Volcanoes are categorized by geologists based on their tectonic setting, the properties of the erupting magma, and the resulting physical shape. Analyzing the volcanoes of Iceland and Hawaii provides a distinct comparison, as both are volcanically active but arise from fundamentally different geological processes. This categorization helps explain the observable differences between the two island regions.
Categorization Based on Tectonic Setting
The most significant distinction between the two locations lies in their underlying geological origin. Hawaii is the classic example of intraplate volcanism, where the volcano forms in the middle of a tectonic plate, far from any boundary. This activity is driven by a relatively fixed mantle plume, or hotspot, which is an upwelling of abnormally hot rock from deep within the Earth’s mantle. As the Pacific Plate slowly drifts northwestward over this stationary plume, a continuous chain of volcanoes is created, with the active islands positioned directly above the hotspot.
Iceland’s tectonic setting is far more complex because it is situated directly on the Mid-Atlantic Ridge, the divergent boundary separating the North American and Eurasian tectonic plates. The plates here are pulling apart, causing the crust to thin and allowing magma to rise through the rift zone. Adding to this is the presence of an underlying Iceland plume, which is a separate hotspot that intersects the divergent plate boundary. This unique combination of a spreading plate boundary and a mantle plume results in an unusually high volume of magma production, allowing Iceland to rise above sea level as the largest landmass on an oceanic ridge.
The constant rifting in Iceland creates a large-scale extensional environment, influencing how magma is injected into the crust. This leads to a greater prevalence of large normal fault scarps within the rift zones. In contrast, Hawaii’s volcanism is driven by the continuous supply of magma from the stationary plume, with the Pacific Plate’s movement providing the mechanism for creating the long volcanic island chain.
Classification by Eruption Style and Magma Type
The eruption styles in both regions are categorized as overwhelmingly effusive, meaning the magma flows out relatively gently instead of exploding violently. This calm behavior is due to the magma being basaltic, a type of molten rock that is low in silica content and therefore has a very low viscosity (it is thin and runny). The low viscosity allows gases dissolved in the magma to escape easily as it rises, preventing a buildup of pressure that would cause an explosive eruption.
Hawaiian eruptions are the archetypal example of this effusive style, characterized by predictable, centralized vent activity. Eruptions typically involve dramatic lava fountains and voluminous, slow-moving lava flows, often described as either ropy Pāhoehoe or clinkery ʻAʻā flows.
Icelandic eruptions share the same low-viscosity basaltic magma but often exhibit a different eruptive geometry due to the divergent boundary. A significant portion of Iceland’s volcanism occurs through fissure eruptions, where magma escapes through long, linear cracks in the Earth’s crust rather than a single central vent. These eruptions can create spectacular “curtains of fire” and result in massive volumes of lava covering large areas, sometimes leading to the formation of extensive lava fields and plateaus.
Classification by Resulting Volcano Form
The accumulated products of the effusive eruptions result in volcano forms that are largely similar in type but differ in scale and associated features. Both regions are dominated by shield volcanoes, which are broad, low-profile mountains built from thousands of thin, fluid lava flows. The Hawaiian Islands are defined by massive shield volcanoes, such as Mauna Loa, which is considered the most voluminous mountain on Earth, rising over 10,000 meters from the ocean floor. These volcanoes have very gentle slopes and feature summit calderas formed by the collapse of the central magma chamber after major eruptions.
Iceland’s landforms are classified by a combination of low-profile shield volcanoes, extensive lava plateaus, and prominent fissure vents. The Icelandic shield volcanoes tend to be smaller than their Hawaiian counterparts, but the sheer volume of lava from the frequent fissure eruptions creates enormous lava plateaus. These flood basalt events, driven by the combination of the rift zone and the plume, have built up much of the island’s landscape over millions of years.