2012 Chapman Conference, Hawaiian Volcanoes: from Source to Surface Kohala Volcano Field Trip Guide, August 22, 2012. Trip Leaders: Ken Hon, Cheryl Gansecki, John Winter Waikoloa Marriott 0.0 mi The Waikoloa area is underlain by lavas erupted from Mauna Loa. Most of these lavas were probably erupted from radial vents on the NW side of Mauna Loa. Just to the south of Waikoloa, the 1859 `a`a flow destroyed a Hawaiian Village and entered the ocean 5 days after the beginning of the eruption. The eruption continued for roughly 1 year producing pahoehoe flows that re-­‐
entered the ocean in the area of Kiholo Bay. The pahoehoe flows near the ocean are sheet flows that have inflated over 10m during emplacement. Travelling out of Waikoloa. 0.2 mi Hualalai volcano can be seen to the southeast. This 9000-­‐foot volcano is in the post shield stage and is erupting alkali basalt. The last eruption was 1800-­‐1801, creating the pahoehoe flows on which the Kona airport is built. In front of Hualalai, the distinctive round hill is Pu`u Wa`a Wa`a, a 100,000 year old trachyte pumice dome, which represents the only silicic pyroclastic eruption on Hawaii Island. The “mesas” below the dome are thick (200-­‐400 feet) trachyte flows (60-­‐64% SiO2) and represent at least 2 distinct flow intervals from Pu`u Wa`a Wa`a. Turn north onto Highway 19. 0.5 mi Kohala volcano is straight ahead, Mauna Kea can be seen to the right. Waimea town sits in the saddle between them. Kohala volcano is the oldest subaerial volcano on the Island of Hawaii. It has a pronounced rift zone that is oriented NW-­‐SE. A strong gravity anomaly underlies this rift zone and continues beneath Mauna Kea connecting with the submarine Hilo Ridge. This continuous gravity anomaly, along with a 1000 m submerged shoreline on both the Kohala and the Hilo Ridge, suggest that they are part of the same volcano. The SW side of Kohala is capped by post-­‐shield alkalic lavas and associated cinder cones (Hawi volcanics) on the upper half. On the lower half of the volcano, the top of the shield stage tholeitic and transitional alkalic lavas are exposed (Pololu volcanics). The shield stage on Kohala volcano ended about 300,000 to 400,000 years ago, which correlates to the 1000m terrace using a subsidence rate of 2-­‐3 m per 1,000 years. The youngest rocks on Kohala are the Hawi series that range from about 260,000 years old to 120,000 years old. Previously reported ages as young as 65,000 years are now thought to be spurious. 1.5 mi Turn east onto Waikoloa road, heading toward Waikoloa Village. More views of Hualalai to the south and Kohala to the north. Mauna Kea is straight ahead. 4.5 mi Contact of Mauna Loa flows with Mauna Kea flows on the west side of Waikoloa village. Mauna Kea flows are covered with variable amounts of windblown ash and glacial loess that forms a weak soil horizon. 12.0 mi Junction with Highway 190, turn left toward Waimea. Ankaramite in Road Outcrops 13.0 mi The lower parts of Mauna Kea volcano are covered with true alkali basalt with silica contents of about 48%. These rocks are called the Hamakua volcanics and represent the earliest part of the post shield volcanism. Eruptions commonly form cinder cones and relatively fluid `a`a flows. Beautiful outcrops of ankaramite are exposed in the road cut at this locality. Ankaramites are alkali basalt with cumulate olivine and clinopyroxene (with less common plagioclase) and generally have lower silica contents, around 48% SiO2. 16.5 mi Turnoff to the Saddle Road to Hilo. The two cones above the highway are part of the younger Laupahoehoe series that is mostly found on the steeper summit cone of Mauna Kea. Hawaiite flows from these cones reach the lower road. As we approach Waimea, the cinder cone on the left is composed of ankaramite tephra that weathered to leave perfect clinopyroxene crystals in the soil. 17.0 mi Mauna Kea volcano can be seen to the right. The lower slopes are flatter and composed of more fluid alkali basalt flows, whereas the upper slopes of Mauna Kea are much steeper and composed of more alkalic hawaiite (48-­‐52% SiO2) and mugearite (52-­‐55% SiO2) lava flows and cinder cones. These are also the most common composition in the Hawi volcanics on Kohala. The top of Mauna Kea is much lighter than the lower slopes because multiple periods of glaciation have removed the darker glass from the outside of glacial terrains. The last glaciation ended about 15,000 years ago and was associated with a 100 m drop in sea level. 