If a picture is worth a thousand words, a geologic map is priceless


Cartography, the art or science of making maps, is alive and well at the Hawaiian Volcano Observatory. Visualizing the dynamic 3-D geologic history of the island on a two-dimensional sheet of paper is no easy task, but it is one of HVO’s fundamental responsibilities. Maps that depict the Earth’s surface in terms of rock age, lithology (composition and texture) and structures (volcanic vents, fissures, faults, and cracks) are called geologic maps.

Nationally, geologic maps are the most requested scientific product produced by state and federal geological surveys. They are tools with many applications: They help people understand the geologic history of an area, manage natural resources, assess hazards and provide information for informed land-use planning and decisions.

Geologic maps help HVO scientists understand the events that have shaped the island and improve their ability to forecast hazards, such as lava flows, explosive eruptions and tsunami, that will impact Hawaii in the future. As time passes, the collective memory of a hazard’s impact often wanes geologic maps serve as reminders of natural events that have faded from public consciousness. Understanding the forgotten past is paramount to preparing for what lies ahead.

Improving the geologic map of Hawaii Island is an ongoing goal of HVO. A combination of remote sensing and field-based techniques are currently used to map the geology of the volcanoes. While remote sensing plays an important role in understanding Kilauea’s current eruption, ground-based field mapping is relied on to decode previous eruptions throughout the island, especially on Mauna Loa.

To date, 90 percent of Mauna Loa’s 2,035-square-mile surface, covered by more than 500 individual flows, has been mapped. The ages for 35 percent of the mapped flows, the oldest of which is more than 36,700 years old, have been constrained, using radiocarbon dating. Even with this wealth of data, HVO geologists working on Mauna Loa still have a long way to go before completing the map.

The summit of Mauna Loa stands at 13,679 feet above sea level, and its slopes accommodate an impressive variety of climatic regimes. This environmental diversity presents a unique set of challenges to mapping the volcano: How do geologists map heavily forested regions? How can we tell the age of a flow for which radiocarbon dating is unavailable? How can we differentiate lava flows that look the same on the surface? Creative solutions have been developed for these and many other questions.

The type and amount of vegetation growing on the surface of a lava flow can help us determine the ages and boundaries of flows, relative to one another. Lava flows that host large-bodied ohia trees and thick groundcover are generally older than those that host less mature and sparse foliage. We use these interpretations in conjunction with field work and aerial photographs to infer the boundaries between covered flows.

U.S. Geological Survey volcanologists Jack Lockwood and Peter Lipman created a visual scale for determining the ages of Mauna Loa’s lava flows at higher elevations above the tree line, where carbon samples (for radiocarbon age-dating) are less likely to be discovered. Their scale relates surface color and weathering characteristics of a lava flow to its age. As a new lava flow is exposed to the elements and begins to break down, its surface color changes from black to gray, to brown, to tan, to orange and, finally, to red. The longer the flow weathers, the greater its color progression. The utility of this technique, however, is influenced by elevation, rainfall and cover.

When different lava flows exhibit similar surface characteristics, geologists look inside the rocks to understand differences. The things we examine include the sizes, shapes and proportions of crystals; groundmass (matrix) texture; vesicle (gas bubble) shape; vesicle lining; and rock chemistry. These physical and chemical properties provide information about the lava’s origin and allow flows to be distinguished from one another.

Kilauea activity update

A lava lake within the Halemaumau Overlook vent produced nighttime glow visible from the Jaggar Museum overlook and via HVO’s webcam during the past week. The lava lake level was relatively stable during the past week and was roughly 130 to 150 feet below the floor of Halemaumau.

On Kilauea’s east rift zone, breakouts from the Peace Day tube remain active at the base of the pali and on the coastal plain. Small ocean entries are active on both sides of the Hawaii Volcanoes National Park boundary. The Kahaualea II flow, fed from a spatter cone on the northeast edge of the Puu Oo crater, continues to advance slowly along the edge of the forest north of Puu Oo. A breakout from the east rim of Puu Oo on Tuesday night sent flows down the cone flank, over portions of the Kahaualea II flow; this flow remains active as of Thursday.

There were two earthquakes reported felt in the past week. On June 16 at 3:50 a.m., a magnitude-3.2 earthquake occurred one mile northeast of Kawaihae at a depth of 16 miles. Also on June 16 at 12:11 p.m., a magnitude-3.2 earthquake occurred six milies east of Waikii at a depth of 20 miles.

Visit hvo.wr.usgs.gov for Kilauea, Mauna Loa and Hualalai activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; or 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.