Hualālai stands at 8,271 ft (2,521 m) with a prominence of 3,071 ft (936 m). It is the westernmost of the five major volcanoes which form the island of Hawaiʻi. Being in the post-shield stage of development, Hualālai is overall much rougher in shape and structure than the more youthful Mauna Loa and Kīlauea. Hualālai's structure is denoted by three rift zones: a well-developed one approximately 50° to the northwest, a moderately developed one to the southeast, and a poorly developed one trending northwards about 3 mi (5 km) east of the summit. There are over 100 cinder and spatter cones arranged along these rift zones. Hualālai has no summit caldera, although there is a collapse crater about 0.3 mi (0.48 km) across atop a small lava shield. Much of the southern slope (above the modern town of Kailua-Kona) consists of lava flows covered by a layer of volcanic ash from 10 to 100 cm (4 to 39 in) thick. In comparison with the other volcanoes of the island of Hawaiʻi, it is the third tallest, third oldest, third most active, and second smallest, making up just 7% of the island.
A major subfeature of Hualālai is Puʻu Waʻawaʻa, Hawaiian for "many-furrowed hill", a volcanic cone standing 372 m (1,220 ft) tall and measuring over 1.6 km (1 mi) in diameter. It extends for 9 km (6 mi), and has a prominence of 275 m (902 ft), north of the summit at 19°46′15″N 155°49′56″W. The cone is constructed of trachyte, a type of volcanic lava that exists at no other volcano on Hawaiʻi. Trachyte flows move more slowly than the typically "runny" Hawaiian lavas, a characteristic caused by its high (over 62%) silica composition (typical basalt is only 50% silica). Geologists hypothesize that Puʻu Waʻawaʻa originally formed during a pumice eruption a little over 100,000 years ago, and has continued to build itself since then, with at least three distinct trachyte flows recognized. The eruptions, although partially covered by flows from Hualālai and Mauna Loa, have built a distinctive structure known as the Puʻu Anahulu ridge.
The westward-facing flank of Hualālai forms a large underwater slump known as the North Kona slump. An area of about 1,000 km2 (390 sq mi), the slump consists of an intricate formation of beaches and scarps 2,000 to 4,500 m (6,600 to 14,800 ft) below the waterline. This area was explored more closely in a 2001 joint Japan-United States project to explore the volcano's flanks, utilizing the Remotely operated vehicle ROV Kaikō. Data collected showed that the lava flows there originated in shallow water 500 to 1,000 m (1,600 to 3,300 ft) deep, and that unlike similar slumps at other volcanoes, the slump at Hualālai formed gradually.
Haulālai is a known source for xenoliths, rock from the Earth's mantle that have been brought up in lava flows. Many prehistoric deposits, as well as those from the 1801 event, contain xenoliths of large size and abundant quantity.
Lava attributed to a shield-stage Hualālai has been found just offshore of the volcano's northwest rift zone. Tholeiitic basalt, indicative of the submarine subphase of the volcano's construction, has been found in wells driven into the volcano at a depth of 75 ft (23 m). These lavas persisted until an estimated 130,000 years ago. Hualālai entered the post-shield stage, the stage it is presently in, about 100,000 years ago. Pumice and trachyte eruptions at Puʻu Waʻawaʻa may be a sign of this change.
Geological mapping of the volcano has indicated that as much as 80% of the volcano's surface has been topped by lava flows during the last 5,000 years, entirely composed of shield alkalic basalt. More than half of this is under 3,000 years old, and about 12% is less than 1,000 years of age. Between the years 1700 and 2016, eruptions originated from six vents; four of these lava flows poured into the sea to the west coast.
Hualālai is the third most active volcano making up the island of Hawaiʻi, behind Kīlauea and Mauna Loa. Although the two larger volcanos have each erupted over 150 times in the last 1,000 years, Hualālai has done so but 3 times. The recurrence of activity at the volcano seems to be every 200 to 300 years.
A recent calm period, with almost no earthquake or magmatic activity at Hualālai, has seen the growth of homes, businesses, and resorts on the mountain's flanks. The most recent major activity at the volcano was in 1929, when an intense earthquake swarm rocked Hualālai, most likely caused by magmatic action near the volcano's peak. Although it has been relatively placid in the recent past, Hualālai is still potentially active, and is expected to erupt again within the next 100 years.
