding water. Lava pillows are formed when flows from the current eruption of Kilauea enter the sea.
Other special formations occur downstream from Boiling Pots. Hidden beneath the thriving vegetation across from the overlook, and exposed along pools further downstream, are spectacular columns of basalt. These vertical multi-sided pillars are of varying height, with some many meters (feet) tall. Most commonly, the pillars are six-sided (hexagonal), loosely resembling a King Kong-sized bundle of pencils that have been irregularly broken across the width of the bundle.
The slow cooling of a thick lava flow formed these columns. The top of the flow was exposed to air (or water) and its bottom to the pre-existing cooler riverbed, so the flow cooled inward from these surfaces. Lava, like most substances, shrinks as it cools. Stresses developed by the shrinking, solidified lava caused cracks to form. These cracks grew perpendicular to the cooling surface of the flow, forming the vertical columns.
Under ideal conditions, if a lava flow has uniform composition and thickness, with level top and bottom cooling surfaces, the flow cools uniformly into perfect vertical columns with precisely six sides. The world of mathematics shows that this six-sided pattern provides the greatest stress relief with the fewest cracks. However, in the real world, since less uniform conditions tend to prevail, columns may often be four- or five-sided as well as six-sided, with fewer columns with three, or seven to twelve sides. Along the Wailuku, in addition to examples of classic vertical hexagonal columns, some columns are curved, deviating wildly from the vertical to create an arching "rosette" pattern.
Lava columns are found in many parts of the world. Although mostly basalt, they can be of other lava types as well. Classic basalt examples include Giants Causeway, Ireland; Fingals Cave, Scotland; the Columbia River basalt of Oregon, Washington and Idaho; Devils Postpile, California; and Devils Tower, Wyoming. Lesser-known examples range from New Jersey to New Zealand, and geologically inclined visitors to Canada, Mexico, Argentina, and Australia can find lava columns in these locations. For a period of time, beautiful basalt columns from an earlier lava pond could also be seen exposed in the east wall of the inner pit of Kilauea's Makaopuhi Crater in Hawaii Volcanoes National Park, but they were buried in the early 1970s during the Mauna Ulu eruption.
Although widespread geographically, because well-developed columns are not abundant, the Wailuku River is an even more special place.
Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Surface activity is mainly visible in the westernmost section of the pali flow field. The Kohola arm of the Mother's Day flow has many breakouts on the coastal flat. The breakouts are distributed between two distinct lobes, one traveling slowly along the northwestern side of the old Kohola, and the other heading more or less due seaward. All breakouts are small and sluggish, but there are plenty of them. The east-side lobe of the main Mother's Day flow also remains visible as a series of incandescent patches from the top of Pulama pali out onto the gentle slope below. Lava stopped entering the ocean at the Highcastle delta, and there is no ocean entry with pillows being formed at this time.
One earthquake was reported felt on the island this past week. A resident of Waiki`i Ranch felt the earth move at 4:56 a.m. on Sunday, August 10. The magnitude-2.7 earthquake was located 7 km (4.2 mi) northwest of Pohakuloa at a depth of 18 km (10.8 mi).
Updated: August 19, 2003 (pnf)