Chronology The activity of the Honolulu Volcanics began less than one million years ago during the late Pleistocene and
Holocene, after volcanic activity at Koʻolau had ceased and the volcano been substantially eroded. The first eruptions occurred within the Koʻolau caldera and the youngest in the far southeastern part of Oʻahu, coinciding with the Koko Rift. There is otherwise little evidence for a spatial pattern in the volcanic activity, with each rift having eruptions widely spaced in time. The lava flows from Honolulu Volcanics have been used to construct a history of variations of
Earth's magnetic field. Eruptions of the Honolulu Volcanics have been correlated to
shorelines generated by sea level variations, which have left both drowned and emergent platforms and terraces on Oʻahu. Some volcanoes formed when sea level was lower than today and thus part of their structures are now submerged, others formed when it was higher and grew on
reefs. These sea level variations are a function of
glacial-
interglacial changes, with higher sea levels associated with interglacials when
polar glaciers expand and retreat. Thus four stages of volcanic activity have been defined, a first during the Kahipa highstand, a second during the Kaʻena and Lāʻie highstands, a third during the Waipiʻo and Waimānalo highstands and a fourth after the Waimānalo highstand. In turn, the Waimānalo stage was correlated to the last interglacial/
Sangamonian interstadial and the Kaʻena highstand to an interglacial 600,000 ± 100,000 years ago. First dating efforts yielded ages of Pleistocene-Holocene based on
sea level variations, while
potassium-argon dating has yielded ages ranging between 800,000 and 60,000 years old. However, the presence of excess radiogenic
argon due to the xenoliths makes dates obtained by potassium-argon dating unreliable and dates older than 800,000 years are especially questionable.
Argon-argon dating has been applied to submarine vents of the Honolulu Volcanics and has yielded ages of 700,000 to 400,000 years for submarine vents northeast of Oʻahu while the southwestern submarine vents have ages clustering around 140,000 ± 50,000 years, in line with the ages of the Koko rift. Volcanic activity occurred in two pulses, one between 800,000 and 250,000 years ago and the other in the last 120,000 years, with a hiatus in between the two pulses. Sometimes the vents of the Sugar Loaf, Tantalus Peak and Koko Rift area are classified separately from the other vents of the Honolulu Volcanics as they are usually less than 100,000 years old. Volcanic eruptions in the Koko Rift occurred between 100,000 and 60,000 years ago; it is not clear whether Koko Rift or Tantalus Rift erupted last, as ages of 35,000 and 85,000 years before present have been obtained on the Koko Rift while the Tantalus Rift has yielded ages of 76,000 ± 1,000 years ago, and the two rifts are clearly unrelated to each other. The average recurrence interval for eruptions in the Honolulu Volcanics is about 35,000 years assuming that the younger ages for the Koko Rift are correct.
Eruption characteristics Many eruptions were highly
explosive, probably due to the magma being rich in gases, threw tephra high into the air, and ripped pre-existent volcanic rock and limestone out. When the rising magma intercepted water—especially close to the coast—
steam explosions ensued which threw up rock debris including older rocks such as coral fragments. Lava flows were erupted mainly during the late stage of the eruption and were typically of small dimensions; the longest flow is long and the largest one at Kaimukī forms a lava shield or
lava dome. Cinder cones form from the fallout of volcanic ejecta material, which is variously described as
cinder,
lava bombs,
pumice and spatter and forms layered deposits. Tuff cones in addition can also contain non-magmatic ejecta. Many of these eruptions, especially that of Diamond Head, probably only lasted for a short time such as a few hours, as has been observed in similar eruptions during historical time. The field has also seen many
Surtseyan or
phreatomagmatic eruptions, especially at nearshore vents, when ascending magma encountered water, such as at Mānana Island, Koko and Punchbowl; these eruptions were explosive and they are the best exposed site of Surtseyan activity in the United States. Some formed small islands when they emerged, and one vent may now be buried beneath coral deposits. At Koko Crater and Diamond Head, the amount of water interacting with the developing volcano varied over the course of the eruption, as more water entered the vent when the eruption enlarged it. Volcanic rocks of Honolulu Volcanics include
agglomerates,
breccia, breccia-tuffs,
scoria bombs and tuffs. Tephra takes the form of both
lapilli and
volcanic ash and some eruptions were followed by
lahars when volcanic ash was swept up by streams; one such deposit is found in Pālolo Valley. During eruptions,
trade winds blew material from the vent; this wind-driven material transport is responsible for the frequently asymmetric shape of the volcanoes such as at Diamond Head where the southeastern rim is the highest part of the crater rim.
Drill cores in Hanauma Bay have found ash layers; a black ash known as "black sand" is widespread across Honolulu and originated in volcanic eruptions of the Honolulu Volcanics, and ash layers cover coral reef deposis both onshore and offshore as far as
ʻEwa Beach. Ash from the Salt Lake Craters was transported by winds to Pearl Harbor and is considered to be responsible for the formation of the
bay by closing off its access to the sea. Activity at specific vents: • At Diamond Head, eruptions commenced underwater and first deposited white rocks formed mostly by reworked corals. Proper tuff layers were emplaced on top of this unit. • The formation of Kaʻau Crater was also accompanied by
phreatic activity that deposited alluvial tuffs. Some eruptions – such as those of the Kaʻau, Mōkōlea and Training School vents – may have occurred as one sequence over a long line. • Intense explosive eruptions took place when magma erupted underwater, forming Hanauma Bay. The eruption took place in several stages; pauses between these stages lasted no more than a few months, and erosion was already underway during the eruption. Hanauma Bay was colonized by coral reefs after it formed, and
marine benches developed within the bay; the exact origin of these benches is often not clear. The eruption of Kahauloa was synchronous to that of Hanauma Bay. • Koko Head was formed by the Koko Tuff. Later
explosion craters, such as Hanauma Bay, developed within Koko Head. Koko Crater was also the source of an ash layer that covers the surrounding terrain, and of
pyroclastic density flows. After Koko Head had formed, a second volcanic explosion generated another crater on its eastern foot and filled
gullies which had been carved into Koko Head by erosion. • Punchbowl Crater formed from the fallout of an
eruption column. Material from the column fell onto the coral plain where the vent was located, forming the crater. • The formation of the Sugarloaf flow was accompanied by tephra fallout. The tephra reached over thickness away in what is now downtown Honolulu, landing on a limestone 123,000 ± 2,000 years old. The flow was erupted on a ridge, west from the Manoa Valley. It is a thick
ʻaʻā flow, an unusual trait given its composition which resembles that of
Nyiragongo volcano which produces fast flowing lava flows. The eruption of the Tantalus vent was apparently synchronous to that of Sugarloaf; both eruptions were highly explosive due to a high volatile content of the magmas, shedding ash over a large area. • Ulupaʻu Head crater contained a
lake, which was once the largest lake of Hawaiʻi with a surface area of . It persisted for a long time during the Middle Pleistocene until the sea breached the crater rim. A number of
bird fossils have been found in the lake deposits.
Rano Kau on
Easter Island resembles the former Ulupaʻu Head lake. Many vents of the Honolulu Volcanics are furrowed;
erosion has cut gullies in the slopes of Diamond Head and Punchbowl Crater.
Wavecut terraces formed in some volcanoes during sea level highstands; it is likely that
wave erosion breached Hanauma Bay, flooding it, either during or after the eruption that created it. == Youngest activity and hazards ==