The USGS has divided the exposed lava flows and tephra erupted by Hualālai volcano during the last 112,000 years into 419 rock units of 8 chronostratigraphic age groups. These are summarized in the table below:
Hualālai last erupted in 1800–1801. This eruption produced very fluid alkalic basalt lava flows that entered the ocean off the western tip of Hawaiʻi island. Although five vents were active at the time, only two produced flows that eventually reached the ocean. The total output volume of the flow is estimated at over 300,000,000 m3 (0.072 cu mi). One volcanic vent, situated high on the slope, produced a large a'a flow, dubbed the Kaʻūpūlehu flow, that reached the ocean as two distinct lobes. On its way down, it overran a village and a valuable 3 mi (5 km) fishing pond. There is a local legend that after the failure of several offerings of animals and other items to the gods, the flow was finally stopped when Kamehameha I threw a lock of his own hair into the fire. The Ka'ūpūlehu flow is also known for the particularly large quantity of mafic and ultramafic xenoliths that came up with it.
The other major outflow from the event reached the sea south of Kiholo Bay, destroying the village of Kaʻūpūlehu. This 1801 flow, known as the Huʻehuʻe flow, formed Keahole Point where Kona International Airport is now located, 11 km (6.8 mi) north of Kailua-Kona. The eruption at Hualālai was concurrent with an eruption at the nearby Mauna Loa. It is theorized that, in the near past, Hualālai has had synchronous eruptions with both Mauna Loa and Kilauea.
Hualālai last erupted in 1801. A severe earthquake swarm shook the volcano in 1929, lasting about a month. This caused $100,000 worth of damage to the Kona district ($1.2 million as of 2010), and two earthquakes with magnitudes of 5.5 and 6.5 were felt as far away as Honolulu. This was probably caused by magma movement near the surface, but there was no surface activity or eruption.
The 2006 Hawaii earthquake, with epicenter just to the north in Kiholo Bay near Māhukona, caused much damage in the area.
Although Hualālai last erupted over 200 years ago, it will erupt again in the near future, as a 200–300 year estimated pause in activity is coming to an end. It presents a distinct hazard to the communities around it as well; for example, in the event of an eruption similar to the 1801 event, Kailua-Kona, which is 15 mi (24 km) from the volcano's summit, could be covered completely in a matter of hours. According to the USGS Lava Flow Hazard Zones, on a scale of 5 to 9, all of Hualālai is listed as threat level 4. For comparison, almost all of Kīlauea and Mauna Loa is listed as threat levels 1 through 3. The flanks of the volcano do not pose a lower threat to the population than the area near the rift zones because the distance is short and the slopes are steep; lava poses as much of a threat as it does near its source.
Since 1991, the Hawaiian Volcano Observatory (HVO) has maintained a seismic recording station 3 km (1.9 mi) east of Hualālai's summit to monitor the volcano. During this time, not a single earthquake swarm or harmonic tremor, indicative of activity at the volcano, has occurred. Although Hualālai does experience several magnitude 4 earthquakes per year, these are attributed to a deep source off the coast of the north-western rift zone and are not related to the movement of magma. The USGS is currently in the process of upgrading its aging monitoring and telemetry equipment, using American Recovery and Reinvestment Act funds. The agency plans to add another seismometer and three more sensors to help monitor activity. In addition, the HVO uses GPS to measure slight changes in tilt and slope of Hualālai, indicative of magmatic movement. A survey has been conducted every two years since 1986, but as of 2010 changes have been recorded.
Hualālai has been a home to native people since ancient times. Centuries ago, the Ahu A Umi Heiau was built on the dry plateau east of the mountain. The Kaloko-Honokōhau National Historic Park lies on the shore west of Hualālai, over the site of an ancient Hawaiian settlement. Although it is called kekaha ʻaʻole wai (lands without water), the rugged volcanic terrain attracted much sea life, making it an appealing place to settle. There are two main attractions within the park: the Kaloko fishpond, an area of loko kuapa (rockwall fishponds) constructed of interlocking rocks across a natural embayment on the coast, and Honokōhau, a former extensive settlement on the south side of the park.
