Dr. Jim Zimbelman is a planetary geologist at the Smithsonian’s National Air and Space Museum. He’s also study leader for our Exploring Iceland tour. Here, we quiz him about what Iceland’s unique geology can teach us about other worlds, as well as our own.
Smithsonian Journeys: Of all the terrestrial and extra-terrestrial imaging and mapping that you’ve done, how does Iceland’s landscape compare in beauty, drama, violence?
Jim Zimbelman: Iceland is unique in my experience, either for sites of other fieldwork here on Earth or areas that I have examined using spacecraft data. No other place on Earth provides such good access to a mid-ocean ridge; tens of thousands of miles of the global ridge system are beneath the surface of the ocean, but an active oceanic ridge is only above water in Iceland. The geology of Iceland is therefore very helpful for those of us trying to understand volcanic terrains on other planets. With regard to the dramatic, no volcanic location I have visited has the combination of lava flows, glaciers, rivers with large waterfalls, and polar desert vegetation that can be found everywhere in Iceland. Fortunately, Iceland is not violent; even major eruptions can be safely viewed from fairly close distances, much like eruptions in Hawaii.
SJ: The huge volume of water that surges over the Dettifoss and Iceland’s other waterfalls and powers down the glacial rivers – where does it come from? Is it “young” water from recent precipitation? Is it ancient water from glacial melt? What “pumps” it to the surface with such force? Is it the volcanoes under the glaciers and lakes?
JZ:The water in the rivers of Iceland is both young and old. Every winter the country is blanketed in new snow that in the spring then feeds large runoffs. However, the glaciers are also melting, releasing water that fell as snow tens of thousands of years ago. Both water sources mix together to make the dramatic rivers of Iceland. The ‘force’ comes from the sheer volume of snow and ice available to feed the rivers; the volcanic bedrock does not need to add additional force (although, when an eruption occurs directly beneath a glacier, the resulting enormous flood when the melt water finally bursts out from beneath the glacier has an Icelandic name – jokullaupe).
SJ: When and how did scientists realize that Iceland straddles two tectonic plates?
JZ: For centuries scientists realized Iceland had a very strong volcanic component to its history. The realization of how Earth’s crust moves in different plates is a fairly recent development. Individual scientists speculated about it early in the twentieth century, but the globe-encircling concept of plate tectonics was only accepted by the worldwide geologic community beginning in the 1960s. Iceland played an important role in this realization, since it allowed scientists to visit (and measure) the separation of two active plates.
SJ: Given the turbulent geologic history of Iceland, how much has the island changed (size, shape, elevation, climate, etc.) since the earliest Norse/Celtic settlements (est. 7th century?)?
JZ: Iceland has not changed all that much since the time of the Vikings. The glaciers are smaller than they were a thousand years ago, but not all that much else has changed since the conditions as described in the Viking sagas.
SJ: Iceland’s fjords seem to be limited to the north and west coasts. Why?
JZ: The fjords are the result of erosion by past glaciers, and the north and west coasts had the steepest topography down which these ancient glaciers could move. Precipitation is usually heaviest on the NW side of the island since storm systems in the northern hemisphere tend to move from west to east.
SJ: How many times in geologic history has the crust of Iceland been recycled? And how old is Iceland?
JZ: I don’t know the age of the very oldest rocks in Iceland, but I have heard discussions of some rocks that are millions of years old. Over 99% of the rocks in Iceland are volcanic in origin, and the rocks at the surface are typically tens to hundreds of thousands of years old. Few of the rocks on the island have been ‘recycled’ in that Iceland does not experience subduction (where the moving plates get shoved into each other), which leads to classic steep-sided volcanoes like Mt. Fuji in Japan or the large volcanic cones along the west coast of the U.S.
SJ:Would you please elaborate on the Icelandic plume, which seems to persist through geologic time, yet wander (or is it the “subaerial” surfaces such as Iceland and Greenland which have wandered?).
JZ: Iceland does indeed sit atop a particularly strong ‘plume’ of magma rising up from the interior of the Earth. The coincidence of this plume with the mid-Atlantic ridge system is, we think, just that, a geologic coincidence. Greenland does ‘wander’ in the sense that it is entirely on the North American plate and it moves along with that plate. Iceland is literally growing as it is pulled apart along the ridge system that runs across the middle of the country. Each year roughly 2 centimeters of new ‘Iceland’ is formed as the two plates spread apart along that ridge.