Volcano Watch: How to build a beach — Pohoiki growth through the years

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. Today’s article was written by University of Hawaiʻi at Hilo staff members Meghann Decker and Lis Gallant, along with students Susan Richfield and Lichen Forster.

Although it might feel like it happened in an instant, the formation of Hawai‘i Island’s youngest black sand beach did not happen overnight.

This week, we explore how the beach at Pohoiki, near the easternmost point on the island, came to be and how it has grown through time.

Pohoiki, which means “little depression,” has been an important ocean access point for people in the Puna District. Before 2018, this area was a rocky coastline of Kīlauea lava flows emplaced between 750 and 1,500 years ago.

The boat ramp was constructed in 1963 and the breakwater in 1979, both by the U.S. Army Corps of Engineers.

The beach at Pohoiki grew rapidly in the year following the 2018 lower East Rift Zone eruption of Kīlauea and has continued to evolve since. The boat ramp was cut off from the ocean, and local warm springs formed in several low-lying areas.

The material that makes the beach at Pohoiki has a distinct black color and bumpy texture. It originally formed as molten lava poured into the ocean, cooled and shattered into sand- to block-sized fragments.

These fragments were then ground down even further by wave action and redistributed by the longshore current.

A longshore current flows parallel to the shore within the zone of breaking waves. They develop when waves approach a beach at an angle and can push sediments along the coastline.

The typical longshore current on the east side of the Hawai‘i Island transports material from the 2018 lava flows north of the beach and deposits it at Pohoiki.

The first area of sediment accumulation in 2018 was around the boat ramp and breakwater.

The beach profile at Pohoiki has also experienced changes because of seasonal ocean swells. Overall, this has resulted in a bigger beach, spanning farther south into areas known as Second Bay and Third Bay.

However, during the summer months, south swells disrupt the longshore current and move material from Third Bay to Second Bay. This results in steepening of the main beach face at Second Bay.

This seasonal reworking of sediment forms internal dune structures at Second Bay. Dune structures are landforms composed of wind- or water-driven particles that typically take the form of mounds, ridges or hills. They can be found in coastal areas, deserts and anywhere with large amounts of loose sediment and strong winds.

Specifically, coastal dune structures form when wind and waves transport material from the beach inland, causing it to accumulate.

Students from the University of Hawai‘i at Hilo recently conducted a ground penetrating radar survey of the beach in Second Bay to study these internal features.

Ground penetrating radar is a technique that uses small radar pulses to detect objects and changes beneath the ground. When these pulses are transmitted into the ground, they encounter obstacles and reflect back toward the surface, where they are captured by a receiving antenna.

It uses low-frequency radio waves no more powerful or harmful than those picked up by household radios.

The survey showed that much of the beach growth was the result of migrating dunes.

The first phase was dominated by primary dune structures built from finer grained material created when the 2018 lava flows entered the ocean. This process of beach growth — called progradation — rapidly resulted in the filling in of Second Bay and the accumulation of a beach face.

Progradation is the process of a shoreline, delta or fan growing toward the ocean through time.

The second phase of beach growth is characterized by continued progradation and sediment accumulation. The dune structures from this phase display cross-bedding.

Cross-beds are formed as dunes migrate from erosion and redeposition of sediment. The first phase dunes also have cross-beds, but at a higher angle; this tells us that the growth of the beach during the first phase was faster and more energetic than growth during the second phase.

The currently active phase of growth at the beach is characterized by stabilization of low-angle dune formation above sea level that is affected by tide changes.

This ground penetrating radar data represents what the beach looks like at one moment in time. With dredging to restore access to Pohoiki boat ramp planned later this year, the shape and structure of the beach will continue to evolve.

Volcano Activity Updates

Kīlauea has been erupting episodically since Dec. 23, 2024, within the summit caldera inside Hawai’i Volcanoes National Park. Its U.S. Geological Survey Volcano Alert Level remains at Watch.

The eruption in Halemaʻumaʻu Crater continued this week with the beginning of Episode 18 the evening of April 18, when lava overflowed from the north vent. The continuous lava fountaining phase of Episode 18, if it happens, is most likely to start between today and this weekend.

Since the end of Episode 17, the summit region has showed inflation suggesting another episode is possible.

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

Mauna Loa is not erupting. Its U.S. Geological Survey Volcano Alert Level is at Normal.

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

• MAGNITUDE-3.3 at 7:13 a.m. April 12 at a distance of 5 miles northeast of Pāhala and depth of 20 miles.

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

Visit the 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.