The islands of Hawaii represent a geological anomaly, rising from the middle of the Pacific Ocean far from the boundaries where tectonic plates typically collide. The archipelago is built by continuous volcanic activity, making it one of the most volcanically active regions on Earth. The distinctive landforms and gentle slopes that define these islands point to a specific class of volcanic structure.
The Defining Type: Shield Volcanoes
The vast majority of volcanoes in Hawaii are classified as shield volcanoes. Their name is derived from their broad, low-profile shape, which resembles a warrior’s shield lying on the ground. These structures are fundamentally different from the steep, symmetrical peaks associated with explosive eruptions, such as stratovolcanoes or composite cones. Shield volcanoes are built almost entirely by the accumulation of thousands of fluid lava flows that spread out over great distances.
The characteristic shape is defined by very shallow slopes, typically ranging between five and ten degrees near the summit. These immense volcanoes are some of the largest mountains on the planet; for instance, Mauna Loa is the largest active volcano on Earth by volume. This massive size and gentle gradient are a direct result of the highly fluid nature of the lava, allowing it to travel far from the central vent before cooling.
Understanding Hawaiian Magma and Eruptions
The material building Hawaiian volcanoes is almost exclusively basalt, a dark, fine-grained volcanic rock low in silica content. This low silica content gives Hawaiian magma its remarkably low viscosity, meaning it flows easily, much like warm syrup. The high fluidity allows gases to escape without building up immense pressure. This is why Hawaiian eruptions are predominantly effusive, or non-explosive, rather than violent.
This gentle eruption style is known as a Hawaiian eruption, characterized by fire fountains and lava rivers that steadily build new land mass. The two main types of lava flows produced illustrate the range of the basalt’s texture. Pahoehoe flows are smooth, sometimes having a ropey texture as the surface cools and folds. A’a flows are rough, blocky, and jagged, forming when the lava is slightly cooler or moves more rapidly, causing the surface to shatter into sharp fragments. Both types of flows contribute to the shield’s structure, but the fluidity of the basalt is the primary reason the volcano grows outward into a broad shield rather than upward into a steep cone.
The Hotspot Origin of the Island Chain
The existence of these volcanoes in the middle of a tectonic plate is explained by the theory of a stationary mantle hotspot. This concept describes a location deep within the Earth’s mantle where an unusually hot plume of material rises toward the surface. This fixed plume provides a continuous source of magma for the volcanoes directly above it.
The Pacific Plate, which makes up the ocean floor beneath Hawaii, constantly moves in a northwesterly direction, but the magma source remains fixed. As the plate slides over the hotspot, new volcanoes are created over the source, while older, inactive volcanoes are carried away. This process generated the 6,000-kilometer-long Hawaiian-Emperor Seamount chain, a trail of progressively older volcanoes stretching across the Pacific.
The island of Hawai‘i, home to active volcanoes like Kilauea and Mauna Loa, is currently positioned directly over the hotspot and is the youngest island. Moving northwest along the chain, the islands become increasingly older, smaller, and more eroded, demonstrating the plate’s continuous movement. Volcanic activity is concentrated at the southeastern end of the chain.
The Life Cycle of Hawaiian Volcanoes
Hawaiian volcanoes progress through distinct evolutionary phases as they are created over the hotspot and carried away by plate motion. The cycle begins with the preshield stage, where a submarine volcano, like the active Lo‘ihi Seamount, grows entirely beneath the ocean surface. This is followed by the main shield-building stage, a period of massive, rapid growth fueled by the highly productive magma source, exemplified by Mauna Loa and Kilauea.
As the volcano moves off the hotspot, it enters the post-shield stage, where eruption rates decrease significantly and the lava composition slightly changes. Mauna Kea is an example of this stage, having experienced its last eruptions thousands of years ago, marking its transition toward dormancy. The final stages involve prolonged erosion and subsidence, with the volcano becoming entirely extinct and eventually sinking back below sea level.