Hawai’i Island, often called the Big Island, stands as the largest and youngest landmass in the Hawaiian archipelago. Its remarkable size difference from the other islands is a direct result of continuous volcanic activity, as it sits directly over a stationary mantle hot spot. This unique geological position means the island is constantly being shaped and enlarged by eruptions from its active volcanoes, most notably Kīlauea. The question of how much new land is added each year is not a simple calculation, as the rate of growth is entirely dependent on episodic eruptions that can dramatically alter the shoreline in a matter of months.
Quantifying New Land: The Average Annual Acreage
The creation of new land on the Big Island is highly variable, making a simple annual average figure misleading. For decades, the eruption of Kīlauea’s Puʻu ʻŌʻō vent (1983–2018) provided a relatively steady, low rate of land accretion. Over that 33-year period leading up to 2016, lava flows added an average of approximately 13 to 15 acres of new land annually to the island’s coastline. However, this average is dwarfed by the massive land creation that occurs during a major eruptive event. The 2018 eruption along the Lower East Rift Zone of Kīlauea, for example, added a total of about 875 acres of new land to the southeast coast in just four months. This single event created more than 50 times the annual average of the preceding decades, demonstrating that the land area of the Big Island expands in sudden, large bursts rather than at a consistent yearly rate.
Geological Mechanisms of Land Creation
New land is primarily formed when highly fluid, basaltic lava flows from Kīlauea’s East Rift Zone travel downslope and meet the Pacific Ocean. Upon contact, the immense heat of the lava causes the seawater to flash to steam, which creates a large, white plume known as laze—a hazardous mix of hydrochloric acid and fine glass particles. The lava then cools rapidly, forming a hard crust as it extends the shoreline. This process builds what scientists call a lava delta, an inherently unstable shelf of land resting on unconsolidated volcanic debris and ocean sediment. These deltas often experience sudden, catastrophic collapses, where large sections of the new land break away and slide into the sea, sometimes generating small, localized tsunamis.
Monitoring and Measuring Volcanic Expansion
Tracking and quantifying new land area falls mainly to scientists at the U.S. Geological Survey’s (USGS) Hawaiian Volcano Observatory (HVO). During active eruptions, researchers use specialized tools to map the rapidly changing coastline. Unoccupied Aircraft Systems (drones) and helicopters are deployed for aerial photography and thermal imaging to define the land-ocean boundary. These high-resolution images are then processed using remote sensing techniques, including Geographic Information Systems (GIS) mapping. This technology allows scientists to calculate the exact acreage of new land added by the lava flow, which is crucial for updating geologic maps.
The Balance of Creation and Loss
The Big Island gains land through volcanism but simultaneously loses it through other geological processes. Coastal erosion is a constant subtractive force, where powerful wave action wears away the shoreline and unstable lava deltas; this retreat is exacerbated by rising sea levels. Another factor in land loss is subsidence, the slow, continuous movement of large sections of the island’s flanks. For example, the Hilina Slump, a massive piece of Kīlauea’s south flank, slides seaward at a rate of several centimeters per year, causing vertical displacement and contributing to the overall lowering of the landmass. Therefore, the true measure of the island’s annual growth is the net change: the balance remaining after subtracting acreage lost to erosion and subsidence from acreage gained by lava flows.