The question of how many acres are added to the Big Island of Hawaiʻi each year does not have a fixed answer, because the island’s growth is a highly episodic geological process. Land accretion is entirely dependent on ongoing volcanic activity, which occurs in bursts separated by periods of little to no change. The island’s size remains stable until a major eruption pushes lava flows beyond the existing coastline. This means some years add zero acres, while others can add hundreds. Historical data provides the clearest picture of the island’s variable expansion.
The Primary Source of New Land
The vast majority of new territory added to the Big Island is sourced from Kīlauea, one of the world’s most active volcanoes. Land forms when molten lava, flowing from the volcano’s rift zones, travels across the existing land and spills into the Pacific Ocean. This outpouring of material cools almost instantly upon contact with the seawater, shattering the lava. The rapid cooling builds up a steep, underwater foundation that eventually emerges above the surface.
The new territory forms an unstable geological feature known as a lava delta or bench. This structure is weak because it rests on unconsolidated lava fragments and sand. Kīlauea’s decades-long eruption cycle, including the Puʻu ʻŌʻō vent, and the 2018 lower East Rift Zone eruption, are the primary events responsible for extending the coastline. The 2018 eruption, for instance, created a significant ocean entry at Kapoho Bay, dramatically reshaping the island’s eastern edge.
Calculating Land Growth
The highly variable nature of Kīlauea’s activity means that calculating an average annual land growth rate is misleading, as it smooths out the peaks and valleys of expansion. Growth is highly episodic. During the 35-year-long eruption of the Puʻu ʻŌʻō vent (1983 until 2018), the volcano added approximately 500 acres of new land to the coastline. This accretion was not steady, with many years adding little to nothing. Smaller events, like the 1955 eruption, contributed only about eight acres. The scale of growth fluctuates wildly, evidenced by the 2018 lower East Rift Zone eruption, which, in just a few months, added an astounding 875 acres of new land to the island.
To accurately quantify the land added, scientists must calculate both the area and the volume of the new material. Area is measured in acres or square miles, while the volume of the lava is calculated in cubic meters to determine the total mass erupted. This volumetric measurement is achieved by comparing high-resolution Digital Elevation Models (DEMs) of the topography before and after the eruption, often created using Light Detection and Ranging (LiDAR) technology. This comparison reveals how thick the new lava flows are, which can be up to 180 feet deep in some places, providing the data necessary to accurately determine the total land mass added during an episodic event.
How New Land is Surveyed and Measured
Documenting the new coastline requires advanced remote sensing technology, as conditions are often too hazardous for ground-based surveys. Geologists rely heavily on aerial platforms, primarily using unmanned aerial vehicles (UAVs) and helicopters for flyovers. These aircraft carry specialized equipment, such as thermal and optical cameras, to map the flow’s extent and track its movement in near-real-time. Satellite imagery, including data from the Landsat program, also provides broad-scale, multi-spectral views of the changing landscape.
The data collected from these sensors are used to create precise Digital Elevation Models (DEMs) using techniques like Structure from Motion (SfM) and LiDAR. This allows scientists to map the exact boundaries of the new land and calculate its area accurately. The measurements must be constantly updated because the newly formed lava deltas are notoriously unstable and prone to sudden, massive collapses back into the ocean. Continued monitoring ensures that the official boundaries of the Big Island reflect the dynamic changes caused by the ongoing volcanic activity.
Ownership of Newly Formed Territory
The territory created by volcanic activity is subject to a distinct set of legal precedents regarding ownership. When lava extends the coastline, the new land that forms seaward of the former high-water mark is generally considered public property. Under Hawaiian common law, the State of Hawaiʻi is the owner of land created below the mean high-water line. This legal principle was affirmed by the Hawaiʻi Supreme Court following the 1955 eruption, which set a precedent for the ownership of future lava-created lands.
This ownership rule clarifies that private property boundaries do not automatically extend with the flow of lava. If a property owner’s land is covered by lava, they still retain ownership of the subsurface plot. However, the new land that extends into the ocean belongs to the state. This legal framework ensures that the newly formed public lands, including the unstable lava deltas, are managed by the State of Hawaiʻi for the benefit of the community.