Iceland is a landmass in the North Atlantic, spanning roughly 103,000 square kilometers, situated far from the continental shelves of Europe and North America. It represents the largest subaerial expression of an oceanic spreading center on the planet. The island’s dramatic landscape, characterized by active volcanoes, vast lava fields, and powerful geothermal features, is a direct result of its position. The island began to emerge from the ocean floor approximately 16 to 18 million years ago, a relatively young age in geological terms.
Iceland’s Setting on the Mid-Atlantic Ridge
Iceland sits directly astride the Mid-Atlantic Ridge (MAR), an immense underwater mountain range that wraps around the globe. This ridge marks a boundary where the planet’s rigid outer shell, the lithosphere, is actively separating. Specifically, Iceland is located where the North American and Eurasian tectonic plates are pulling apart from one another. This type of boundary is known as a divergent plate boundary, where new crust is constantly created as magma rises to fill the widening gap between the plates.
At this boundary, the North American plate moves westward while the Eurasian plate drifts eastward, causing the island to expand by about 1.8 to 2.5 centimeters each year. Seafloor spreading typically occurs deep beneath the ocean surface, where magma quickly cools to form new oceanic crust. Along most of its 16,000-kilometer length, the Mid-Atlantic Ridge remains submerged. The existence of Iceland as dry land, where this process can be observed on the surface, requires an additional factor.
The Power of the Iceland Mantle Plume
The reason Iceland is an island rather than a submerged ridge is the presence of a persistent upwelling of superheated rock deep beneath the crust, known as the Iceland mantle plume. This plume is believed to originate from the boundary between the Earth’s core and mantle, far deeper than the processes driving normal plate separation. Acting as a stationary heat source, this plume provides a large supply of molten material that exceeds the magma volume available at a typical mid-ocean ridge.
The heat from the mantle plume causes the surrounding rock to melt more extensively than usual, leading to an abnormally thick crust beneath Iceland, which can reach up to 46 kilometers deep in the central region. This magma production has built an elevated plateau that rises significantly above the level of the Mid-Atlantic Ridge on either side. Geophysical evidence, such as high concentrations of the primordial helium isotope He-3 in the erupted lavas, supports the theory that the magma originates from a deep, undegassed source within the mantle.
Constructing the Landmass: Rift Zones and Volcanism
The continuous interaction between the separating tectonic plates and the magma supply from the plume constructed the island. The plates are not pulling apart uniformly across the whole island, but rather along narrow, seismically and volcanically active corridors called the neovolcanic zones. These zones are characterized by parallel fissures and faults that run through the landscape, showing the crust being stretched and torn apart. The most famous of these rift valleys is the Þingvellir Graben, a sunken area where the boundary between the two major plates can be walked.
The island’s landmass was built over millions of years by eruptions of highly fluid, basaltic lava, often referred to as flood basalts or plateau lavas. These eruptions were not isolated peaks but large outpourings from linear fissures that spread across vast areas. Solidified flows created the thick volcanic pile that forms the foundation of the island. The oldest rocks are found in the eastern and western edges, having been carried away from the central rift zone by the spreading plates. Today, the active volcanic zones remain the primary areas for new land creation, with eruptions occurring from fissure swarms that can extend for tens of kilometers.
A Geologically Active Present
The geological processes that formed Iceland are still active today. The crust continues to spread at a measurable rate, with the movement of the plates continuously monitored by high-precision Global Positioning System (GPS) instruments. This constant stretching and the movement of magma beneath the surface result in frequent, though typically shallow, earthquake activity, particularly along the rift zones and associated transform faults.
The heat generated by the mantle plume and shallow magma chambers is harnessed for energy production. High-temperature geothermal fields, where temperatures can exceed 200°C at shallow depths, are found directly within the active volcanic zones. This heat source allows Iceland to utilize geothermal energy for a significant portion of its heating and electricity needs.