Geology of Island of O‘ahu• Composed of two different shield volcanoes

– Wai‘anae Volcano, > 4.0 to 2.8 Ma• Shield-building and postshield alkalic cap stages• No rejuvenation stage!

– Ko‘olau Volcano, 2.5 to 1.5 Ma (shield stage)• Shield-building and rejuvenation stages• No postshield alkalic cap!

• Both volcanoes have also passed through erosion and coral reef growing stages– amphitheater-headed valleys– Abundant raised reefs

• Both volcanoes have experienced giant submarine landslides

Ko‘olauWai‘anae

O‘ahu

Penguin

Bank

Wai‘anaeSlump

Ka‘ena Ridge Ka‘enaSlump

Wai‘anaeVolcano

Ko‘olauVolcano

Island of O‘ahu

Island of Oahu – Physical features

Pacific Ocean

Ulupau crater

Formation of Waianae Volcano 4 million years (MY)

ago

Waianae shield building, and Koolaus’ emergence 2.5 MY

Making up of Oahu

Present day Oahu

Oahu, 2 MY ago

Wai‘anae Volcano, Shield Stage• Lualualei Member

– >4.0 to 3.55 Ma– normal tholeiitic basalt, ~50% silica

• Kamaile‘unu Member– 3.55 to ~3.1 Ma– transitional caldera-filling sequence

• wider variety of minerals and rock types

• some are very silica-rich, up to 66%

• dying magma chamber?

Waianae Volcano, Postshield Stage

• ~3.1 to 2.8 Ma• Mostly composed of hawaiite, an intermediate,

alkalic igneous rock• Broken into two members

– Pālehua (older) and Kolekole (younger)

– separated by an erosional surface• Kolekole lavas are found on top of mudflow deposits

– Kolekole is more mafic (has less silica) than Pālehua • may be related to the Wai‘anae Slump, a giant submarine landslide

– removal of mass would lower pressure within the volcano, promoting decompression melting and higher degrees of partial melting

Ko‘olau Volcano, Shield Stage• Tholeiitic basalt, but unusually high Si, Al, Na,

and Fe– old idea: chemical variations within the hot spot

plume– New data from drilling shows that unusual

chemistry disappears with depth and that deeper rocks are more like other shield-stage lavas in Hawaii

• therefore, unusual chemistry is only in the top layer of Ko‘olau

• may represent decline of melting in the volcano– was on its way toward a postshield stage that never happened

The Ko’olau erupted along vertical cracks that extended from beyond Kahuku in the north to past Makapu’u in the south. A central vent and summit depression, or caldera, formed near the present region of Kaneohe and Kailua. The major volcanic activity stopped here about three million years ago. The Ko’olau volcano was also partially built by lava rising to the surface through thousands of cracks in preexisting flows. Lava cooled in these cracks and formed nearly vertical sheets of dense basaltic rock called dikes. Many such dikes, each about one meter (3 ft) in width, can be seen in the high cuts made for the Pali Highway near Castle Junction and along the Kaneohe - Kailua section of the H-3 highway near Mokapu Blvd. These dikes retained infiltrating rainwater and created a storage area as well as the artesian springs at lower elevations.The top of the Ko’olau volcano was about 3.8 km (2 mile) above sea level when it ceased building. The old crater region covers the area from Waimanalo to Kaneohe. Also located in the site of the old crater is Olomana Peak, an isolated mountain that sweeps up into a sharp 500 m (1,643 ft) peak.

Nu‘uanu Landslide

• Longest in the Hawaiian Islands, 230 km long• Traveled 80 km past axis of the Hawaiian Deep

– in other words, moved uphill

• Moved as a giant debris avalanche– a more catastrophic type of landslide

– moved as isolated blocks

– the Wai‘anae Slump, by comparison, moved as a single unit (similar to Kīlauea)

• Age?– Paleomagnetism gives an ambiguous age, > 2.58 or 1.8 Ma,

but well within the shield stage of Ko’olau

Geologic History of Koolaus Volcano: A. Formation about 2.5 million years ago

B. Giant Landslide about a year ago:Collapse ofKoolaus’Caldera due to a giant slump into the ocean, Nuuanu Landslide

Ko’olau Volcano, Rejuvenation Stage• Called the Honolulu Volcanic Series

• Very low silica rocks, ~36-42% SiO2

– very low degrees of partial melting

– come directly from mantle, no magma chambers

• Ages range from 800-600 Ka (poorly constrained) at the start to as young as 30 Ka

• Sometimes erupt simultaneously along fissures– Koko Fissure

– Sugarloaf-Roundtop-Tantalus

• Both “wet” and “dry” eruptions, even in same fissure• Vents do not lie on shield’s rift zone, but rather their

alignments are perpendicular to it

true alignments…or wishful thinking?

Map of a very large lava flow field (magenta) recently discovered on the Hawaiian arch north of O'ahu and Kaua'i. Note the size of this flow compared to the islands (adapted from Clague et al. 1990). Age may be recent.