Michael Garcia Dept. of Geology & Geophysics University of Hawai‘i at Mānoa

Early explorers saw two stages of volcanism on O‘ahu: Young Diamond Head and eroded Ko‘olau Mountains

Ko‘olau Mountains Mt. Lē‘ahi (Diamond Head) Diamond Head Crater

View from the air of the classic landmark of Honolulu 2.2 to >3.3 Ma

Honolulu 2.9 to Volcanism 4 Ma

Rejuvenated volcanism only SE O‘ahu (Haskins and Garcia, 2004) Kalihi Vents (2) (Many) Nu‘uanu Vents (2)

Punchbowl Crater Tantalus Vents (3)

Rocky Hill Craters (3)

Airport (3) U.H. Mānoa Cone Mau‘umae Cone

Kaimuki Shield Waikiki

Diamond Head Crater Photo by P. Mouginis-Mark, SOEST Ko‘olau Mountains Volcanic hazards related to next Honolulu eruption would be catastrophic

Punchbowl Crater

Downtown Honolulu Photo by P. Mouginis-Mark

Koko Rift: Site of youngest Honolulu volcanism Site of 13 separate eruptions from Koko Head to Rabbit Island

* Submarine vents

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* ** Digital elevation map of O‘ahu with bathymetry of offshore Some Basic Facts on Honolulu Volcanism • At least 40+ distinct vents • Monogenetic eruptions (each vent erupts only once) • Many are young (<100,000), some 10,000’s of years Voluminous lava flows (100+ m thick) that flooded valleys (Mānoa, Nu`uanu, Kalihi) • Extremely explosive creating large tuff cones (Diamond Head, 1.2 km wide crater) with extensive tephra deposits Collaboration with Prof. Tagami from Kyoto University

Where

Tephra

Lava 41 samples from 32 separate vents New Age Results When

2nd

Two pulses at 0.8-0.35 and 0.1 Ma Ko`olau melting history ~3.5 Ma R

2.2 Ma 0.8 Ma O`ahu now

Plate motion Is Honolulu volcanism over? Depends on model

Secondary zone Plume

(Ribe & Christensen, 1999) Talk Highlights Honolulu volcanism was a violent chapter in O‘ahu’s history

It began 1.4 Myrs after death of the Ko‘olau volcano

Volcanism for 800,000 years from isolated vents and fissures

Future eruption?

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Questions? Ages of volcanism for Hawaiian Islands ? Two Periods of Hawaiian Rejuvenation Volcanism: Shield and GAP Rejuvenated Stages

• Gap present 1.0 Shield • Duration variable Stage • Rejuvenation absent on some shields; examples? • Duration of volcanism increasing to north?

(Ozawa et al. 2005) ? Rejuvenation Notable features

• Synchronous GAP rejuvenated volcanism • Gap variable in duration • Volcanism may Shield be pulsed as in Stage Canary Islands • More on W. Maui? • Long Kauai stage • Implications for (Ozawa et al. 2005) origin of Honolulu volcanism? Flexural Arch Model The flexural arch under southeast O`ahu 100,000 years ago

Trigger for the second pulse?

Rapid Trends of Tantalus and Koko rifts consistent with stress field soading: expected from the flexural arch Sinking 2.5 mm/yr Correct model? If so, volcanism probably done Models for Rejuvenated Volcanism

1. Conductive heating of the lithosphere by the plume 2. Upwelling from flexural related to rapid loading 3. Second zone of melting in rising plume (cartoon by B. Taylor, SOEST Dean)