Volcano Watch: Cracks in the 2018 Kīlauea lava delta; what do they mean?

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

Kīlauea’s 2018 eruption transformed the lower Puna District of Hawaiʻi Island — including its coastline, where 875 acres of new land were added in a large lava delta.

The area is still subject to sudden, dynamic changes, as evidenced by recent observations of new ground cracks near Pohoiki.

The new cracks are alarming and have led people to wonder what they mean.

There is, fortunately, a near-perfect analog for the 2018 lava delta located nearby to provide evidence of what its future might be. This older lava delta was formed during the 1960 eruption of Kīlauea, which infamously destroyed Kapoho.

It was emplaced in a similar coastal environment to the 2018 lava delta, as both eruptions saw lava flows inundate a shallow marine platform before extending into deeper ocean waters.

Both lava deltas are exposed to the full force of the Pacific Ocean as a result of reaching deeper ocean waters. No other islands or nearshore coral reefs protect them.

Because of regular volcanic activity, 90% of the coastline along Kīlauea is less than 1,000 years old and lacks a significant fringing reef.

The only exceptions are areas impacted by fault-related subsidence that causes the coastal plain to sink below sea level — such as Waiʻōpae tidepools in Kapoho, covered in 2018 by lava.

Wave action has had a substantial impact on the 1960 lava delta.

Mapping using aerial imagery shows the coastline along the delta eroded landward 200 feet or more in many places. Most of this retreat occurred in the first few decades after the eruption, with the rate slowing thereafter.

In no place has the coastline receded past where it sat before the eruption.

No major collapse has ever been reported along the 1960 lava delta, so the erosion is likely more gradual.

Most lava deltas consist of a solid surface atop loose submarine debris known as hyaloclastite, which forms when molten lava fragments hit the water.

This material is prone to subsidence because of compaction of the sediments with time and erosion on the coastline because the loose debris is washed away underwater by longshore currents, destabilizing the solid lava above.

The 2018 lava delta is likely in the early stages of the same processes.

The beach at Pohoiki Bay is built from lava that fragmented in 2018 when it entered the ocean and was transported along the coast to the bay.

Recently spotted cracks in the delta demonstrate the instability of solid lava along the coastal edge as the hyaloclastite sand and debris compacts and erodes away. Collapses of this lava are likely to be much smaller than collapses of active lava deltas, with at most a few tens of feet (several meters) slumping into the ocean.

Though these erosional collapses are relatively small, nobody wants to be standing on the delta’s coastline if a small section of lava slumps into the ocean, tossing them into the pounding surf.

Small collapses could even produce small, highly localized rogue waves affecting areas such as Pohoiki Bay and/or Kumukahi.

These growing cracks highlight the changing and hazardous nature of the coastline along the 2018 lava delta.

Lower Puna residents can be assured that any signs of instability along the 2018 coastline — though we can expect further erosional changes and minor hazards remain — do not represent an indication of renewed volcanic activity in the area.

Hawaiian Volcano Observatory continues to closely monitor the lower East Rift Zone of Kīlauea; geophysical data streams have shown no volcanic unrest there since 2018.

Volcano Activity Updates

KĪLAUEA volcano, Hawai‘i Island

U.S. Geological Survey Volcano Alert Level: Watch

Kīlauea has been erupting episodically since Dec. 23, 2024, within its summit caldera.

Episode 31 of the ongoing eruption Aug. 22-23 within Halemaʻumaʻu Crater — and confined inside Hawai‘i Volcanoes National Park — lasted just fewer than 13 hours.

Sulfur dioxide emission rates are elevated in the summit region during eruptive episodes. No unusual activity has been noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.

WHAT’S NEXT? Summit region inflation since the end of episode 31 suggest another episode is possible and forecasting models suggest the likely window begins late next week.

MAUNA LOA volcano, Hawai‘i Island

U.S. Geological Survey Volcano Alert Level: Normal

Mauna Loa is not erupting.

Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

EARTHQUAKE report

One earthquake was reported felt during the past week in the Hawaiian Islands:

• MAGNITUDE-3.5 at 12:44 p.m. Aug. 27 located 6 miles southeast of Pāhala at a depth of 18 miles.

Visit Hawaiian Volcano Observatory’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.