Hualālai (pronounced in Hawaiian) is an active shield volcano on the island of Hawaii in the Hawaiian Islands. It is the westernmost, third-youngest and the third-most active of the five volcanoes that form the island of Hawaii, following Kīlauea and the much larger Mauna Loa. Its peak stands above sea level. Hualālai is estimated to have risen above sea level about 300,000 years ago. Despite maintaining a very low level of activity since its last eruption in 1801, and being unusually inactive for the last 2,000 years, Hualālai is still considered active, and is expected to erupt again sometime in the next 100 years. The relative unpreparedness of the residents in the area caused by the lull in activity would worsen an eruption's consequences.
The area near Hualālai has been inhabited for centuries by Hawaiian natives, dating back to before recorded history. The coast to its west in particular had several royal complexes. The volcano is also important ecologically, is home to many rare species and several nature reserves near the summit, and is a popular hiking attraction. Today the coast near Hualālai is dotted by vacation resorts, some built on historic flows, and a National Historical Park.
Geology
Structural features
thumb|150px|left|An expedition down a lava conduit in Hualālai volcano
Hualālai stands at with a prominence of . It is the westernmost of the five major volcanoes that form the island of Hawaii. 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 northward about east of the summit. Over 100 cinder and spatter cones are arranged along these rift zones. Hualālai has no summit caldera, although there is a collapse crater about 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 thick. Of the volcanoes on the island, it is the third-tallest, third-youngest, third-most active, and second-smallest, making up just 7% of the island.
Hualālai 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. Hualālai entered the post-shield stage, the stage it is now in, about 100,000 years ago. Pumice and trachyte eruptions at Puu Waawaa may be a sign of this change.
Geological mapping of Hualālai indicates that as much as 80% of its surface has been topped by lava flows during the last 5,000 years,
A recent calm period, with almost no earthquake or magmatic activity at Hualālai, has seen the growth of homes, businesses, and resorts on its flanks. The most recent major activity at the volcano was in 1929, when an intense earthquake swarm rocked it, most likely caused by magmatic action near its peak. Although it has been relatively placid in the recent past, Hualālai is still potentially active, and is expected to erupt again in the next 100 years.
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!Age Group!!Age (years before present)!!Rock type!!Surface area (percent)!!Number of rock units!!Location of exposures!!Comments
|-
|8
|less than
|alkali basalt
|6
|3
|northwest flank
|Vent cinder deposits with black pahoehoe basalt lava flows formed during AD 1800–1801 eruption. Five vents along NW rift zone. Abundant xenoliths of ultramafic intrusive plutonic rocks.
|-
|7
|200–700
|alkali basalt
|3
|4
|southern flank
|Vent lava spatter and lava flows, cinders and tuff at Waha Pele. Spatter cone erupted pahoehoe and ʻaʻā basalt lava flows. Cone collapsed causing violent phreatic eruptions of tuff. Lava flow eruption resumed building larger cone, with eruption of long ʻaʻā lava flows.
|-
|6
|750-1,500
|alkali basalt
|8
|14
|mostly central summit area and southeast flank
|Dark grey to black vent lava spatter (including a 650-meter-long spatter rampart), cinders, ʻaʻā and pahoehoe basalt lava flows, including Hualalai's longest lava flow (22 km). Collapse of vents produced pit craters. Hawaiian, Strombolian, and sub-Plinian activity.
|-
|5
|1,500–3,000
|alkali basalt
|38
|49
|mostly northern flank
|Spatter deposits, ʻaʻā and pahoehoe basalt lava flows. Very active 2,400–1,900 years before present.
|-
|4
|3,000–5,000
|alkali basalt
|25
|98
|mostly northern flank and summit
|Spatter deposits, ʻaʻā and pahoehoe basalt lava flows.
|-
|3
|5,000–10,000
|alkali basalt with minor amounts of picritic basalt, hawaiite and ankaramite
|15
|185
|mostly southern flank and summit
|Spatter deposits, extensively weathered ʻaʻā and pahoehoe lava flows.
|-
|2
|10,000–25,000
|alkali basalt with minor amounts of picritic basalt, hawaiite and ankaramite
|5
|63
|northeastern and southwestern flanks
|Lava spatter, cinders, extensively weathered ʻaʻā and pahoehoe lava flows and palagonite tuff. Spatter and cinders contain abundant xenoliths of mafic and ultramafic plutonic rocks.
|-
|1
|more than 100,000
|trachyte
|less than 1
|3
|northeastern flank
|Trachyte cone of Puu Waawaa (current prominence approximately 430 meters). Block and ʻaʻā trachyte lava flow of Puu Anahulu and trachyte pyroclastic deposits. 5.5 km<sup>3</sup> in volume, the largest-volume single eruption on Hawaii (Big island). K-Ar age is 106,000 ± 6,000 years before present.
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1800–1801 eruption
thumb|right|Na One [[pit crater of Hualālai volcano]]
Hualālai last erupted in 1800–1801. This eruption produced very fluid alkalic basalt lava flows that entered the ocean off the western tip of Hawaii 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 . One volcanic vent, situated high on the slope, produced a large ʻaʻā flow, dubbed the Kaʻūpūlehu flow, that reached the ocean as two distinct lobes. This 1801 flow, known as the Huehue flow, formed Keahole Point where Kona International Airport is now located, north of Kailua-Kona. The eruption at Hualālai is believed to have been concurrent with an eruption at the nearby Mauna Loa. It is theorized that in the near past, Hualālai has been active around the same time as both Mauna Loa and Kilauea, although precise dating is impossible.
Future monitoring
Hualālai is expected to erupt again in the near future, as a 200- to 300-year estimated pause in activity is coming to an end. The volcano's flanks 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 1971, the Hawaiian Volcano Observatory has maintained a seismic recording station 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 Kaloko-Honokōhau National Historical 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.