23.0 mi Turn left on highway 19 and head west. We are in Waimea, which was a cattle ranching town. Hawaiian cowboys are called paniolos and have a proud tradition reflected in this town. The Parker Ranch that extends over much of the west side of Mauna Kea was once the largest privately owned cattle ranch in the US until the last descendant passed away. The cones around Waimea are all Kohala cones composed mostly of hawaiite and mugearite lavas. 25.0 mi Turn right on highway 250 and head toward Hawi from Waimea. 27.7 mi Mamaewa gulch has been eroded through a thick stack of Pololu volcanics ranging in age from 420,000 years to 360,000 years. The capping flow just to the west is a hawaiite of the Hawi volcanics that is 160,000 years old. A prominent red soil is generally found between the Pololu and Hawi volcanics everywhere on Kohala. This is the largest stream valley on the south side of Kohala. Stream valleys on the south and west sides of Kohala are much smaller than those on the north side due to the extreme difference in precipitation. Streams on the south and west sides also begin at the top of Kohala (where it rains more frequently) and erode downward. This is in direct contrast to the large stream valleys on the north side of Kohala that begin at landslides near the coastline and work their way toward the summit. Pu`u Kawaiwai Quarry 28.7mi This is a large hawaiite cinder cone complex of the Hawi Volcanics, comprised of four aligned vents and thick piles of stratified tephra. The cone has been heavily quarried and is still sometimes used as a shooting range; watch for shells. The quarry wall nearest the road shows the profile of a crater that was buried as the original cinder cone grew. Activity shifted downslope to the three other vents as spatter and cinders filled the crater. Lava bombs can be seen in the quarry walls and scattered on the surface of the cone. Lava flows associated with this cone make a broad fan that spreads downslope until it is overlapped by younger Mauna Kea lavas. The cone is leased from the state by Parker Ranch, who granted us permission to visit. Puu Makela benmorite dome 31.3 mi Puu Makela differs in several ways from most of the Hawi volcanics. The silica content is 8-­‐10% higher (57% SiO2) than hawaiites, making this rock a benmoreite (basically an alkali andesite with lots of sodium). The benmoreite also has the unusual characteristic of having amphibole, a hydrous mineral that is uncommon in Hawaii. Eruption of this material produced very viscous lava that formed a dome, another rare feature in Hawaii. The dome sits just above the road and the thick benmorite flows extend almost all of the way to the lower highway. 36.0 mi Continue driving northwest on Highway 250 toward Hawi. The road crosses Kohala’s rift zone and follows along if for some time. Although the shield stage vents are buried by post shield Hawi vents, their location can be easily inferred by the elongate shape of Kohala, a strong gravity anomaly, the alignment of alkali Hawi vents, and the presence of a well-­‐defined axial graben. The graben and associated summit facing faults on the north side of Kohala are thought to be related to the large submarine landslide on that flank of the volcano. They may either represent the headwall of the slide or, more likely, a slump structure that formed inland of the slide headwall, which is now underwater. 44.7 mi Turn right on highway 270 and head toward Pololu Valley Soils are sparse on much of the Big Island and those that are present generally owe their existence to the weathering of wind blown ash and glacial loess. Rocks beneath these soils are general fresh to mildly weathered. In contrast, lava flows found on the north side of Kohala volcano show deep weathering, frequently producing spheroidally weathered basalts. The north side of Kohala is the only part of the island where significant weathering of the basement lavas has occurred. The deep soils in Kohala caused this area to experience much more damage during the October 2006 M6.5 EQ than the epicentral area around Waikola. Pololu Valley 52.0 mi Pololu valley is on the western edge of a series of large valleys that have cut headward into Kohala volcano from the sea. These valleys are thought to have formed when a large submarine slump