(BIVN) – The cyclic eruption styles of Kīlauea and Mauna Loa volcanoes are illustrated with new maps, part of an update to the USGS Hawaiian Volcano Observatory “Geology & History” webpages.

These pages now include maps and links to revised tables summarizing volcanic activity over the past ~200 years at each volcano, HVO wrote in a short statement announcing the updates. You can also access the updated Kīlauea and Mauna Loa maps and tables directly online.

courtesy USGS HVO

The updated Kīlauea page shows how geologic mapping and dating of the volcano’s surface tells of cyclic eruption styles, transitioning from explosive eruptions to effusive phases. The scientists write:

Long periods of explosive (tephra-dominated) and effusive (lava-flow-dominated) activity have alternated at Kīlauea for the past 2,500 years. Scientists infer that the eruption style is determined by the amount of magma being supplied to the volcano. When magma supply is high, the summit caldera fills and feeds voluminous lava flows from summit and rift zone vents. When the magma supply drops, the caldera collapses. When the caldera floor is deep enough to be at or near the water table (about 500 m (1640 ft) deeper than present), water can seep into the vent to trigger steam explosions. Eventually magma supply increases, and effusive eruptions dominate as many lava flows fill the caldera and erupt from the rift zones.

courtesy USGS HVO

On the Mauna Loa page, scientists write about how recent lava flows spread alternately from the summit area and rift zones, saying:

Detailed geologic mapping and dating of lava flows above sea level have shown that about 90 percent of Mauna Loa’s surface is covered with flows erupted within the past 4,000 years. Hundreds of lava flows were erupted during this time, but they did not cover the volcano evenly. By tracing the flows back to their vents and knowing their ages based on radiocarbon dating, geologists have recognized a general pattern in the frequency of lava flows spreading from the summit area and the rift zones during the past few thousand years.

Neither Mauna Loa or Kīlauea volcanoes are erupting at present.