During eruptions, ascending magma often underwent interactions with water and thus caused steam explosions and the formation of particular volcanic structures such as tuff cones.

[5] Vents span the area southeast of a line between Mokapu Peninsula and Pearl Harbor, and extend from the ridges of Koʻolau volcano to below sea level and to the coast plain of southern Oʻahu.

[8] The volcanic system includes well-known landmarks of Honolulu such as Diamond Head, Koko Head, Punchbowl Crater,[3] Rabbit Island, Tantalus,[9] Hanauma Bay (notable as a snorkeling site)[10] and the Mokapu Peninsula,[11] which is the location of Marine Corps Base Hawaiʻi.

[17] Most cinder cones on Oʻahu are quite large, over 76 metres (250 ft) high and up to 0.80 kilometres (0.5 mi) wide.

[21] The vents of the Honolulu Volcanics follow northeastward-trending[6] alignments that are at right angles to the rift zone of the Koʻolau volcano.

[25] It is not clear whether these alignments are in any way related to the structure of the previous Koʻolau volcano, instead of being controlled by the crust of the Pacific Ocean,[6] but the trends along the Koko and Tantalus Rifts are parallel to that of the flexural arch[a] of Hawaiʻi Island.

[27] There is also a hypothetical "Diamond Head fault" that may be associated with earthquakes on Oahu that occurred in 1948, 1951 and 1961–1981, but it is not parallel to these alignments and its very existence is questionable.

[28] Submarine vents are also known,[29] including a 300-metre (980 ft) high solitary cone with two ridges off northeast Oʻahu, which is covered by pillow lavas and volcaniclastic sediments.

[38] Inland from Diamond Head lie the Kaimukī and Mauʻumae cones,[51] which appear to come from a shared fissure.

[7] The Mokapu peninsula was formed by Honolulu Volcanics and includes the three volcanic vents of Puʻu Hawaiʻiloa, Pyramid Rock and Ulapaʻu Head; additional vents form islets off the peninsula,[58] such as Moku Manu[59] and Mōkōlea Rock.

[66] The Salt Lake Tuff is associated with these craters and covers an area of at least 13 square kilometres (5 sq mi);[67] Honolulu International Airport and Hickham Air Force Base lie south and southwest from the vents respectively.

This volcano constitutes the tholeiitic stage of Hawaiian volcanism,[70] and developed possibly during Miocene to Pleistocene time.

[57] Cinders have red-black colours which can grade to yellow when they are hydrothermally altered, due to the formation of the glassy rock palagonite.

[93] In some vents, such as Diamond Head, the rocks are so heavily altered that their original composition[29] and texture can no longer be reconstructed.

[79] Groundwater contained in Honolulu Volcanics rocks, while not voluminous, is important in some areas such as Maunawili Valley.

[74] Some groundwater in Honolulu Volcanics rocks is saline, and has been used both as a water source for a sea-life park at Makapuʻu and for the discharge of salty wastewaters.

The first eruptions occurred within the Koʻolau caldera and the youngest in the far southeastern part of Oʻahu, coinciding with the Koko Rift.

[22] There is otherwise little evidence for a spatial pattern in the volcanic activity, with each rift having eruptions widely spaced in time.

[108] The lava flows from Honolulu Volcanics have been used to construct a history of variations of Earth's magnetic field.

[109] Eruptions of the Honolulu Volcanics have been correlated to shorelines[110] generated by sea level variations, which have left both drowned and emergent platforms and terraces on Oʻahu.

[123] 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.

[135] 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.

[152] Wavecut terraces formed in some volcanoes during sea level highstands;[111] it is likely that wave erosion breached Hanauma Bay, flooding it,[14] either during or after the eruption that created it.

[154] Future eruptions of Honolulu Volcanics are possible,[155] but the likelihood of a new event in the next hundred or thousand years is considered to be so small as to be negligible;[156] it is probably comparable to the Kohala peninsula on Hawaiʻi, the lowest risk area of the active island.

[156] Any future eruption is likely to occur in the southeastern sector of Oʻahu and will be of small volume,[156] involving the emplacement of cinders, lava flows and mudflows[71] with characteristics similar to those of past Honolulu Volcanics eruptions:[6] The Mokapu Peninsula has been prospected for the presence of geothermal power resources, but the presence of such resources was judged to be unlikely.