Sitting in the middle of the North Atlantic Ocean is the island country of Iceland. Visiting Iceland is on pretty much every geologist’s “bucket list” because of its unique position on a plate boundary where two plates are moving apart from each other (i.e., diverging). During a trip there with other geologists in July, I explored the landscapes of one of the most geologically-active places on our planet. This post describes the tectonic setting of Iceland; the second post will explain the types of volcanoes; and the third post will discuss the glaciers and their often-spectacular interactions with volcanoes.
This Google Earth map shows the location of Iceland in the North Atlantic Ocean between Greenland and Norway. It was Norwegians who first settled the island in the year 840. Iceland sits on the Mid-Atlantic Ridge (pink lines, dashed where transform faults connect ridge segments), where the North American plate is separating from the Eurasian plate (green arrows) and new ocean crust (blue color) is being created. The tectonic plates include both continents and the newly-created oceanic crust. As the plates move apart, the Atlantic Ocean continues to widen at a rate of about 2 cm/yr (1 inch/year). Mid-ocean ridges are where ocean crust is created around the world. As two plates pull away from each other, hot magma rises and erupts to form volcanic rock. The thing is—nearly all of these ridges are located about 2500 meters (8200 feet) beneath the ocean surface. So why has Iceland grown so much taller? That’s because it is also sitting on a hot spot—a hot plume bringing heat and molten rock (i.e., magma) from deep within the earth. For more information about hot spots, see my post: https://landscapes-revealed.net/plate-tectonics-101-what-happens-at-hot-spots/). Hawaii is the classic example of a hot spot volcano. It is not at the boundary between plates but in the middle of a plate (Pacific), so it is only partly like Iceland. The orange numbered circles on the map above—numbers are millions of years ago—show points under the North American plate where the hot spot was located during the past 50 million years. Keep in mind that hot spots remain stationary while plates move over them. Fifty million years ago, Greenland was over the hot spot. Since then, the North American plate has moved to the northwest until the hot spot was under its current location beneath the Vatnajökull ice field (0 million years ago). Iceland began to emerge from the sea about 16 million years ago, when enough lava had accumulated to raise the pile that high.
This figure shows the position of the Mid-Atlantic Ridge (plate boundary) on Iceland. The colors show the ages of the volcanic rocks that make up the island. Notice that the oldest rocks are on the western and eastern edges of Iceland. As the plates have continued to move apart, the older rock gets carried away from the plate boundary and new volcanic rock forms in between. The orange color is volcanic rock that formed during the past ~800,000 years. You may wonder why there is a piece of older rock (yellow color) between the WVZ and the EVZ. This piece is a “micro-plate” that was created as the plate boundary “jumped” from the WVZ to the EVZ. The white areas are land that is covered by ice. The sandur deposits (grey color) are sedimentary deposits carried seaward during vast releases of water from under the ice when volcanoes there erupted—events called jökulhlaups. RR=Reykjanes Ridge (the Mid-Atlantic Ridge segment south of Iceland); RVB=Reykjanes Volcanic Belt (where fissures have been active in 2021 and 2022); WVZ=West Volcanic Zone (where Þingviller National Park is located); EVZ=East Volcanic Zone; MIB=Mid-Iceland Belt (a volcanic zone that connects the WVZ and the EVZ); SISZ=South Iceland Seismic Zone (another connector between the WVZ and the EVZ); NVZ=North Volcanic Zone; TFZ=Tjörnes Fracture Zone (transform fault connecting the NVZ and the KR); KR=Kolbeinsey Ridge (the Mid-Atlantic Ridge segment north of Iceland); ÖVB=Öræfi Volcanic Belt (overlies the hot spot today); and SVB=Snæfellsnes Volcanic Belt. It is thought that, as Iceland moved northwestward over the hot spot, the main locus of faulting and volcanism moved eastward from the SVB to the WVZ, and today to the EVZ and NVZ. Nevertheless, all of these segments continue to be active.
Here a group of fellow geologists bridge the plate boundary between the North American plate (left side) and the Eurasian plate (right side) at a site along the Reykjanes Volcanic Belt (RVB; see map above) with its many active volcanic fissures, including the fissure that erupted in 2021 and again just a month ago in 2022. In reality, this is just one fault in a rift graben that is 5–6 km (3–3.6 miles) wide. The lava was all erupted during the past ~10,000 years. For more information about divergent plate boundaries, where plates move away from each other), see my post: https://landscapes-revealed.net/plate-tectonics-101-what-happens-when-plates-move-away-from-each-other/). Figure is from: https://steemit.com/geology/@sooflauschig/geology-of-iceland-part-3-an-island-being-ripped-apart.
Rift grabens are landscape features formed where pieces of Earth’s crust are being pulled apart. This extension (red arrows) causes crustal blocks to sink downward between two normal faults, whose relative motion is indicated by the white arrows. Figure by Sue Selkirk, Arizona State University: http://lroc.sese.asu.edu/posts/56.
