How Hawaiian Volcanoes Grow and Age
Hawaiian volcanoes pass through long life stages as the Pacific Plate moves over the Hawaiian hot spot. A volcano begins deep below the ocean surface, grows through repeated eruptions, may rise above sea level, builds a broad shield, slows down over time and is eventually shaped by erosion. Some older volcanoes may even erupt again after a long quiet period.
Geologists often describe the main eruptive stages of Hawaiian volcanoes as preshield, shield, postshield and rejuvenated. The older simple idea of birth, maturity and old age is still useful for understanding the story, but the scientific stage names help explain how lava chemistry, eruption frequency and volcano shape change over time.
The Birth of Hawaii's Volcanoes
Hawaiian volcanoes begin deep beneath the ocean as submarine volcanoes. Lava erupts onto the seafloor and builds underwater mounds that grow larger over thousands and even hundreds of thousands of years. As the volcano keeps erupting, it may eventually grow closer to the ocean surface.
When a submarine volcano gets near the surface, interactions between hot lava and seawater can produce steam-driven explosions and volcanic fragments. If growth continues, the volcano can eventually rise above sea level and become an island.
Scientists have identified an active undersea volcano southeast of the Big Island called Kama'ehuakanaloa, formerly known as Loihi. It is still below the ocean surface and is not expected to become an island anytime soon. Its future growth depends on eruption rate, collapse, subsidence and other geologic processes.
The Shield Stage
The shield stage is the main growth phase of a Hawaiian volcano. During this stage, frequent eruptions of fluid basaltic lava build a broad, gently sloping volcanic mountain. Most of the volcano's volume forms during this stage.
Shield-stage eruptions can last a long time and send lava across large areas. The broad shape of Hawaiian volcanoes comes from these repeated lava flows, which spread outward rather than piling up into steep cones.
Mauna Loa and Kilauea on the Big Island are the best-known examples of active Hawaiian shield volcanoes. Their eruptions and magma movement continue to help scientists understand how Hawaiian volcanoes grow.
The Caldera Stage
Many Hawaiian shield volcanoes develop summit calderas during their active growth. A caldera can form when magma drains from beneath the summit and the ground above collapses. Over time, eruptions may partly refill the caldera, and later collapses may reshape it again.
Caldera development is not a single event. It can happen repeatedly while the volcano remains active. Kilauea and Mauna Loa both show how summit areas can change through eruption, collapse, refilling and renewed activity.
The Postshield Stage
After the main shield-building phase slows, a Hawaiian volcano may enter the postshield stage. Eruptions become less frequent, and lava chemistry can change. Lava may be thicker, and eruptions may build cinder cones, steeper flows and rougher landscapes on top of the older shield.
Mauna Kea is an example of a Hawaiian volcano in the postshield stage. Its upper slopes include cinder cones and lava flows that are different from the smoother shield-building flows of younger volcanoes such as Mauna Loa.
Haleakala, also known as East Maui Volcano, is another example of a volcano that has moved beyond its main shield-building stage. Although eruptions are rare today, Haleakala is not considered extinct.
Erosion and Old Volcanoes
As a Hawaiian volcano moves away from the hot spot, eruptions slow or stop. Rain, waves, streams, wind, landslides and time begin to reshape the island. Valleys deepen, sea cliffs form, ridges sharpen and coral reefs may develop around older shorelines.
This is why older Hawaiian islands look so different from the Big Island. Kauai, Oahu, Molokai and other older islands have had far more time for erosion to carve valleys, cliffs and rugged mountain ridges.
Rejuvenated Volcanism
Some Hawaiian volcanoes may erupt again long after their main shield and postshield activity has ended. This is called rejuvenated volcanism. These late eruptions are usually smaller and less frequent, but they can create cones, craters and lava flows on older islands.
Examples of rejuvenated volcanic features can be seen on older Hawaiian islands. These eruptions help explain why some older islands still have younger-looking volcanic cones or craters even after their main shield volcanoes stopped building the island.
A Long Volcanic Story
The life of a Hawaiian volcano stretches across enormous time spans. It begins deep below the ocean, rises through repeated eruptions, builds an island, slows down, erodes and may sometimes erupt again much later.
This long process explains why the Hawaiian Islands form a chain of different ages and shapes. The Big Island is young and volcanically active, while the older islands to the northwest show what happens after rain, ocean, wind and time reshape volcanic land.
