How Hawaiʻi's volcanoes work

A volcano is a place where molten rock, called magma underground and lava once it reaches the surface, escapes from inside the Earth. Despite the popular picture, the Earth is not a ball of liquid under a thin crust. The mantle, the thick layer between the crust and the core, is almost entirely solid rock. It is hot enough to flow very slowly over millions of years, like warm wax, but it only melts in special places.

Three ways to melt solid rock

• Lower the pressure. Hot mantle rock that rises toward the surface is squeezed less, and some of it melts even though it hasn't gained any heat. This decompression melting feeds mid-ocean ridges and hotspots like Hawaiʻi.
• Add water. Where one tectonic plate dives beneath another, it carries seawater down with it. The water lowers the melting point of the mantle above it. This builds the volcanoes of the Pacific "Ring of Fire", such as Mount St. Helens, Mount Fuji and the Andes.
• Add heat. Hot magma rising into the crust can melt the rock around it, producing magmas rich in silica.

Magma is lighter than the solid rock around it, so it rises, collecting in reservoirs a few kilometers underground. When pressure in a reservoir builds, magma breaks a path to the surface and an eruption begins.

Why some volcanoes explode and others flow

Two things decide how violent an eruption is: how sticky the magma is (its viscosity) and how much gas it holds. Silica-rich magma is thick and traps gas until the pressure blasts it apart, which is why Mount St. Helens and Pinatubo exploded. Hawaiian magma is basalt, low in silica and very fluid. It reaches the surface at about 1,150 °C (2,100 °F), and its gas usually escapes easily, so Hawaiian eruptions are mostly effusive: lava fountains and flowing rivers of lava rather than towering ash columns.

Hawaiian words describe lava around the world. Pāhoehoe is smooth, ropy lava with a glassy skin. ʻAʻā is rough and clinkery, formed when lava cools, thickens and tears as it flows. Fountains also spin thin strands of volcanic glass called Pele's hair and droplets called Pele's tears.

Kinds of volcanoes

Shield volcanoes, like Mauna Loa and Kīlauea, are built from thousands of fluid lava flows into broad, gently sloping domes shaped like a warrior's shield lying on the ground. Stratovolcanoes, like Mount Rainier, are steep cones of alternating lava and ash from more explosive eruptions. Cinder cones are small hills of loose fragments around a single vent, and a caldera is the large basin left when the ground above an emptied magma reservoir collapses, as at Kīlauea's summit.

Volcanoes are where the Earth releases its internal heat, and they appear where the planet's plates are pulling apart, colliding, or, more rarely, over a hotspot. Roughly 1,300 to 1,500 volcanoes on land are considered potentially active. Most volcanic activity, though, happens out of sight on the seafloor, along some 65,000 km (40,000 miles) of mid-ocean ridges where new ocean crust is made.

Over the planet's history, volcanism has done far more than destroy:

• It built the surface. All of the ocean floor, and much of the continents, began as volcanic rock.
• It helped make the air and oceans. Gases released from the young Earth's interior, mostly water vapor, carbon dioxide and nitrogen, contributed much of the early atmosphere and the water that filled the oceans.
• It helps regulate climate. Over millions of years, carbon dioxide from volcanoes warms the planet, while the slow weathering of rock pulls carbon back out of the air. This balance has helped keep Earth habitable. On today's timescales, though, human activity releases far more carbon dioxide than all the world's volcanoes combined (Figure 4).
• It can cool the planet briefly. Large explosive eruptions inject sulfur gas into the stratosphere, where it forms a haze that reflects sunlight. After Pinatubo in 1991, global temperatures dropped by about 0.5 °C (0.9 °F) for a year or two.
• It feeds life. Volcanic ash weathers into some of the most fertile soils on Earth, and new land like Hawaiʻi gives plants and animals entirely new places to evolve.

Hawaiʻi is unusual. It sits in the middle of the Pacific Plate, more than 3,000 km (2,000 miles) from the nearest plate boundary, yet it is one of the most volcanically active places on Earth. The explanation is the Hawaiian hotspot: a long-lived upwelling of unusually hot rock, often called a mantle plume, rising from deep in the mantle. As that hot rock nears the surface, decompression melting produces a steady supply of basalt magma.

