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Coastal Geology Group
- Studying coastal change.
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Introduction

The Hawaiian Hot Spot lies in the mantle under, or just to the south, of the Big Island of Hawai‘i, where two active subaerial volcanoes and one active submarine volcano reveal its productivity. Centrally located in the Pacific plate, the hot spot is the singular source of the Hawaiian Island Archipelago and its northern arm, the Emperor Seamount Chain. This system of islands and associated reefs, banks, atolls, and guyots spans over 30º of latitude across the Central and North Pacific Ocean to the Aleutian Trench, and contains at least 107 separate shield volcanoes (Clague and Dalrymple 1987). The trail of islands increases in age with distance from the hot spot and reflects the dynamic nature of the Pacific plate, serving as a record of its paleospeed and direction over the Hawaiian hot spot for the last 75-80 my (Clague and Dalrymple 1987). A major change in plate direction is marked by the northward kink in the chain at the end of the Hawaiian Ridge approximately 3,500 km from the site of active volcanism (Moore 1987). Today the Pacific plate migrates northwest at a rate of about 10 cm a year away from the hotspot at the southeast corner of The Big Island (Moore 1987). The eight main islands in the state: Hawai‘i, Maui, Kaho‘olawe, Läna‘i, Moloka‘i, O‘ahu, Kaua‘i, and Ni’ihau, make up 99% of the land area of the Hawaiian Archipelago. The remaining one percent exists as small volcanic and carbonate islands offshore of the main islands, and to the northwest.

Building Volcanoes in Hawai‘i

The main bodies of the Hawaiian Islands are great shield volcanoes built by successive flows of pähoehoe and ‘a‘ä basalt lavas that evolve through a recognized sequence of morphological and chemical stages (Clague and Dalrymple 1987).

Alkalic basalts dominate the earliest submarine pre-shield building stage; with a transition to tholeiitic basalt that occurs as the volcano grows and approaches sea level. Loihi seamount, on the southeast flank of Mauna Loa Volcano on the Big Island, is an example of this stage. Loihi rises over 3000 m from the sea floor and erupts hydrothermal fluids at its summit and south rift zone. Loihi’s current growth rate may allow it to emerge above sea level in the next few thousand years.

The subaerial main shield building stage produces 98-99% of the lava in Hawai‘i. Kïlauea Volcano on the southeast flank of Mauna Loa on the Big Island, is actively undergoing shield building. Kïlauea Volcano alone has produced ~2km ³ of lava since 1983 (Garcia et al. 2000). The main shield building stage produces tholeiitic lavas of basalt and volcanic glass with olivine, clinopyroxene, and plagioclase as the primary crystal components.

Declining eruption rates accompany the onset of a post-shield building stage. This stage marks the end of active volcanism and is dominated by massive flows of ‘a‘ä, and marked by a shift back to alkalic lavas that typically cap the top of the shield.

In some cases, after lying quietly in erosional conditions for thousands-to millions of years, some Hawaiian volcanoes experience a rejuvenation stage of eruptions, from a distinctly different source. These silica-poor alkalic lavas typically contain combinations of nepheline, olivine and clinopyroxene with either melilite or plagioclase. Post-erosional rejuvenated eruptions are more explosive than the less viscous eruptions of the shield building stage, commonly producing cinder and tuff cones around the active rift zones, and vents atop the eroding older shields. Three edifices on the island of O‘ahu: Diamond Head, Koko Head, and Koko Crater are the result of this process.

Although this is the classic sequence for Hawaiian shields, an individual volcano may become extinct during any of the phases, ending the evolution of that particular shield (Moore 1987).

Sand and Detritus at the shoreline

Hawaiian carbonate sand is mainly a creamy white calcareous mix, derived from the tests of microorganisms, weathered coral, calcareous marine algae, and mollusk shells (Harney et al. 2000). The black and green sand beaches on the islands are derived from eroded volcanic material. Where volcanism is active, black sand is produced at sites of littoral volcanic explosions at the coast, where lava flows into the ocean. Lava entering the ocean is instantly quenched from ~1300˚ C, resulting in glass that is crushed by waves into sand-sized grains. In Hawai‘i, basalt is generally a secondary source of sand, but a primary source of detritus as the basaltic islands erode and material is transported to the coast. Green sand beaches contain enough olivine to acquire the color of the mineral, and occur where olivine is significantly weathered out of basalt in the backshores of beaches. Ferromagnesian olivine and other basaltic minerals are relatively unstable in the tropical climate of Hawai‘i, eroding quickly folloing exposure. Calcareous beaches are dominant on all the older Hawaiian Islands where significant coral reef communities have been able to develop (Moberly and Chamberlain 1964). Though there are flurishing reef systems around the older Hawaiian islands much of the sand may have been produced at earlier times (up to 5000 yr B.P. in Kailua Bay on O‘ahu) (Harney et al. 2000).

The coastal plain of most Hawaiian Islands holds major land based sand reservoirs, of variable volumes, in the zone between approximately 1 m below sea level and 2 – 3 m above sea level. These sands have been radiocarbon dated in the range 1000 to 4000 yrs BP. Their position is a function of late Holocene high sea levels dating from the same period, and persistant eolian deposition under seasonal winds (Fletcher and Jones 1996; Grossman et al. 1998). Shore-normal reef channels that are partially filled with sand augment beach sediment budgets. Longshore transport dominates sediment movement along the coast in distinct littoral cells. Both offshore sand reservoirs as well as erosion of coastal plain sandstones play important roles in the seasonal and storm-related erosion and accretion of Hawaiian beaches. Offshore sand reservoirs exist in natural channels cut in fringing reefs around the islands during lowered sea levels, and in innumerable shallow ref flat depressions formed by karst action during periods of subaerial exposure (Moberly 1964).

