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

This geologic map of Mauna Loa shows surface flows that have been mapped as of 2013. The flows are color-coded to reflect age. Warm colors indicate younger flows, with red indicating flows erupted since 1832. Cooler colors indicate older flows, such as at South Point, which is covered by flows ranging from 7,000 to more than 21,000 years old (blues and purple).

Cartography, the art or science of making maps, is alive and well at the Hawaiian Volcano Observatory (HVO). Visualizing the dynamic three-dimensional geologic history of the island on a two-dimensional sheet of paper is no easy task, but it is one of the fundamental responsibilities of HVO. 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 our ability to forecast hazards, such as lava flows, explosive eruptions, and tsunami, that will impact Hawai`i 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 Hawai`i Island is an ongoing goal of HVO. A combination of remote sensing and field-based techniques are currently utilized 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 5,271 sq-km (2,035 sq-mi) 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 over 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.