The hotspot stays roughly in place while the Pacific Plate slides over it toward the northwest at about 7 to 10 cm (3 to 4 inches) a year. Each volcano grows while it sits over the hotspot, then is carried away from its magma supply and slowly dies. A new volcano starts to grow behind it. The result is a conveyor belt of volcanoes that get older the farther northwest you go.

The chain doesn't stop at Kauaʻi. Northwest of the main islands, the volcanoes have eroded and sunk into low islands and atolls such as Nihoa, Midway and Kure, then into undersea mountains called seamounts. The full Hawaiian-Emperor chain runs about 6,000 km (3,700 miles) to the deep-sea trenches off Kamchatka, where its oldest seamounts, around 80 million years old, are being carried down into the mantle.

Partway along, the chain bends sharply. The northern Emperor Seamounts run almost due north, while the Hawaiian chain runs northwest. The bend formed about 47 million years ago. Most researchers link it to a change in the direction the Pacific Plate was moving, and some argue the hotspot itself drifted too. The question is still debated.

Every Hawaiian volcano passes through the same stages as it rides away from the hotspot. You can see every stage along the chain today.

1. Pre-shieldA young volcano grows on the deep seafloor with small, gas-rich eruptions.Kamaʻehuakanaloa
2. ShieldOver the hotspot, huge volumes of fluid basalt build the main volcano, about 95% of its total size.Kīlauea, Mauna Loa
3. Post-shieldAs magma supply falls, eruptions become rarer and more explosive, capping the shield with steeper cones.Mauna Kea, Hualālai, Haleakalā
4. ErosionRain, waves and landslides carve deep valleys and sea cliffs, and the island sinks under its own weight.Kauaʻi's Nā Pali, Oʻahu's Koʻolau
5. RejuvenationAfter a long quiet, small eruptions sometimes return, building craters and cones.Diamond Head, Hanauma Bay
6. Atoll and seamountThe volcano sinks below the waves, leaving a coral atoll, then a submerged seamount.Midway, Kure, the Emperor Seamounts

Because they keep growing for so long, Hawaiian volcanoes are enormous. Measured from its base on the seafloor, Mauna Kea rises about 10,200 m (33,500 feet), taller than Mount Everest. Mauna Loa, its neighbor, is the largest active volcano on Earth by volume.

The Island of Hawaiʻi sits over the hotspot today, which is why it is the youngest island and still growing. The U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) watches six volcanoes it considers active:

Inside Kīlauea

Kīlauea is one of the most active volcanoes in the world. Magma rising from the hotspot is stored in reservoirs beneath the summit caldera before it erupts at Halemaʻumaʻu crater or travels sideways underground into two rift zones, long cracked belts that run east and southwest from the summit. Many of Kīlauea's biggest flows, including those of 1983–2018 and 2018, erupted far down the East Rift Zone, many kilometers from the summit.

Kīlauea's south flank is also slowly sliding toward the sea, pushed by magma wedging into the rift zones. Sudden slips of the flank cause Hawaiʻi's largest earthquakes, including the magnitude 7.7 Kalapana earthquake in 1975 and the magnitude 6.9 earthquake of May 4, 2018.

How scientists watch it

HVO was founded at Kīlauea's summit in 1912 by geologist Thomas A. Jaggar, and it is the oldest volcano observatory in the United States. Today it combines seismometers that record earthquakes, tiltmeters that sense the ground swelling as magma fills the summit and deflating as it erupts, GPS and satellite radar that measure the ground moving by fractions of an inch, gas sensors and webcams. During the current eruption, the tiltmeter record of the summit re-inflating is a key part of how HVO forecasts the time window for each new fountaining episode.

Living with an active volcano also means living with vog, volcanic smog. Sulfur dioxide from Kīlauea reacts in the air to form a haze of tiny sulfate particles. The trade winds usually carry it around the island to the Kona side, and it can irritate the lungs, especially for people with asthma.

Kīlauea's current summit eruption has been running since December 23, 2024, as a series of short, intense lava-fountaining episodes separated by pauses of days to weeks. The chart below updates from HVO's figures as each new episode is added. Follow the eruption on the episode archive or watch it on the live webcams.

