GG103 Geology of Hawaiian Islands

I. Hawaiian Swell A. The geologic province of Hawai‘i is much more than simply islands sticking out of the sea. The entire province comprises three components that collectively make up the Hawaiian Swell: 1. Hawaiian Arch: a broad feature ~350 km across that rises about 1000 m above the deepest part of the Hawaiian Moat 2. Moat (or Deep): a series of deep troughs with axes lying about 100 km off shore. They are best defined on the north side of the ridge, and can be followed discontinuously up to about Gardner Pinnacles. 3. Hawaiian Ridge: rises ~4500 - 9200 m above the surrounding seafloor. Individual islands lie on the Hawaiian Ridge B. The structure of the Hawaiian Swell can be related to two main processes 1. Thermal effects (uplift) associated with the intrusion of hot material into the underlying mantle. (Hot material is less dense (more buoyant) than cold material). 2. Flexure (bending) associated with loading (volcanic construction) onto the surrounding seafloor

II. Hawaiian Ridge - Emperor Seamounts A. Conprises both the Hawaiian Ridge, trending WNW, and its continuation along the Emperor Seamounts, trending ~N-S. The overall geometry suggests a quasi-continuous feature with a bend west of Midway. B. Individual volcanoes lie on top of the Hawaiian Ridge. 1. They appear to lie on several curved loci, which generally step to the left along the Hawaiian Ridge and step to the right along the Emperor Seamounts. 2. No one really understands these lineaments; they may somehow be related to regional stresses in the Pacific Basin. In this scenario as magma is produced it becomes channeled along zones of weakness in the Pacific plate. C. There is a highly irregular volume distribution along the Hawaiian Ridge - Emperor Seamounts. 1. Huge masses at Maui-Big Island, Gardner Pinnacles and the Midway Complex 2. Thin and feeble between Kaua‘i and Gardner, and from Midway to Kanmu 3. It is apparent that production has not been constant at every location along the Hawaiian Ridge. D. Age progression

1. Hawaiian legends describe Pele changing house from west O‘ahu to Koko Rift to Haleakalā to Halema‘uma‘u Crater in Kīlauea. 2. J. D. Dana used geomorphology to estimate progressive ages of extinction of the principal volcanoes of the Hawaiian chain based on the amount of erosion each volcano had experienced. 3. Modern radioactive dating, mainly using the K-Ar technique, shows a relatively uniform age-distance relationship with an average slope of 86 mm/yr. 4. The regular age progression along the Hawaiian-Emperor Chain shows that this province is the result of some regular process that has been fairly systematic for at least the last 80 m.y. a. It also is a kind of clock, i.e. if you know where you are, you know the age of the rocks beneath you. b. All clocks use movement to measure time. c. What is moving in the Hawaiian Chain? E. The Hawaiian Ridge generally deepens to the WNW 1. The deepening is not very linear, i.e. depth is fairly constant between distances of 1000 to 2500 km from Kīlauea. 2. Several processes may be active in affecting the deepening. a. erosion b. subsidence, primarily related to cooling c. reef growth 3. Subsidence is very important. Most Hawaiian volcanoes have subsided 2-4 km since reaching the sea surface. What we see above sea level today are just the highest tops of ancient volcanoes.

III. Pacific Basin Structure and Plate Tectonics A. East Pacific Rise (EPR) 1. linear volcanic mountain range comprised of young volcanic rocks, generally similar to those found in Hawai‘i. 2. It is a region of extension or rifting. Opposing sides of the EPR are moving away from each other and new crust is being formed by volcanic processes in the divergent zone. 3. This process, known as seafloor spreading, results in an age symmetry about the EPR with progressively older ages farther from the ridge axis. 4. The EPR is one example of mid-ocean ridge; others occur in the other major ocean basins, e.g. Mid-Atlantic Ridge, Southeast Indian Ridge, etc. 5. The recognition that seafloor spreading is an active process on Earth provided important explanations for many long-standing observations in the Earth Sciences

a. As early as 1620 A.D., Sir Francis Bacon recognized that the continents could be fit together by "closing up" the Atlantic Ocean. b. Alfred Wegner proposed the hypothesis of Continental Drift from studies in 1915 - 1921. c. The seafloor-spreading hypothesis (1963) suggests that, rather than simply drifting around, the continents are riding on a region of crust that is in dynamic motion. 6. The Seafloor Spreading Hypothesis has several important implications a. The surface of the Earth is in relative motion. This requires that these surface regions must somehow be decoupled from the deeper parts of the Earth. Hence the concept of plates. b. If the Earth is expanding around mid-ocean ridges then the Earth as a whole must either be getting larger or else there must be complementary zones of convergence somewhere. B. The unifying theory of Plate Tectonics 1. The basic concept is that the Earth can be divided into plates (thin relative to their areal extent) that move with respect to one another. 2. Plate boundaries are of three main types. a. divergent, e.g. mid-ocean ridges, also called spreading centers; where two plates move away from each other. b. convergent: where two plates come together. i. where convergence is between two oceanic plates (e.g. Marianas) or between an oceanic plate and a continental plate (e.g. Andes, Japan, Aleutians), one plate slides under the other in a process known as subduction. Arc volcanism is produced above the down-going subducted plate. ii. where convergence occurs between two continental plates neither plate is subducted but great buckling ensues (e.g. Himalayas) c. transcurrent: where two plates slide past one another. i. Transform faults are a type of transcurrent plate boundary linking two spreading centers. The healed manifestations of transform faults on older plates are called fracture zones. ii. some transcurrent faults link spreading centers to subduction zones, e.g. San Andreas Fault zone, or two subduction zones, e.g. Alpine Fault of New Zealand. 3. The recognition and confirmation of plate tectonic theory has been enthusiastically accepted by most geologists, because a. For the first time we had an explanation for the great mountain ranges of the world and the distribution of volcanoes and active seismic zones. b. It explains many details of why arc volcanoes are so different from Hawaiian volcanoes

c. It tells us what moves to create the Hawaiian clock i. If the rate of motion of the Pacific Plate has been relatively constant over the last 80 m.y., then the source of magma for Hawai‘i must lie beneath the base of the plate and to have been more or less stationary relative to the deeper parts of the Earth. ii. The nature of the "source of magma" for Hawai‘i is not completely understood but the term hotspot has been used to denote such relatively stationary, deep-seated sources of magma. d. When the concept of hotspots is incorporated into Plate Tectonic Theory, especially seafloor spreading, explanations for most of the features of the Pacific Basin emerge and it is apparent that the Hawaiian chain is not unique. i. Linear island chains, aligned parallel to absolute plate motions are associated with the Galápagos Samoan, and French Polynesian island groups ii. Great scars striking across the Pacific Basin are Fracture Zones related to ancient spreading centers now subducted beneath North America. iii. It has been suggested that the Mid-Atlantic Ridge, and associated splitting of the American, Eurasian and African plates, occurred around hotspots associated with Iceland, Azores, Ascension, Tristan da Cunha and Bouvet Islands.

IV. Questions A. What drives plate tectonics? B. How thick are plates? C. Has plate tectonics always been operating on Earth? When did it start? Does it operate on other planets of our solar system? D. Why did the Pacific Plate change motion 42-43 m.y. ago? E. What are hotspots? How big are they? F. How does Hawaiian volcanic history change as the Pacific Plate approaches the hotspot, moves over it and then back off of it again?