Monday, January 25, 2016

Monday Map -- Cook Inlet Tectonics


At the boundaries of two plates, one of the things that can happen is for one plate to slide under another.
That’s what’s happening on the southern coast of Alaska, where the Pacific Plate, moving northwesterly, is sliding under the North American Plate and, specifically, Alaska. The Pacific Plate, under the Gulf of Alaska, is sliding under Southcentral Alaska and the Being Sea. This is important. It gives us things like the Great Alaska Earthquake of 1964, the eruption of Mt. Katmai, the Aleutian Island arc and the Denali Fault. It has also given us a pretty nifty image of the Pacific Plate sliding under Alaska – the technical term is “subducting.” Here’s a plot of earthquake activity in Cook Inlet over that last 50 years:1The size of the round dots is proportional to the strength of the quake; the color of the dot is the depth of the epicenter of the quake. You can see that the blue dots – the deeper quakes – trend to the westerly region, and the red/purple/pink dots – the shallower quakes – to the east side. To better understand what this is showing, we’ll choose a cross-section, roughly from Homer on the east shore of Cook Inlet to the slopes of the Alaska Range, and view that cross-section from the side. If you prefer, think of it as a trench dug 250 kilometers deep, and looking at the wall of the trench. Here’s what you’d see:This is Pacific Ocean floor being pushed under the North American Plate. It could not be shown more clearly. Earthquake activity generally ceases at depths greater than 200-250 kilometers because rock heats enough from pressure and the earth’s internal temperature that is deforms plastically instead of fracturing and generating a quake. The approximately 60 degree angle of descent is what geology would predict and what is generally observed. Deeper than 200 kilometers, the subducted Pacific Plate melts and, for various reasons involving chemical composition, water content and density, rises as the volcanic arc of Augustine, Iliamna, Redoubt and Spurr, among many others.

Sunday, January 24, 2016


image courtesy USGS
The 2016 Old Iliamna earthquake struck in the Cook Inlet region of Alaska near Iliamna at 1:30 AM AST on January 24, 2016. The quake was centered approximately 162 miles from Anchorage, and 65 miles from Homer. The earthquake registered 7.1, and was felt across a wide area of Southcentral Alaska, the Kenai Peninsula and as far away as Juneau roughly 700 miles southeast of the epicenter. Damage, mostly moderate with pockets of heavier damage, was experienced across a wide area of Southcentral Alaska. On the Kenai Peninsula, four homes were destroyed in Kenai due to gas leak related fires. Businesses had damage to merchandise, and the Kalifornsky Beach Road was heavily damaged. There were also power outages in Homer, as well as moderate property damage. There was a voluntary evacuation of the Homer Spit.  -- from Wikipedia
Saturday night--hadn't been sleeping long--when I woke to the sound of a crash, followed closely by the stampede of half-a-dozen cats upstairs and away.  After a few seconds, I could feel the shake-shake-shake of the ground.  

"What happened?" 


The gentle shaking had almost subsided when the second wave hit, a series of increasingly sharp jolts that got our attention.  Denny grasped my hand and pushed up closer to the wall.  I could hear things falling over upstairs and thought, if we have a third shock, we will be in trouble.  

We waited a moment.  The shaking seemed to subside--though when every pendant thing in the house is swinging, it is hard to really tell when the shaking stops.  We got out of bed and went out to see what damage had been done.  We could see we still had lights, so I checked the propane tank--still sitting pretty--and the plumbing connections on the second floor--no spurting water--then did what every 21st-century first-worlder would do, checked to see if we had internet and got on Facebook to report in.  (Being old Alaskans, we checked the Tsunami Warning Center and the Volcano Observatory as well.)

As news reports came in, Homer seemed to be the closest town to the epicenter and although we had had a pretty interesting ride, only a few unsecured things, like framed photos and stacks of DVDs had toppled.  I think under the glacial soils there is a lump of bedrock that makes up Bluff Point, so we are on firmer ground than the folks in the lowlands and alluvial fans.

Although they were farther away from the epicenter than we were, the Kenai area suffered more significant damage.  A foot-wide crack split K-Beach road a mile or so from the Kasilof end, and one neighborhood in Kenai experienced a gas-line leak and explosion.


Monday, January 18, 2016

Monday Map: The Farallon Plate

I like maps.  I like the visual display of information, and the Zen-like moments of consideration as I absorb the information on a visceral level.

This is one of those that stuns me.  It shows the remnants of the Farallon Plate underneath North America.  If you aren't a geology nerd, this may bring a ho-hum moment.  And that's okay.  How boring the world would be if we all shared the same few interests. But this picture excites me in the same way an actual photograph of the surface of Pluto excites me.  It gives substance to scientific theories and helps me visualize a complex process.

Also, I have lived most of my life around the scraping of this plate, and feel a strange sense of kinship and longing for that ancient, lost land.

A software model by NASA of the remnants of the Farallon Plate, deep in Earth's mantle.

From Wikipedia
The Farallon Plate was an ancient oceanic plate that began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart during the Jurassic period. It is named for the Farallon Islands, which are located just west of San Francisco, California.
Over time, the central part of the Farallon Plate was completely subducted under the southwestern part of the North American Plate. The remains of the Farallon Plate are the Juan de Fuca, Explorer and Gorda Plates, subducting under the northern part of the North American Plate; the Cocos Plate subducting under Central America; and the Nazca Plate subducting under the South American Plate.
The Farallon Plate is also responsible for transporting old island arcs and various fragments of continental crustal material rifted off from other distant plates and accreting them to the North American Plate. These fragments from elsewhere are called terranes (sometimes, "exotic" terranes). Much of western North America is composed of these accreted terranes.

The Farallon Plate had a lasting effect on the landscape of the western America, as this geology blog remarks:

The Farallon plate went under at a shallow angle, and hence the subduction process was long and complicated. The plate scraped along the underside of the continent for awhile, and the resulting volcanoes formed the Sierra Nevada in California. Due to motions in the mantle, underneath the crust, the continental plate moved westward over the subducted plate; the progress of the Farallon plate under the continent was instrumental in forming the Rocky Mountains. And all these millions of years later, the plate is evidently still descending, and as it moves it affects the flow of molten rock in the mantle beneath parts of the eastern US.

The new paper describes how the resulting stresses in the crust could have triggered the New Madrid earthquake, and perhaps could contribute to further seismic activity in the area in the future.
 The Sierra Nevada, the Basin and Range, the tilted strata of the Southwest--all the landscapes I will dwell on are the direct consequence of this ancient collision and the islands that came in with it   I wonder what populations of life originated on the Farallon Plate and rode the terranes in until they merged with the North American Plate.  Geologic times are long.

Saturday, January 16, 2016

Thursday, January 14, 2016

Curious George

Georgie relaxes in front of the wood stove.

Wednesday, January 13, 2016