Background: Mantle convection, decompression melting, and mid-ocean ridges

Iceland is unique in that it is the only place in the world where a mid-ocean ridge protrudes above sea level. Iceland straddles the Mid-Atlantic Ridge, with the North American Plate to the west and the Eurasian Plate to the east, and ~2 cm/yr relative motion in opposing directions. Much, although probably not all, of the magmatism at Iceland is a result of mantle decompression beneath this divergent plate boundary.

Location of Iceland on the Mid-Atlantic Ridge. Although there are several islands that lie close to the Mid-Atlantic Ridge, Iceland is the only part of the ~16,000 km long ridge that is exposed above sea level. Decompression of the mantle beneath the plate boundary, possibly with the help of a hot mantle plume, results in nearly constant volcanic activity on Iceland.

Credit: U.S. Geological Survey

Mantle Convection and Plate Tectonics

In Module 1, we learned that tectonic plates move across Earth’s surface relative to a more or less fixed reference frame of mantle plumes. So what are these plates, and why do they move? The Earth’s lithosphere – which consists of the crust and the rigid upper portion of the mantle – is broken up into 15 major plates, plus several micro-plates. The tectonic plates move with respect to one another – some moving apart at divergent boundaries, some coming together at convergent boundaries, and some sliding past each other at transform boundaries. The lithospheric plates ride on top of the flowing, plastic mantle asthenosphere.

Earth’s Tectonic Plates, Wikimedia Commons. Some plates, such as North America and Eurasia, move away from each other at divergent boundaries. Others, such as Nazca and South America, move toward each other at convergent boundaries. Less commonly, two plates may slide past each other at a transform boundary – the most famous example being the San Andreas fault zone, which lies between the North American and Pacific plates.

Credit: Wikimedia Commons

The geothermal gradient inside the Earth is such that temperature increases with depth. The higher temperatures at the core-mantle boundary (~2,900 km depth) relative to the lithosphere-asthenosphere boundary (~100-200 km depth) drive convection in the plastic, flowing asthenosphere. This works very similarly to water convecting in a pot that’s being heated on the stove – hot fluid is less dense and therefore it rises, while the dense cooler fluid sinks. Just remember that the “fluid” in the mantle is actually a flowing solid; it is more viscous and flows much more slowly than water in a pot – at a rate of millimeters per year. Most geoscientists agree that there is a close relationship between mantle convection and plate tectonics, although it remains unclear to what extent the convecting mantle “drags” the lithospheric plates along its surface, or if instead the sinking of lithospheric plates at subduction zones serves to initiate convection cells in the mantle. In the most general sense, we can imagine that tectonic plates move away from each other at places where the mantle is rising, and together at places where the mantle is sinking.

Mantle convection cell, Wikimedia Commons. In this generalized model, relatively cool mantle from near the lithosphere-asthenosphere boundary sinks at convergent plate boundaries (i.e., subduction zones), while relatively hot mantle from near the core-mantle boundary rises at divergent plate boundaries (e.g., mid-ocean ridges).

Credit: Wikimedia Commons

New oceanic crust is more or less continuously being formed at mid-ocean ridges, which are a type of divergent plate boundary. It was mentioned above that the North American and Eurasian plates are moving away from each other at ~2 cm/yr at Iceland. As it turns out, this is a pretty representative average spreading rate for the Mid-Atlantic Ridge, which means that if you were to fly from John F. Kennedy International Airport in New York to London’s Heathrow Airport today, the trip would be 1 meter longer than if you had taken the same flight 50 years ago!

Map of the world’s oceans showing the ages of the sea floor. Red indicates the youngest crust (<10 million years old), while purple indicates the oldest crust (160-180 million years old; older crust exists only in a small part of the eastern Mediterranean Sea). Note that the youngest crust is found at the mid-ocean ridges, where it is being formed. The relatively narrow band of young crust at the Mid-Atlantic Ridge reflects slower spreading in the Atlantic compared to the Pacific.

Credit: National Geophysical Data Center, National Oceanic and Atmospheric Administration (NOAA)