Geologic History of Hawai'i

Hawai’i is located thousands of kilometers from the nearest plate boundary, but it is volcanically active and geologically very young. The oceanic crust on which the Hawaiian Islands reside is nearly 90 million years old, yet the oldest of these islands was formed a mere 5 million years ago. In fact, the youngest is less than a half million years old. The Hawaiian Islands are the surface expression of a mantle hot spot—a place where a large slab of crust rides over the top of a rising plume of hot rock in the underlying mantle. The Hawaiian hot spot has its origin deep within the Earth, near the core-mantle boundary, where an area of anomalously high temperature creates a thermal plume: a zone of solid mantle material that moves slowly upward toward the surface. Thermal stress drives the motion of the plume; the hotter material rises because it is less dense and therefore more buoyant than its surroundings. As magma created by the plume erupts onto the seafloor, repeated lava flows build a massive volcano that eventually reaches the surface of the ocean and becomes a volcanic island.

The Hawaiian hot spot lies beneath the Pacific plate, where it maintains a relatively fixed position within the mantle. The overlying Pacific plate moves to the northwest at the tectonically rapid rate of 8.5 centimeters per year (3 inches per year). The northwest motion of the plate eventually carries each island away from the hot spot, creating a chain of volcanic islands whose age increases with increasing distance from the hot spot (Figure 1.13). The Hawaiian Islands are the southernmost part of a string of islands and undersea mountains stretching from the middle of the North Pacific to the Aleutian Islands of Alaska. The Emperor Seamounts, the northernmost end of these undersea mountains, are the oldest. The youngest islands are currently over the hotspot, and still volcanically active, whereas the older, extinct islands reveal the track of oceanic crust moving over the hotspot.

The Hawaiian hot spot has been active for at least 80 million years, based on the age of the oldest island, Meiji Seamount, now far to the north near the Aleutian trench. The hot spot remains active today, erupting new lava at Kīlauea volcano and the still-submerged Lō’ihi seamount—the youngest volcano in the chain.

As each volcano forms above the hot spot and moves away, it undergoes a sequence of constructive and then destructive geological processes. These processes, detailed below, have created a chain of volcanoes that extend 5800 kilometers (3600 miles) from the Hawaiian Islands to the Emperor Seamounts.

Figure 1.13: Interaction of the Pacific plate and the Hawaiian hot spot produces a chain of volcanic islands that increase in age with increasing distance from the hot spot. The Emperor Seamounts are the continuation of the Hawaiian Islands, and formed while the plate was moving directly north.

Lifecycle of Hawaiian Volcanoes

For more than 50 years, geologists have recognized that Hawaiian volcanoes exhibit a relatively consistent set of features reflecting events in the island’s lifecycle. These events are typically divided into nine stages, although (as with all natural processes) there may be overlap and gradation between them. Over time, the volcanoes exhibit changes in morphology and behavior, as well as chemical changes in the lavas they erupt. Early lavas are derived from near the edge of the thermal plume, a magmatic source that is chemically different from lavas derived from the center of the plume.

As one of the best-developed examples of an oceanic hot spot system, the Hawaiian Islands provide a unique window into geologic processes—a time machine through which we can look either forward or backward at the creation and evolution of the islands.

(1) Deep Submarine Stage

While not all Hawaiian volcanoes go through every stage of development described here, we assume that all volcanoes begin with a deep submarine stage, where magma erupts through the seafloor and solidifies. During the deep submarine stage of volcano formation, the magma is made up of alkalic basalt, which is derived from the edge of the thermal plume. Alkalic basalts contain around 47% silicates, and are relatively high in sodium and potassium. On the seafloor, they erupt as dense lava flows form the initial volcanic shield. Lō’ihi, currently forming off the south coast of Hawai’i Island, is an example of the deep submarine stage.

(2) Shallow Submarine/Early Shield Stage

After layers of early dense lava flows have accumulated, they create a broad submarine shield. When lavas erupt on the deep sea floor, the pressure of the overlying water column keeps gases like oxygen, carbon dioxide, and hydrogen sulfide dissolved in the magma. However, as the growing volcanic shield approaches the ocean surface, the gases begin to form bubbles in the magma, or exsolve. These bubbles can be preserved in the chilled lava as vesicles. The exsolving gasses can also contribute to a more violent style of eruption. When the summit of the growing volcano approaches the sea surface, the interaction of seawater with the erupting magma can be quite spectacular, characterized by jets of steam, shattered rock, and vaporized lava. No eruptions of this type occur in modern Hawai’i, although from similar eruptions observed in Iceland and elsewhere in the Pacific, we hypothesize that they must have been part of Hawai’i’s past.

