Geology and Geologic Time through Photographs

Archive for the tag “plate tectonics”

Iceland –where you can walk a mid-Atlantic rift –and some other geology photos

While Iceland hosts an amazing variety of awesome landscapes, what stands out to me most are its incredible exposures of the Mid-Atlantic ridge. To the north and south, the ridge lies beneath some 2500m of water, forming a rift that separates the North American plate from the Eurasian plate. The rift spreads apart at a rate of some 2.5 cm/year, forming new oceanic lithosphere in the process. But in Iceland, you can actually walk around in it!


Geologic map of Iceland as compiled from references listed below.

If you look at the geologic map above, you can see two rift zones near Iceland’s mid-section, and that the rocks become older in either direction away from those zones –just what you’d expect. New material erupts in the rift zones as they pull apart, separating the two major tectonic plates. In Iceland there’s a third microplate between the two rifts. The process continues through time, creating volcanic activity with ages somewhat symmetrical about each of the rifts. From the map, you can also see that most of Iceland’s big thermal areas lie within the rift zones.

Geothermal power plant, Iceland

Geothermal power plant in the Western Rift Zone, just east of Reykjavik

It’s more complicated though. A hot spot beneath Iceland causes increased volcanic activity, which through time, thickened the Icelandic crust to greater than 40 km in places. By comparison, most of the oceanic crust beneath the North Atlantic ranges from 4-7 km thick. Icelandic magma, rising from its source in the underlying mantle, must therefore pass through a lot more rock, which gives it plenty of time and opportunity to evolve into more silicic varieties. If you look at the geologic map’s legend, you can see that besides basalt (which is the rock of the ocean floor), you also see a lot of andesite and even some rhyolite. In fact, Iceland contains numerous stratovolcanoes, several of which have erupted silicic ash and pumice in the last couple thousand years, including Hekla and Snaeffelsjökull, shown on the map.

Snæfellsjökull volcano and glacier, Iceland

Snæfellsjökull volcano and glacier. There’s a moss-covered basalt flow in the lower part of the photo; the upper reaches of the volcano are more silicic.

What I found so instructive was the physical layout of the rift zones. There’s not a single, discrete crack that marks where all the lava comes up. Each zone is at least several km in width –and marked by numerous discontinuous fissure zones, active at different times than each other. Near Reykjavik, these features are beautifully expressed—and accessible—at Thingvellir National Park as well as along the south coast of the Reykjanes Peninsula.


Western Rift Zone at Thingvellir, view northward. Besides the prominent fissures on the left (west), numerous other ones cut the interior of the rift, 3 of which are marked by arrows. The rift continues up the valley behind the arrows.

And the lava flows –they too erupt at different times and different places –and not always from an existing fissure. Sinton and others (2005), for example, map more than 30 different post-glacial flows at various places within the western rift zone. Most of the recent activity, however, lies within the eastern rift zone. Since the year 2000, some six separate eruptions occurred within or at the margins of the eastern rift zone. These eruptions include the 2010 eruption of Eyjafjallajökull, which took place beneath a glacier. The water-magma reactions created a gigantic ash cloud that disrupted European air traffic for nearly a week –my oldest daughter got stranded in Ireland!

Eyjafjallajokull Volcano as seen from Heimaey

Eyjafjallajokull Volcano as seen from island of Heimaey


Through time, the Mid-Atlantic ridge has split N and S America from Europe and Africa  (USGS)


These observations are important because we tend to present rifting and its accompanying volcanism as a steady, continuous process. We ask students to imagine opposing conveyor belts moving outwards from a central area to help visualize the process. But more accurately, the conveyor belts are partially broken and rusty and so move in only fits and starts –and mostly, the conveyor belts seem stuck. If you walked through the rift zone on any typical day, you might perceive that the whole process had simply stopped. And that’s the point. At human time scales, this rifting process is inexorably slow, almost imperceptible. But through geologic time it creates the enormous changes we can see.


To make Iceland even more complex –and interesting—the Snaeffelsnes Peninsula, north of Reykjavik, marks an earlier position of the rift. The rift migrated some 6-7 million years ago to today’s western rift zone. The peninsula’s main volcano, the ice-capped Snaeffelsjökull stratovolcano is still potentially active, having last erupted sometime around 200 AD.

