I’m beginning to love geology.
One of the first choices I had to make when the fall semester began was which of two classes with conflicting schedules I wanted to keep. I decided to go with geothermal surface exploration, because it’s something I know nothing about. Hey, I’m here to learn, right?
It turns out that what’s happening on the surface of the earth can tell you a lot about what’s going on deeper down. Sometimes it’s subtle, as when certain species of grasses grow in areas where the ground is warmed and the soil chemistry changed by the presence of a magma reservoir miles beneath the surface. Sometimes it’s not so subtle, as when steam and other gases come pouring up out of the ground. What’s really interesting is why some places are better suited for extraction of the earth’s heat than others.
Iceland basically sits on the solid surface of a gigantic bubble of molten rock created by one of the largest magma plumes in the world. There’s a lot of heat down there – but it’s normally miles and miles below the surface. You could drill down to get it, but molten rock is a bit beyond our ability to handle safely, at least for now.
The rock sitting on top of the magma is still pretty hot, though, even if it isn’t quite hot enough to actually melt. The melting point of rocks varies depending on their composition, but below about 2000o F, what used to be magma begins solidifying. That’s important, because solid rock breaks; molten rock doesn’t. Water can flow into fractures in the rock, even miles underground – and when it does, the water gets hot, too.
Geothermal power systems use the energy in the underground water (which is usually laden with dissolved minerals and gases, and referred to as geothermal fluid or brine) and the steam created when it boils as it nears the surface and starts to depressurize. What you’re looking for when you try to find a place to build a geothermal plant is a place where there’s a lot of hot rock, and a lot of fractures in the rock where you can extract the brine.
Iceland has a lot of fractured rock. The country is basically being torn in two by the pulling apart of North America and Europe, leaving a long rip that runs from the northeast to the southwest. The rip is part of the Mid-Atlantic Ridge (MAR), the longest mountain range in the world. The MAR basically runs almost halfway around the world, from the north Atlantic Ocean all the way to the south.
When you look at a map of the MAR, you can see that it’s not a smooth, unbroken line of mountains: like a tear in a long strip of paper, there are places where the rip veers off to one side or another. The actual process is different, but it leaves marks all along the ocean floor: offsets to the east and west, and fracture zones that are more or less perpendicular to the MAR. In Iceland, there’s a similar zone called the South Iceland Seismic Zone (SISZ): lots of earthquakes happen there, and the rocks there are especially fractured. Mount Hengill, the 2,000-year-old volcano that Icelanders use to power the capital city, sits at the intersection of the SISZ and the western edge of the MAR where it comes ashore in Iceland.
A bit to the east of Hengill is a place called Bitra. It’s one of those places that’s easy to get to if you know it’s there. Just off the Ring Road on the way from Reykjavik to Hveragerði, Bitra is a place where the earth is in obvious distress: huge columns of steam are visible for miles all around, and the very ground itself is in some places bleached and discolored by the presence of hot, slightly-acidic gases.
It’s such an obvious place for a geothermal plant that planning for a 135-megawatt power station were actually pretty far along, until they stalled a few years ago due to strong environmental opposition and lack of a compelling need to develop the site.
My surface exploration class, led by Björn Sverrir Harðarson, a geologist from Iceland Geosurvey (ISOR), visited Bitra as part of a field trip to observe the formations around Reykjavik. We stopped near a hillside shrouded in steam and hiked a short distance to the hill itself. There were dozens of fumaroles there, holes in the ground where the brine had worked its way up through the fractures in the rock, decompressing and boiling into steam as it neared the surface.
The steam had a distinct odor of rotten eggs, caused by hydrogen sulfide from the geothermal fluid reservoir far below. The soil was mostly red, orange, and yellow clays – material that had at one time been solid rock, but was now in the process of decomposing as the hydrogen sulfide formed dilute sulfuric acid. Parts of the ground were grey, almost white, bleached by the acidic brine spitting up from small springs.
Iceland is home to a lot of unusual landscapes, with stories that involve literally inhuman forces or timetables – or both. Hiking those landscapes, and understanding a bit of how they were made, is fascinating stuff.