(3) Subaerial Shield-Building Stage

The submarine and subaerial shield-building stages are the main constructive phases of Hawaiian volcanism. By the time the volcano has emerged above sea level, the lavas have undergone a change in chemistry. Movement over the center of the hot spot produces lavas with fewer alkali elements and more silicates, creating tholeiitic basalt, the second major type of Hawaiian lava, This highly fluid lava erupts voluminously over 0.5–1 million years, continuing to build a classic shield-shaped volcano. These volcanoes erupt from shallow magma reservoirs located a few kilometers beneath their summits, and also from radial rift zones that channel magma lower onto the flanks of the active volcano. High-volume eruptions that rapidly empty the magma reservoir often lead to summit collapse and the formation of a volcanic caldera: the large steep-walled crater that crowns many shield volcanoes. While a summit caldera is characteristic of the shield-building stage, some volcanoes do not appear to have one (e.g. Hualālai, West Moloka’i), and the well-developed summit caldera on Lō’ihi shows that caldera formation may also occur earlier in the life cycle of a volcano. Mauna Loa and Kīlauea, which both have large summit calderas, are examples of volcanoes in the subaerial shield-building stage.

(4) Post-Caldera Stage

Most summit calderas are eventually filled and buried by continuing eruptions of tholeiitic lavas. As the volcano moves away from the center of the hot spot, the eruptions change composition again, returning to alkalic basalt. These later eruptions cap the older shield, and their slightly more explosive nature gives rise to lava fountains that produce cinder cones on the summits and flanks of the volcanoes. These cones give older volcanoes a bumpy profile, in contrast to the smooth, younger shield. Alkalic cap lavas often contain numerous phenocrysts: early-formed crystals that are entrained in the magma as it erupts. The growth of crystals changes the composition of the remaining liquid magma, and late-stage eruptions can sometimes produce unusual lavas that are so silica-rich they are no longer called basalts. Mauna Kea and Hualālai are in the post-caldera stage, and the Pu’u Wa’awa’a cinder cone on Hualālai’s north flank produces high-silica eruptions called trachyte.

(5) Erosional Stage

While the first four stages in the volcanic life cycle are predominantly constructional, it is important to recognize that erosional processes begin as soon as a volcano rises above the sea floor. Submarine landslides that drape the flanks of Lō’ihi, and prominent fault scarps (pali in Hawaiian) on Kīlauea and Mauna Loa are evidence that even young volcanoes begin to experience gravitational collapse. Once eruptive activity slows, erosion and weathering become dominant forces in shaping the evolution of the islands. Soils form, and streams—absent on very young volcanoes—begin to incise the surface of the shield. Hawaiian volcanoes are also subject to episodic, catastrophic, erosional events, when mega-landslides rip apart the unsupported seaward flanks of the islands. These landslides scatter enormous blocks of material for hundreds of kilometers (miles) across the seafloor and generate tsunamis with local run-up heights up to 300 meters (980 feet) above sea level. Kohala and Haleakalā are examples of erosional stage volcanoes.

Figure 1.14: Sunrise view of Hawai’i Island from the summit of Haleakalā, Maui. Four of Hawai’i’s five subaerial volcanoes are visible. On the leftmost side is Mauna Kea, in the post-caldera stage, and smaller, eroded Kohala peeks through the clouds directly in front. Mauna Loa, a giant shield volcano is on the right, and on its flank is smaller Hualālai, also a post-caldera volcano. Although the topography appears gentle and subdued, both Mauna Kea and Mauna Loa rise more than 4,000 meters (13,100 feet) above sea level.

(6) Reef-Building Stage

Hawai’i’s tropical latitude permits the growth of reef-building corals. The reefs are found principally on the leeward side of each island where there is little river runoff and thus very clear water. Many of these corals live at or near sea level, making the reefs excellent indicators of sea level rise and fall. As the islands age, the coral reefs expand, eventually encircling each island. Volcanoes as young as Mauna Loa are host to incipient reefs, while the older islands of O’ahu and Kaua’i have extensive, well-developed fringing reefs.

(7) Rejuvenated Post-Erosional Eruptions

After a long period of quiescence, most Hawaiian volcanoes experience a final episode of eruptive activity. This rejuvenated stage typically produces cinder cones and associated volcanic ash deposits. Many of the cinder cones are located near the shoreline. One hypothesis for the origin of these late-stage eruptions is that mass loss due to erosion decompresses the magma below, triggering further melting and eruption. Rejuvenated volcanism is found on all islands except Hawai’i.

(8) Atoll Stage

As the volcanoes drift farther from their hot spot origin, they cool and eventually become extinct. The lithosphere on which the islands sit also cools, becoming denser. This leads to a gradual subsidence of the volcanic ridge, causing the islands to slowly sink. As subsidence drags the islands down from below, erosion by waves, streams, and mass wasting works to diminish the islands from above. Fringing reefs expand to encircle the eroded volcano. Ultimately, the volcanic edifice is eroded to sea level and only the circular reef remains, forming an atoll. Many of the Northwestern Hawaiian Islands are in the atoll stage.

(9) Guyot Stage

Continued subsidence pulls even the coral atolls below sea level. Eventually, the living coral reefs literally “drown,” becoming flat-topped seamounts called guyots. The Emperor Seamounts exemplify this final stage in the life cycle of Hawaiian volcanoes. Ultimately, each of the Emperor Seamount and Hawaiian islands will return to the mantle via subduction at an active plate boundary, millions of years in the future.