Terrigenous sediment in localized coastal stream and river deltas may be reworked and provide a locally important source for neighboring beaches. In general, the carbonate beaches of Hawai‘i are the leading frontal edge of coastal plain sand deposits dating to the late Holocene Kapapa Stand of the sea (+2 m), although localized exceptions to this rule abound.

Wind and Waves

The central pacific location of the Hawaiian Islands exposes them to wind and ocean swells from all directions. The Islands’ relative locations however, may provide rain, wind, and wave shadows, that create unique exposures of coastline that are either protected from, or vulnerable to, wind and wave impact. One example of this type of relationship occurs in the Maui Group of islands, Maui, Läna‘i, Moloka‘i, and Kaho‘olawe. Here, Maui, elongated northwest-southeast, and Moloka‘i, elongated east-west, provide substantial protection for the north and east coasts of the smaller islands that lie in their lee: Läna‘i and Kaho‘olawe. The north coasts of these islands lack significant sea cliff development and are low-lying. This morphology directly reflects these islands protection from prevailing winds and strong seasonal storms in the north Pacific (Macdonald et al. 1986).

The north shores of the Hawaiian Islands are best known for world-class surfing conditions that persist in the winter between October and March. Long period swells (12-20 seconds) derived from energetic sub-polar and mid-latitude storms in the north Pacific bring waves to the north coasts (282º-45º). These waves average 1.5 – 4.5 m in height, with extraordinary extreme wave heights of up to 15 m, and an annually recurring maxium significant waveheight of approximately 7.5 m. North shore waves break over near and offshore fringing reef systems, with faces twice the height of the waves, creating tubular and A-frame waves, and strong (dangerous) rip currents at the shoreline (Fletcher et al. 2002).

Hawai‘i is located in the tropical latitudes and northeast trade winds dominate, consistently blowing15–30 km/h, with periods of exceptionally strong and gusty winds up to 65–95 km/hr. Short period wind swell (6-8 seconds) generates waves that break over fringing and outer reef systems exposed on the windward side of the islands , heralding average wave heights of 1–3. m, with greater heights associated with particularly strong wind events. The trade winds prevail approximately 70% of the year, with maximum intensity and consistency between April and September.

Exposed south and west facing shores (258º-147º) are subject to Kona storms that dominate mainly during periods of weakened trade wind activity. Kona storms occur less than 10% of the year, generating long and moderate period waves (6-10 seconds) that are steep faced typically reaching heights of 3–4.5 m.

South and southwest (210-147º) coasts are also impacted by storms of the south Pacific that are associated with southern hemisphere winter season: April through October. These strong storms blow over long fetches and generate waves that travel thousands of kilometers to reach Hawaiian shores, arriving 53% of an average year. These long period swells (12-20 seconds) occur most regularly and with the most power in the summer when waves averaging 0.3–1.8 m high, bring abundant recreational opportunities and, refreshing surf to the south shores during the hot summer months.

Sea Level

Local relative sea level in Hawai‘i is not only dependent on global eustatic trends (which at the time of this writing is +2 mm/yr.), but is also affected by subsidence of the oceanic lithosphere, which responds elastically to volcanic loading over the hotspot. It is estimated that one half of the upward building of Hawaiian volcanoes is reduced by subsidence and that most of the volcanoes have subsided 2–4 km since emerging above sea level (Moore 1987).

The main Hawaiian Islands span ~5 my in age, and are at differing distances from the hot spot foci of subsidence, thus they are each at unique stages of subsidence. The result is that the islands experience different relative sea levels. These relative differences in sea level are demonstrated by modern tide gage rates and support the view that subsidence is active over the hot spot (Moore 1987).

Evidence such as: submerged wave cut notches and benches, raised coral reef, subaerial marine terraces, and entire coastlines of alluviated river valleys can be found at different elevations and locations around the state. These types of evidence, found around the islands, indicates that Hawaiian shorelines have been drowned and reestablished with eustatic trends and island-specific vertical movement. The fossiliferous marine conglomerate 80 m above sea level on south Läna‘i, are typical examples of indicators of past stands of the sea (Rubin et al. 2000).

Holocene sea level has been influenced both by eustatic postglacial meltwater as well as equatorial oceanic siphoning associated with the changing postglacial geoid (Mitrovica and Milne 2002). These lead to a highstand approximately 2 m above present ca. 3000 yr BP followed by se level fall culminating in the pre-modern period. Tide gages record sea level rise since only 1900 in Hawai‘i.

Erosion

The Hawaiian Islands are experiencing widespread but locally variable coastal retreat in response to a history of human interference with sand availability and the inferred influence of eustatic rise. In pristine coastal areas calcareous sand stored on the low-lying coastal plain is released to the beach as sea level rises, allowing the beach to maintain a wide sandy shoreline even as it migrates landward.

Coastal property in many areas of Hawai‘i is at a premium, and the encroachment of the Pacific Ocean onto multimillion-dollar residential and commercial lands and development has not gone unnoticed by landowners. The response in many cases has been an armoring of the shoreline. Artificial hardening of the shoreline is a form of coastal land protection that occurs at the expense of the beach where there is chronic recession, preventing waves from accessing the sandy reservoirs impounded behind the constructed coast. Thus, efforts to mitigate coastal erosion have created a serious problem of beach erosion and beach loss along many shorelines in the state, particularly on the most populated and developed islands (Fletcher and Jones 1996). The need to address this issue is acknowledged by the state and local communities, and the hope is that a broadly scoped management plan will keep the Hawaiian shorelines in balance: between the natural coastal morphology and human resource needs.

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