This weekly newsletter delves into a different captivating topic from the world of geoscience, exploring Earth’s fascinating phenomena in bite-sized, easy-to-understand segments each week.
Monday 4th of March, currently as you read this the Reykjanes Peninsula in Iceland is in a dynamic state of volcanic activity. The last time this area of Iceland was this active was 800 years ago which included decades of eruptions. In this newsletter, I want to go into the geophysics behind why the town of Grindavik is in danger and why this is happening. Before we proceed, let’s catch up on the past events and current developments in the Reykjanes, as of Monday, March 4th. The current on-going volcanic eruptions known as the Sundhnúkur eruptions at Sundhnúkur crater north of the town of Grindavik, 26 mi away from Reykjavík, started on the 18th December 2023 initially caused by lava spewing from a fissure in the ground. As of 14th of January 2024, a second fissure has erupted further north than Grindavik accompanied by a large number of earth and microquakes which are associated with a magmatic intrusion which is estimated to be 9.3 mi long starting at Kálfellsheiði and following the line of the Sundhnúkur crater chain occurring around 800 m from the surface. As of the 2nd of March 2024 the micro-seismic activity has stopped meaning it is likely that the magma flow has stopped for the time being — if you want to read more about the current status in Iceland, click this link. So, let’s get into why this situation is happening but first it’s important to understand why the geographical location of Iceland is significant.
Geographical Location
Iceland is located on the Mid-Atlantic Ridge which is a tectonic plate boundary which separates the Eurasian and the North American plates. Tectonic plates are large areas of the Earth’s crust and uppermost mantle which slowly move on the asthenosphere (denser and weaker layer beneath the lithospheric mantle) known as continental drift. The plate margin at the Mid-Atlantic Ridge is known as a constructive plate margin as the plates are diverging; Iceland was formed by the coincidence of the spreading boundary and a hotspot. A hot spot originates from a region deep within the Earth’s mantle, where heat rises through convection. This heat causes rock to melt at the base of the lithosphere, where the upper mantle meets the Earth’s crust. The resulting molten rock, or magma, frequently intrudes through fractures in the crust, forming volcanoes. Hot spot volcanism stands out because it doesn’t arise from the boundaries of Earth’s tectonic plates, where most volcanic activity occurs. Instead, it emerges from exceptionally hot centres called mantle plumes (upwelling of magma within Earth’s mantle). When two plates move apart at a constructive plate margin the lithosphere becomes thinner leading to the upwelling of mantle rock and due to the reduced pressure exerted by the crust onto the mantle leads to decompression melting leading to the formation of diapirs which feed magma chambers along the ridge. All of this is responsible for ridge push which occurs when gravity causes the ridge at a constructive plate margin to push on the lithosphere causing the plates to move apart.
Constructive plate margins are usually located under the sea and the Mid-Atlantic Ridge is a perfect example of this. When these plates diverge, geologists call this process seafloor spreading and interestingly the magnetism of the mid-ocean ridge aided scientists in identifying the process of seafloor spreading. Effectively, basalt which is the once-molten rock which forms new oceanic crust is magnetic and using magnetometers scientists discovered that the magnetism of the ocean floor can be divided in to matching stripes on each side of the ridge which are symmetrical as shown in the image below. The Earth’s magnetic field’s polarity occasionally flips and these reversals are recorded below. The significance of the magnetic stripes show that the Mid-Atlantic Ridge at the seafloor is spreading at a rate of about 0.02 m per year.
Iceland was formed around 60 million years ago when mantle plumes appeared along the Mid-Atlantic Ridge meaning Iceland is one of the youngest countries in the world. This is very important as it demonstrates why Sundhnúkur eruptions are happening but why do earthquakes also happen?
Sundhnúkur Eruptions and Earthquakes
As I mentioned earlier, the Sundhnúkur eruptions are actually caused by a fissure known as fissure volcano. Fissure volcanoes get their name because they form along fissures compared to a central vent and occur when magma fills dikes which breakthrough the surface. Fissure eruptions usually involve lava which travels long distances. This is a big threat to Grindavik and authorities are trying to mitigate the flow of lava reaching the town by building up large embankments to divert it. Effectively, a fissure is an elongated fracture in the surface from which lava erupts. Remember lava is molten rock that breaks through the Earth’s surface whereas magma is molten rock underground. At an eruptive fissure occasionally lava will actually flow back into the fissure from which it came known as drainback.
Usually at the Mid-Atlantic Ridge, earthquakes tend to be shallow and not very large and often are not a major natural hazard to people however they can be very dangerous. Simply, the earthquakes are formed when large amounts of magma tries to breakthrough the surface of the crust, fracturing the plates slightly causing small earthquakes as the fractures occur near the surface because the rock is weaker as its new. This is further exacerbated at volcanic rift zones in which majority of volcanoes in Iceland occur. Rift zones are regions of vulnerability within a volcano that typically develop early in its formation, resulting from the volcano’s expansion and settling processes. With the current state of Iceland, we don’t know the certainty of an eruption at this fissure zone and therefore the future of Grindavik isn’t guaranteed.
Conclusion
I hope everyone in Grindavik is safe and the future is promising. In the future I will be going into more detail regarding conservative and destructive plate margins with a focus on subduction so if you enjoyed this read make sure to check back weekly to learn more. I’d recommend staying up to date with what is happening in Iceland and if any big changes happen I will be sure to let write about it.
Thank You
Thank you for reading, all information is taken from reputable sources and are linked below. All images are free to use under copyright laws.
National Geographic ‘ The Geological Society ‘ LibreTexts Geosciences ‘ British Geological Survey ‘ USGS