Holocene lava flow near Snaeffelsjokull

Holocene lava flow near Snæfellsjökull


So here’s a bit of a photo dump. I visited the SW third of Iceland during early September, 2018 with my friends Christine and Charlotte and shot nearly a zillion photos. What a landscape! While these are some of my favorite photos, I posted more than 100 others on my geology photo website and they’re all freely available to download. Just type “Iceland” into the search!

click on any photo (including the ones above) to see it larger and in a separate window


Christine and Charlotte one evening

Gravel bar in braided river, Iceland.

One of my favorites, a gravel bar near the south coast



We met up with a Geo group from Colorado College (where Christine teaches) -these are glacial deposits.

Columnar-jointed basalt, Iceland

The CC group took us to these basalt columns near Vik –columns in 3D!

Eldfell cinder cone and Heimaey, Iceland

Eldfell cinder cone and town on Heimaey. In 1973, the cinder cone erupted, destroying parts of the town and nearly blocking the harbor. Icelanders stopped the lava using seawater!


Airfall deposits from earlier eruptions on Heimaey –with projectiles!

Rift valley within W Rift Zone, Iceland

One of the fissures in the W Rift Zone of Thingvellir NP.

Feeder dike intruding tephra (Vertical)

Christine and a feeder dike

Rocky coastline and breaking wave,  Iceland

Breaking wave on the S Coast


and moss…



Some References:
Sinton, J., Gronvold, K., and Saemundsson, K., 2005, Postglacial eruptive history of the western Volcanic Zone, Iceland: Geochemistry, Geophysics, Geosystems, v. 6, no. 12.

Great general reference (thanks Tom!)  Gudmundsson, A.T., 2007, Living Earth, Outline of the Geology of Iceland, Mál og menning, Reykjavik, 408p.

References used for map:
Jóhannesson, H., 2014, Geological map of Iceland. Bedrock Geology, 1:600,000. Icelandi Institute of Natural History.

Geothermal sources of energy in Iceland. Water and Fire:

Islam, Md. Tariqul, 2016, Rheological response to tectonic and volcanic deformation in Iceland, Thesis, University of Gottenburg,


Washington’s waterfalls–behind each one is a rock!

Of all the many reasons why waterfalls are great, here’s another: they expose bedrock! And that bedrock tells a story extending back in time long long before the waterfall. This posting describes 9 waterfalls that together paint a partial picture of Washington’s geologic history. The photos and diagrams will all appear in my forthcoming book Roadside Geology of Washington (Mountain Press) that I wrote with Darrel Cowan of the University of Washington.


Rainbow falls along WA 6 in the Coast Range


And waterfalls in heavily forested areas are especially great because they may give the only view of bedrock for miles around! Take Rainbow Falls, for example–the small waterfall on the left. It’s in Washington’s Coast Range along State Highway 6–a place where a roadside geologist could otherwise fall into total despair for lack of good rock exposure. But this beautiful waterfall exposes a lava flow of the Grande Ronde Basalt, which belongs to the Columbia River Basalt Group. Significant? Yes!

This lava erupted in southeastern Washington and northeastern Oregon between about 16 and 15.6 million years ago and completely flooded the landscape of northern Oregon and southern Washington. We know how extensive these flows are because we can see them–and they cover the whole region. The photo below shows them at Palouse Falls in the eastern part of Washington. Take a look at my earlier blog post about the Columbia River Basalt Group? (includes 15 photos and a map).

Read more…

Oregon’s rocky headlands: geologic recycling through erosion and uplift and erosion…


Crashing waves at Heceta Head, Oregon

You can’t avoid thinking about erosion while standing on one of Oregon’s rocky headlands. The waves keep coming, one after another, each crashing repeatedly against the same rock. Impossibly, the rock appears unmoved and unchanged. How can it not erode?

The answer, of course, is that headlands do erode, quickly, but on a geologic time scale. We just miss out because we live on the much shorter human time scale. And the erosion belongs to a cycle in which coastal uplift causes eroded and flattened headlands to rise and become headlands once again, all subject to more ongoing erosion and uplift.

