GEOLOGY OF THE HAWAIIAN

ISLANDS

KILAUEAThe tholeiitic shield lavas on Kilauea have been

divided into two groups. The older flows are only exposed in the fault scarps of the Hilna Pali system on the south flank of Kilauea. These lie below an extensive ash horizon called the Pahala ash.

• Puna Basalt—Tholeiitic basalts that overlie the Pahala ash and cover most of the surface of Kilauea. Mostly less than 10,000 yrs old.

• Hilina Basalt—Older tholeiitic basalts exposed in fault scarps on the Hilina Pali. May be as old as 100,000 years but mostly younger.and education.

MAUNA LOAAll of Mauna Loa volcano is also tholeiitic shield

lavas. The oldest lavas are exposed on the south end of the volcano in the Ninole Hills above Pahala and in the Kahuku fault scarp west of South Point. The Kahuku basalt is exposed in the landslide scarp and the Ninole Basalt is the remnant of a “Hilna Pali” system on the southeast side of Mauna Loa.

• Kau Basalt—Tholeiitic Basalts that cover the surface of Mauna Loa. >>185 m thick.

• Ninoole and Kahuku Basalts—Older tholeiitic basalts that underlie the Pahala Ash and may be as old as 200,000 years.

Hualalai• Hualalai Volcanics—Alkali basalts and rare

hawaiite that covers the surface of Hualalai volcano. Less than 100,000 years old. Youngest flows were erupted in 1800-1801. Many flows are less than 10,000 years old.

• Waawaa Trachyte Member—Trachyte cone and flow on north side of volcano. Dated at 105,000 years old. Trachyte has also been found underlying the alkali basalt in drill holes.

• There is no tholeiitic basalt on the surface of Hualalai, but it has been found in drill holes and is exposed on the submarine rift zone of Hualalai.

Mauna Kea• Divided into 2 formations. Lapahoehoe Volcanics are

the more evolved post shield alkalic rocks that form the steep cap on Mauna Kea. The Hamakua volcanics are akalai basalt overlying the tholeiitic shield basalts.

• Lapahoehoe Volcanics—Youngest eruptions about 3,600 years ago and 8,000 years ago. Both post date the youngest glacial deposits on the summit. Lapahoehoe Volcanics are mostly evolved hawaiite that built steep cones and stubby flows. The oldest flows are thought to be less than 200,000 years. An older glacial deposit is interlayered with the lower parts of this unit.

• Hamakua Volcanics—Alkali Basalt flows and a glacial deposit that overlie the tholeiitic shield flows. The oldest alkali basalts date back to 275,000 to 375,000 years ago. The tholeiites are older than this (remember the submarine rift zone is covered by a 400,000 year old reef with no lava flows).

KOHALA• Composed of 2 units, a post shield stage of hawaiite with

mugearite, benmoreite, and trachyte (more silica rich lavas) and a lower unit composed of tholeiitic basalt overlain by transitional alkalic basalts near the top.

• Hawi Volcanics—One of the most evolved sequence of post shield lavas in the Hawaiian Island. These are more silica rich than most and consist of hawaiite, mugearite, benmoreite, and trachyte as steep cones, domes, and thick flows capping the main ridge of Kohala Volcano. Ages range from 60,000 to 260,000 years old.

• Pololu Basalt—Upper alkali basalts may have filled caldera or landslide structure near summit and cap the tholeiitic shield stage lavas. Range in age from 300,000 years to 450,000 years.

HALEAKALA––MAUILargest and most complete seqence of lavas preserved on

a single volcano in the Hawaiian Islands. Contains all three stages of volcanism.

• Hana Volcanics—Rejuvenated ultra alkaline volcanics that form most of the cones on the rift zones and inside of Haleakala crater. These flows also have filled the large Kaupo and Keanae valleys making them appear flat floored. The youngest eruption was 1790. The Hana volcanics may have begun erupting 200,000 years ago.

• Kula Volcanics—Postshield alkali basalt and hawaiites that cap most of the rest of the surface of Haleakala and also make up most of the walls of the summit crater. Range in age from 450,000 years to 750,000 years.

• Honomanu Basalt—Shield stage tholeiitic lavas only exposed on the north side of Haleakala near sealevel. Youngest flows are 750,000 years old.

WEST MAUI• The tholeiitic shield lavas on Kilauea have been divided

into two groups. The older flows are only exposeLahaina Volcanics—Rejuvenated ultra alkalic basalts (basanites) found in small patches. 1.3 million years old?

• Honolua Volcanics—Postshield stage lavas of mugearite, trachyte, and hawaiite. Caps many ridges on the volcano. 1.5 million years old?

• Wailuku Basalt—Tholeiitic shield stage volcanics, found in canyons and in Iao Valley which may have been the caldera. Capped by transitional alkalic basalts at top. Probably greater than 1.5 million years old.d in the fault scarps of the Hilna Pali system on the south flank of Kilauea. These lie below an extensive ash horizon called the Pahala ash.