Kamakahonu, Holualoa Bay, and Keauhou Bay were favored retreats of Hawaiian royalty long before the westernization of Hawaii. It was here that Kamehameha I rested after his eight-year campaign to unite the Hawaiian isles. His death in 1819 triggered social chaos. Mokuaikaua Church, built for missionaries in 1837 of lava rock and crushed coral, still stands today. Huliheʻe Palace, where many of Hawaii's last kings spent their time, has been maintained as a museum since 1927.
Today, the coast west of Hualālai is a popular location for vacation resorts, since the rain shadow of the mountain causes many sunny days. The first, Kona Village resort, was built in 1961. Since then the Four Seasons Resort and the Kūkiʻo golf course and vacation home complex have also been built on the 1800 flow. Both the Kona Village Resort and the Four Seasons Resort were damaged by the tsunami generated by the 2011 Sendai earthquake. The Hawaii Belt Road traverses the western slopes with an upper route called the Mamalahoa Highway and lower route named for Queen Kaʻahumanu.
Much of the Kona coffee crop grows on Hualālai's western slope near the town of Holualoa. The family of early coffee merchant Henry Nicholas Greenwell owned a large ranch on the western side of the volcano. The road from Kailua-Kona up the slopes of Hualālai is named for Frank "Palani" Greenwell. Hawaii Route 200 known as the Saddle Road, crosses the plateau north of Hualālai, where the Pohakuloa Training Area provides a remote training ground for the United States Army and United States Marine Corps.
Hualālai's many interesting features, most especially its volcanic cones, make it a popular destination for hikers. Although it is relatively easy to climb, much of the land at and around the summit of the volcano is owned by Kamehameha Schools, which routinely denies access to hikers attempting to climb it. As most, if not all, routes up to Hualālai pass through the estate, hiking on Hualālai is more or less illegal. However, the laws are not stringently enforced, and many hikers slip through anyway. One of the most popular mountaineering features is Luamakami and its sister Puhia Pele, two pit craters on Hualālai that are the deepest on the island. Puhia Pele, also known as "Pele's Pit", has been explored to a depth of 862 ft (263 m), and Luamakami is known to be even deeper. The walls are scalable with the proper technical equipment.
Although some of Hualālai is bare volcanic rock, most of it is covered by some form of vegetation. Bushes, ferns, and grass are common, and even a few ōhiʻa lehua trees (Metrosideros polymorpha) grow along the summit. Many of the collapse craters in particular have vegetation, and a few even have respectably-sized "vertical forests" inside, including several Eucalyptus tree groves. The volcano is populated by many birds and animals; the coast in particular attracts many fish and sea-dependent animals, such as the green sea turtle (Chelonia mydas) and the black-winged stilt (Himantopus himantopus). Hualālai averages 18.27 in (46 cm) of rainfall per year. The summit gets more rain than the coast and is typically obscured in heavy cloud cover and vog.
Several ecological reserves lie on the flanks of Hualālai. The Puʻu Waʻa Waʻa forest sanctuary was established in 1992 (along with the Laupahoehoe sister reserve on Mauna Kea) as a testbed for long term ecological research about Hawaiian moist forest and dry forest biomes, and lies within a mile of the volcano's summit on its northwestern flank. Elevation differs from sea level near the coastal edge to 6,300 ft (1,920 m) near the summit. Median annual rainfall is about 46.7 in (119 cm). Plentiful lava flows from the 19th century provide unique niches for vegetative and soil growth in the region. The southern section of the reserve, closest to the summit, has been split into a bird sanctuary.
The Honuaula forest reserve on the southwestern flank of the volcano at 19°30′25″N 155°54′41″W, preserves an extensive koa (Acacia koa) forest stand, with smaller Naio (Myoporum sandwicense) and Māmane (Sophora chrysophylla) trees and an undergrowth of ʻĀkala (Rubus hawaiensis) and various ferns. The reserve measures 655 acres (265 ha) and protects an ecosystem that has since been largely deforested in the surrounding area. The Wai Aha spring reserve on the lower slopes of the mountain is somewhat swampy and is home to the flowering evergreen ōhiʻa (Metrosideros polymorpha), the woody climber ʻIeʻie (Freycinetia arborea), and a dense undergrowth of ʻAmaʻu (Sadleria cyatheoides).