Wave energy is most intense at headlands because the incoming wave typically feels the ocean bottom near the headland first, which causes the wave to refract. As shown in the aerial photo below, this refraction focuses the wave energy on the headland.


Wave refraction causes wave energy to focus on the headland. Arrows are perpendicular to wave fronts.

As you can see in the next few images, headlands don’t erode evenly. They erode irregularly, as the waves exploit any kind of weakness in the rocks such as faults and fractures, or if they’re sedimentary, bedding surfaces. The products of this erosion are as beautiful as they are interesting: sea stacks, sea arches, sea caves… The list goes on and on.

Headland and lighthouse, Heceta Head, Oregon

Aerial view of Heceta Head, Oregon.

From the above photo, you can see that sea stacks are simply the leftover remains of a headland as it retreats from erosion. That’s a critical point, because some sea stacks, especially the one with the arch in the photo below, are a long way from today’s coastline.

Sea stacks and sea arch, southern Oregon

Sea stacks and sea arch, southern Oregon

Those rocks, 1/4 to a 1/2 mile away used to be a part of the coastline? The land used to be way out there? YES!!! For me, that’s one of the very coolest things about sea stacks –they so demonstrate the constant change taking place through erosion.

Taken to its extreme, erosion renders headlands into wave-cut platforms, such as the one below at Sunset Bay. Being in the intertidal zone, these platforms make great places for tide-pooling–and ironically, for people-watching too. Geologically, they form important markers because they’re both flat and form at sea level. When found at higher elevations, they indicate uplift.

Wave-cut bench, Sunset Bay, Oregon

Wave-cut bench at Sunset Bay, Oregon

In fact, looking carefully at the photo above, you can see a flat surface on the other side of the bay. It’s an uplifted wave-cut platform! Called a marine terrace, it’s covered by gravel and sand originally deposited in the intertidal zone. Those deposits rest on bedrock that, at an earlier time, was also flattened by the waves. The photo below shows a better view of this terrace from the other side.


Breaking wave at Shore Acres State Park, Oregon. Tree-covered flat surface in the background is an uplifted marine terrace.

These uplifted marine terraces can be found up and down Oregon’s coastline. Researchers recognize several different levels, the oldest being those uplifted to highest elevations. The one in the photo above at Shore Acres State Park is called the Whiskey Run Terrace and formed about 80,000 years ago. You can see a similar-aged terrace below as the flat surface beneath the lighthouse at Cape Blanco, Oregon’s westernmost point. An older, higher terrace forms the grass-covered flat area on the right side of the photo.

Cape Blanco, Oregon

Cape Blanco, Oregon looking NE. The flat surface beneath the lighthouse is the ~80,000 year-old Cape Blanco Terrace, probably equivalent to the Whiskey Run Terrace at Shore Acres; the flat area on the right side of the photo is the higher Pioneer Terrace,  formed ~105,000 years ago.

Researchers take the approximate ages of the terraces and their elevations to calculate approximate rates of uplift. In this area, Kelsey (1990) estimated a rate of between 4-12 inches of uplift every 1000 years. That might seem slow, but over hundreds of thousands of years, it can accomplish a great deal.

And look! The uplifted terraces? They’re on headlands! Of course, because they’ve been uplifted! And the headlands are now eroding into sea stacks and then platforms –to be uplifted in the future and preserved as marine terraces that sit on top headlands. And on and on, as long as the coastline continues rising.


Blowhole near Yachats, Oregon. Incoming wave funnels up a channel eroded along a fracture and explodes upwards on reaching the end.

Some links and references:
Kelsey, H.M., 1990, Late Quaternary deformation of marine terraces of the Cascadia Subduction Zone near Cape Blanco, Oregon: Tectonics, v. 9, p. 983-1014. (Detailed study of Cape Blanco, including uplift rates).

Miller, M., 2014, Roadside Geology of Oregon, Mountain Press, Missoula, 386p. (General reference which details the concepts and includes several of the photos used here).

Earth Science Photographs–free downloads for Instructors or anybody: my webpage!

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