The eight main Hawaiian Islands are made up of 15 volcanoes, which are the youngest in a linear chain of more than 129 volcanoes (above and below sea level) that stretches for about 6,100 km (3,800 mi) across the north Pacific. The ages of the volcanoes are systematically younger toward the southeast, and the bend in the chain separates the older Emperor Seamount Chain from the younger Hawaiian Ridge. The oldest dated volcano near the northern end of the Emperor Seamount Chain is 81 million years. The bend between the two chains is dated at 43 million years. Volcanism on Kaua‘i Island ended about 3.8 million years ago, making it the oldest of the main Hawaiian Islands. On the Island of Hawai‘i, the youngest of the main Hawaiian Islands, Kīlauea and Mauna Loa are historically the two most active volcanoes, with frequent eruptions. The submarine volcano Lō‘ihi, off Hawai‘i Island's southeastern shore, is the youngest volcano in the Hawaiian Ridge.
All of these volcanoes were built in an assembly line-like process that is driven by plate motion and a "hot spot," or plume of hot material, deep within the Earth. Heat from the Hawaiian hot spot partially melts mantle rock at depths starting about 200-400 km (125-250 mi) below Earth's surface. This melting produces magma that rises upward through the overlying Pacific Plate. As the plate moves west-northwest, each volcano moves with it from its place of origin above the hot spot. The age and orientation of the volcanic island chain records the Pacific Plate's direction and rate of movement. The pronounced 43-million-year-old bend between the Hawaiian Ridge and the Emperor Seamounts marks a dramatic change in plate motion direction.
Based on the chemistry of the magma erupted by Hawaiian volcanoes, scientists have identified an idealized sequence of eruption stages for each volcano. This sequence may last 4 to 6 million years.
The initial stage of volcano growth is the submarine preshield stage, characterized by infrequent, small-volume eruptions. Pillow lava constructs a steep-sided volcanic pile with a shallow summit caldera and two or three riftzones radiating from the summit. Calderas continue to develop and fill repeatedly throughout the preshield and shield stage and go hand-in-hand with the high rate of magmatism that builds the Hawaiian Islands. Rift zones are prominent features of Hawaiian volcanoes during all but the final eruptive stage. The preshield stage lasts for perhaps 200,000 years, yet produces only a small portion of the final volume of the volcano. As the volcano grows, the composition of the lava changes, and eruptions become more frequent and voluminous. When the transition is complete, the volcano enters the shield-building stage.
The only current example of a preshield stage volcano is Lō‘ihi Seamount. Chemical analyses of rocks collected during submersible dives at Lō‘ihi show that eruptions are transitioning to the subsequent shield-building stage. At all other Hawaiian volcanoes, preshield eruption products have been buried by products of later eruptive stages.
The shield-building stage is the second, and most voluminous, stage of Hawaiian volcanism. More than 95 percent of a Hawaiian volcano's volume is erupted during this stage, which may last up to 2 million years. The oceanic crust of the Pacific Plate, unaccustomed to the enormous weight of the volcanoes building atop it, subsides greatly during this stage—as much as 3 mm per year, based on current subsidence rates recorded on the Island of Hawai‘i. Early shield-building eruptions are entirely underwater, but the rate of growth exceeds the rate of subsidence, and eventually the volcanoes emerge above sea level.
Mauna Loa and Kīlauea volcanoes are in the shield-building stage. It is unknown exactly how long Mauna Loa has been producing shield-stage lavas, but the earliest preshield flows erupted onto the seafloor sometime between about 0.6 and 1 million years ago. Kīlauea transitioned from the pre-shield stage about 155,000 years ago.
The postshield stage caps the volcano with a carapace of lavas, but these rocks form only a small part of the total volume of a volcano. Not all Hawaiian volcanoes go through this stage (e.g., Ko‘olau Volcano on Oahu, and Lāna‘i volcano), whereas other volcanoes have erupted thick postshield-lava sequences (e.g., WAIANAE Volcano on O‘ahu and Mauna Kea Volcano on the Island of Hawai‘i). During this stage, high lava fountains produce large amounts of cinder and short, thick, ‘a‘ā flows that steepen the slopes of the volcano as they pile up near summit and rift-zone vents. These lavas commonly fill and overflow the shield-stage caldera (read more about the formation of Kīlauea Caldera). During the postshield stage, the eruption rate decreases to zero typically within 500,000 years, but this stage may continue for as long as 1 million years.
The end of the postshield stage is followed by a period of erosion and subsidence, during which deep canyons may form along the flanks of the volcano. As the volcanic islands erode and subside, fringing coral reefs grow.
The fourth and final stage is rejuvenated, or renewed, volcanism. Periods of erosion may precede or be interspersed with rejuvenated stage eruptions, which contribute much less than 1 percent to the cumulative eruptive volume of a volcano. Lavas commonly erupt through reefs that form offshore as erosion progresses. Eruptions that occur near the shoreline produce volcanic maars, and eruptions further inland form lava flows that funnel down eroded stream valleys (e.g. Mānoa Valley on O‘ahu). Rejuvenated stage activity has low eruption rates and may occur over several million years.
Eruptive features on Ko‘olau volcano on the island of O‘ahu are classic examples of rejuvenated-stage volcanism. The most iconic example is Diamond Head Crater (Lē‘ahi in Hawaiian), which is about 400-500 thousand years old and located east of Waikīkī Beach. Lavas erupted during the rejuvenated stage are also abundant on the Islands of Kaua‘i, Ni‘ihau, and Moloka‘i, as well as West Maui.
Following the rejuvenated stage, a long period of erosion and subsidence eventually reduces the volcanic island to sea level. Basalt is a porous rock that easily succumbs to erosion, especially with the high amounts of rainfall in Hawaii. Erosion processes include gradual degradation due to weathering and massive landslides that displace large parts of islands. The largest of these landslides can remove entire flanks of volcanoes – such landslide deposits or debris fields litter the sea floor around the Hawaiian Islands. Smaller landslides, especially on the rainy, windward sides of the islands, are associated with deep erosional canyons or sea cliffs. Landslides can occur during any of the eruption stages.
As the volcanoes erode and islands subside, they become coral atolls, such as those west of La Perouse Pinnacles, the westernmost and oldest subaerial remnant of a former Hawaiian island. These coral reefs begin to die as they sink below water depths with enough sunlight to keep the reef healthy. After the reef dies, the islands continue to subside. Once they are below sea level, these flat-topped, coral-capped mounds are called "guyots." Although a few volcanoes in the Hawaiian Ridge-Emperor Seamounts volcanic chain apparently never grew above sea level, nearly all the volcanoes older than 30 million years are guyots. Most of these volcanoes stood thousands of feet above sea level, just as the volcanoes that make up the present-day islands do today.