created a headwall near the shoreline. The formation of large cliffs creates waterfalls at the coast that notch the volcano and begin cutting headward. Notice the tops of the valleys do not reach the summit of Kohala, but instead end in huge amphitheaters. This erosion pattern is also seen in the southwest of the US. Much of the erosion on Kohala appears to have taken place after the end of the shield stage and emplacement of the Pololu volcanics. Some of the younger Hawi flows drape the valleys and can be found on the valley floors. The flat bottoms of the valleys are created by filling of deeper V-­‐shaped valleys that formed during the last glaciation, when sea level was 330 feet lower. All of these valleys were heavily populated by Hawaiians with perhaps 20,000 people living in them. Waipio valley on the eastern side is the largest and has an extensive set of agricultural and water works that were constructed for taro farming. Pololu Valley experienced about 55 feet of runup during the devastating 1946 tsunami. LUNCH at John Winter’s House. 53.0 mi Keokea Beach Park. Possible stop to look at spheroidally weathered basalt and a beautiful bay. 59.0 mi Return to intersection of 250, continue on 270 towards Kawaihae. About 2 miles out of Hawi we cross the rift zone of Kohala volcano. 65.0 mi Mahukona Beach Park is an abandoned port for the Kohala Sugar Company, destroyed by WWII and the 1946 tsunami. Remains of the sugar mill, railroad and lighthouse can still be found in the area. The harbor is now a good spot for snorkeling and viewing marine life. The name “Mahukona,” an old land division name meaning “false harbor,” is used for the submarine volcano due west of here. Offshore is Mahukona volcano, the oldest volcano that is part of the island of Hawaii. The summit region of Mahukona is marked by a submerge coral terrace at 1100-­‐1200 meters, below and to the west of the submerged shoreline of Kohala volcano. Mahukona is a small volcano, but it was clearly emergent at 400,000-­‐500,000 years ago. 66.0 mi One mile to the south, Lapakahi State Historical Park is the site of an ancient Hawaiian village, inhabited until the mid-­‐nineteenth century and now partially restored. This area also has many sea caves (old lava tubes) and abundant marine life. Both Mahukona and Lapakahi are located within flows of the Pololu Volcanics. Outcrops of Megatsunami Deposits. 70.0 mi Trail along fence line to Stearn’s outcrop of 100,000-­‐year-­‐old tsunami deposits. About a 1 mile round trip hike, bring water. This locality preserves coral rubble deposits just above sea level with marine sands and fossils. Uranium series dating gives ages of around 100,000 to 150,000 years for the corals in this deposit. They are also partially overlain by Hawi lava flows. Given the 2-­‐3 m/1000 year subsidence rates of Hawaii, these corals would have been deposited at 200-­‐300 m above sea level. Scattered higher outcrops inland of this site give maximum elevations of 400 m. The deposits are interpreted as being emplaced by a megatsunami related to the Alika II debris flow on the seafloor west of Mauna Loa. NOTE: Please be very careful to stay north of the prominent fence, the property owner is very security conscious. Be respectful of private property. Also please do not sample or break any of the outcrops. They are extremely rare and need to be preserved for future research. RETURN TO HOTEL via Kawaihae and Highway 19. This field trip guide drew heavily on the work of Mike Garcia on Kohala volcano published in guidebook form for the 1987 Hawaii Symposium on How Volcanoes Work. Gary McMurtry also provided a wealth of information on the giant tsunami deposits. Figures and ages are from the excellent pamphlet by Dave Sherrod that accompanies the Geologic Map for the State of Hawaii. McMurtry, G. M., D. R. Tappin, G. J. Fryer, I. P. Wilkinson, M. Williams, J. Fietzke, D. Garbe-­‐Schoenberg and P. Watts, 2004, Megatsunami deposits on Kohala Volcano, Hawaii, from flank collapse of Mauna Loa, Geology 32, 741-­‐744. Porter, S.C., Garcia, M.O., Lockwood, J.P., and Wise, W.S., 1987, Guidebook for Mauna Loa-­‐Mauna Kea-­‐Kohala Field Trip: unpublished guidebook for the Hawaii Symposium on How Volcanoes Work, 40 p. Sherrod, D.R., J.M. Sinton, S.E. Watkins, and K. M. Brunt. 2007. Geologic map of the State of Hawai'i. U.S. Geological Survey Open-­‐File Report 2007-­‐1089, 83 p.