Northwest of the “bridge between two continents” site shown above, at the southern end of the Western Volcanic Zone (WVZ; see map above) is Þingvellir (pronounced Thingvellir) National Park, where the entire ~6-km-wide rift graben along the plate boundary is more clearly visible. This view is southward across the rift graben. The area also has important historical significance. The white buildings are the original site of Iceland’s parliament, called the AlÞingi, that was first established here in the year 930. Because of rifting and sinking in the valley, in 1789 the AlÞingi was moved to Reykjavik, Iceland’s capital city.
This view is northward toward Þingvallavatn (vatn=lake). Like the “bridge” site, this narrow fissure is often shown as the plate boundary. But it is only the eastern edge of the plate boundary, which encompasses the whole extent of the rift graben shown above.
Þingvellir National Park is one of three stops on the “Golden Circle” tour that is the most common tourist circuit, since it can be reached from Reykjavik on a single day trip. The other two stops are Gullfoss and Geysir geothermal area.
The geologic action of crustal blocks moving along faults, volcanic flows with different resistances to erosion, and extreme glacial outwash events have produced a wealth of spectacular waterfalls in Iceland. Gullfoss (=golden waterfall) is one of the most famous. At the near end, water is flowing into a narrow fissure—don’t try to kayak on this one! In the early 20th century, investors wanted to build a hydroelectric power plant that would have destroyed Gullfoss. However, Sigríður Tómasdóttir, daughter of the farmer who owned the waterfall, loved the beauty of the place and protested so intensely—even walking barefoot 120 km to Reykjavik—that the power plant was never built. Visitors can pay their respects at a memorial to her at the falls.
The Geysir geothermal field is the source of the word we use in English—geyser. Naturally-heated underground water is common in places where the crust is being pulled apart and hot molten rock (i.e., magma) rises easily toward the surface. Consequently, geothermal activity is prevalent along the length of the divergent plate boundary in Iceland. The people in this photo are awaiting Strokkur, a geyser that consistently erupts a 20–50-meter-high (65–165 feet) jet of boiling water every 5–10 minutes.
Another feature in the “Golden Circle” region well worth visiting is the Hellisheiði geothermal plant. The plant is located next to a 2,000-year-old volcano and is the largest in Iceland. The plant includes education exhibits about the plant’s processes that generate electricity from steam and provide hot water to Reykjavik where it is used directly and as a heating source. The plant also captures carbon via a process they have developed to re-inject mineral-rich water and remove carbon dioxide.
Probably the most popular feature in the “Golden Circle” area is the Blue Lagoon, where people can soak in 38°C (100°F) water. Most don’t realize they are soaking in waste water from an adjacent geothermal plant—see smoke stacks in the distance! This plant’s location close to the ocean makes the water too mineral rich to be used directly in homes. So the naturally hot water is used to heat other water for homes and the mineral-rich water is pumped into the lava field, which accidentally created a lagoon that has become quite a money maker. They also extract minerals from the water to make beauty products sold in the gift store—making lemonade out of lemons!
On our tour, we continued on the perimeter road to circumnavigate the island. On the north side, we stopped at the second largest city in Iceland—Akureyri—that is located on a fjord where we saw humpback and minke whales. The biggest attraction for geologists is the rift graben at the northern end of the plate boundary where it transitions from land back out to sea (see Google Earth map above).
This view across the rift graben is along the Northern Volcanic Zone (NVZ), near where the plate boundary heads back out to sea. The view is to the south, with the Atlantic Ocean located to the right of the photo. Unsurprisingly, a geothermal plant, and a site with fumaroles and mud pots, are located in this rift graben. The mountain on the left side of the photos is a “moberg”, the Icelandic name for a flat-topped mountain produced when a volcano erupts under a glacier.
Another impressive landscape feature on the north side of the island in Dettifoss (=collapsing waterfall), the largest waterfall in Iceland and perhaps in all of Europe. We were fortunate to be there on a sunny day when the mist created spectacular rainbows.
Our geology tour was organized by the Association for Women Geoscientists, using the small Iceland Geology Tours company run by Tamie Jovanelly (geology professor—blue jacket, center) and her husband Joe Cook (nurse and logistics manager extraordinaire—red jacket). As you can see, participants were men as well as women—a compatible group who delighted in the geologic wonders of Iceland.
Next up: the volcanoes of Iceland.
Iceland, the formation and evolution of a young, dynamic, volcanic island—a field trip guide: Geological Society of America Field Guide 54, 2019, by Jordan, Carley, and Banik.
Iceland: tectonics, volcanics, and glacial features, 2020, by Tamie Jovanelly.
Iceland, Third Edition (Classic Geology in Europe series), 2022, by Thordarson & Höskuldsson.