Hawaiian lava flows usually move slowly enough for people to walk away from them, so Hawaiʻi's volcanoes have destroyed far more homes than lives. The exceptions are the rare explosive eruptions and the earthquakes, landslides and tsunamis that come with a growing volcano. Native Hawaiian oral history preserves memories of eruptions centuries old, and scientists have used those accounts alongside the geologic record. These are some of the events that most shaped life on the islands.

• Explosion or collapse
• Lava flows
• Lava fountains
• Earthquake or tsunami
• Science and the park

1. About 1500Kīlauea's summit collapsesAfter a long eruption that sent lava toward the sea, the ground above Kīlauea's summit reservoir collapsed into a deep caldera. For roughly the next three centuries, Kīlauea was dominated by explosive eruptions.
2. 1790The deadliest eruption in U.S. historyAn explosive eruption at Kīlauea's summit, recorded in the Keanakākoʻi ash, killed members of a party of warriors and their families led by Keōua, who was at war with Kamehameha. Estimates of the dead range from about 80 to several hundred. It remains the deadliest known volcanic eruption in what is now the United States.
3. 1801Hualālai's last eruptionLava from Hualālai flowed to the sea on the Kona coast, under where Kona International Airport stands today.
4. 1868Earthquake, tsunami and landslide in KaʻūDuring a Mauna Loa eruption, a great earthquake of about magnitude 7.9 struck the south coast. A tsunami and a mudflow it triggered killed 77 people and wiped out coastal villages.
5. 1880–81Mauna Loa threatens HiloLava from Mauna Loa's Northeast Rift Zone flowed for months toward Hilo and stopped within what are now the city's limits.
6. 1912The Hawaiian Volcano Observatory opensThomas Jaggar began continuous monitoring at Kīlauea's summit. Hawaiʻi Volcanoes National Park was established four years later, in 1916.
7. 1924Halemaʻumaʻu explodesSteam-driven explosions hurled boulders from Halemaʻumaʻu, killing one man, and the crater more than doubled in width.
8. 1950Mauna Loa's fast flowsLava from the Southwest Rift Zone raced to the sea in hours, crossing the main highway in three places and destroying homes in South Kona.
9. 1959–60Kīlauea Iki and KapohoFountains at Kīlauea Iki reached 580 m (1,900 feet), long the highest measured in Hawaiʻi. Weeks later, an eruption in lower Puna buried the village of Kapoho.
10. 1975The Kalapana earthquakeA magnitude 7.7 earthquake on Kīlauea's south flank triggered a tsunami that killed two campers at Halapē.
11. 1983–2018The 35-year Puʻuʻōʻō eruptionKīlauea's longest rift eruption in centuries covered about 144 km² (56 square miles), added land to the coast and destroyed 215 structures. In 1990 it buried the town of Kalapana, including about 100 homes, a store and a church.
12. 1984Mauna Loa againFlows from Mauna Loa's Northeast Rift Zone came within about 7 km (4.5 miles) of Hilo.
13. 2018Lower Puna eruption and summit collapseFissures opened in the Leilani Estates subdivision. Lava destroyed 716 homes, covered 35 km² (13.7 square miles) and filled Kapoho Bay, while the summit caldera collapsed through some 60,000 earthquakes.
14. 2020A lava lake returnsA new summit eruption boiled away the water lake that had formed in Halemaʻumaʻu after 2018, beginning a series of summit eruptions.
15. 2022Mauna Loa wakes upMauna Loa erupted for about two weeks, sending lava toward the Daniel K. Inouye Highway, its first eruption since 1984.
16. 2024–todayEpisodic fountains at KīlaueaSince December 23, 2024, Halemaʻumaʻu has erupted in dozens of lava-fountaining episodes (Figure 8). See the episode archive.

Each of these events changed how people live with the volcanoes. Hawaiʻi County maps lava-flow hazard zones from 1 (highest) to 9 (lowest) that affect where and how people build, and HVO's forecasts give emergency managers time to close roads and evacuate neighborhoods.

Explosive eruptions elsewhere show what Hawaiʻi's mostly gentle volcanoes are not, and why monitoring matters. Volcanologists compare explosive eruptions with the Volcanic Explosivity Index (VEI). Each step up the scale means about ten times more ash and rock thrown out.

Death tolls for older eruptions are estimates.