KAHO‘OLAWE

Kanapou Volcanics—Post shield hawaiite and alkali basalt capping shield stage tholeiites. Upper alkalic basalts are about 1 million years old.

LANA‘I

Lanai Basalt—Shield stage tholeiites. Some of which have been thought to be caldera filling, but recent evidence suggests landslides on the west side of Lanai may have destroyed the summit area. Upper units date at about 1.3 million years.

EAST MOLOKA‘I• East Molokai Volcano makes up most of the island, but

its north half was severed by the Wailau slide. The Kalaupapa Penninsula is a rejuvenated shield volcano with lava tube system (very rare in the rejuvenated series) that grew up on the north side of the volcano after the landslide.

• Kalaupapa Volcanics—Rejuvenated alkali basalt and basanite volcano on north side of Molokai. About 350,000 years old.

• East Molokai Volcanics—Consists of upper post-shield stage mugearite, hawaiite, and trachyte overlying shield stage tholeiites including a caldera filling stage. Upper member ranges from 1.35 to 1.5 million years, lower member ranges from 1.5 to 1.75 million years old.

WEST MOLOKA‘I

• West Molokai Volcanics—Post shield hawaiite and alkali basalt overlying shield stage tholeiites. Ages range from about 1.5 to 2 million years ago. Prominent scarp on east side of this volcano marks the boundary with East Molokai volcano and may represent a major landslide scar.

KOOLAU ––OAHU• Honolulu Volcanics—Rejuvenated basanites,

nephelinite, melilitite, and alkali basalts that range in age from 30,000 years to 0.9 million years old. Numerous small flows, cinder and spatter cones, and a prominent group of tuff cones characterized by Diamond Head and Punch Bowl Craters. Salt Lake Crater contains numerous pyroxene xenoliths from the mantle as well as some rare lherzolite fragments (normal mantle rocks).

• Koolau Basalt—Shield stage tholeiitic basalts with some transitional alkalic basalt near top. Caldera filling lavas and dikes are exposed in the Kailua area, though much of the eastern part of the volcano has slid into the ocean.

• Ages range from 1.8 to 2.7 million years old.

WAIANAE ––OAHU• Kolekole Volcanics—Rejuvenated lavas in small

patches mostly at the southern edge. Age uncertain, but probably less than 2 million years.

• Waianae Volcanics—Upper sequences of post shield stage hawaiite and lesser amounts of alkalic basalt that cap the Waianae Range. Ranges in age from 2.5 to 3.0 million years. Lower part of consists of transitional alkalic basalt at top and filling caldera? Includes an almost “rhyolite” rock called rhyodacite, the most silicic lava in the Hawaiian Islands. Overlies thoeliitic shield lavas that are > 450 m thick. Age of this lower sequence runs from about 3-4 million years.

KAUAI• The geology of Kauai is currently open to some debate

as to whether there were one or two volcanoes. The older literature suggests only one, but more recent unpublished work suggests there may have been two. By sheer size alone, I’d vote for two. In the meantime, we’ll just consider the island one big lump of volcanics and not worry about how many volcanoes make it up.

• Koloa Volcanics—Rejuventated lavas of basanite, nephelinite, melilitite, and alkalic basalt that cover much of the eastern lower part of the island. Dates range from 600,000 years to 2.5 million years.

• Waimea Canyon Basalt—Shield stage tholeiitic lavas locally overlain by small amounts of post-shield stage hawaiite. The summit “swamp” of the island is capped by a series of flat lying basalts that are thought to have filled in a summit caldera that measures more than 10 km across by far the largest in the Hawaiian Islands, if it is real (which I personally have doubts about).

NA PALI––WEST KAUAI• To the south Waimea Canyon is the west side of a fault

bounded graben that contains another sequence of thick flows, which contrast with the thin bedded flows that make up the west side of the island. This may be an early slide structure that was filled in, but it’s origin is uncertain. On the southeast side of the island there is another sequence of flat lavas interpreted as a smaller “flank” caldera, another unique occurrence in the Hawaiian Islands. Confusing place this is. Anyway, the shield stage lavas range from about 3.5 to 5 million years. This is probably too big an age span for a single volcano, but the dates are not very reliable. The main flank lavas on the west side range from about 4.3 to 5 million years, a bit better age range.

• old.

NIIHAU• Kiekie Basalt—Rejuvenated akalic basalts and

pyroclastic rocks on the flat coastal plain near sealevel. Age uncertain, but thought to be around 2 million years old. The only cone is the partially submerged tuff cone that make up Lehua Island on the northwest side of the island.

• Paniau Basalt—Tholeiitic shield basalts with rare post-shield alkalic basalt. Makes up higher northeast end of the island and is the apparent remnant of the shield volcano. The sharp eastern side of the island and extensive landslide deposits to the east of the island on the seafloor suggest that much of the island is now on the seafloor.