Anyone for a South Pacific vacation after your Alaska Highway trip?  You can view some South Pacific geology in west-central Idaho on your way home!

As we drove north down the Little Clearwater River from our pine-forested lunch stop near Pinehurst, Idaho, we came around a curve and the vegetation changed completely.  The change was so abrupt that my wife commented that it looked like “it must rain a lot less here.”  But how could that happen in such a short distance?

The answer was that it couldn’t, so, having read about the area before our trip, I responded that it wasn’t the precipitation that changed, but the geology.  When we came around that curve, the pine-forested slopes of granite and basalt that we had been driving through became barely vegetated cliffs of metamorphic rock.  We had gone from the granite of the Idaho Batholith and Columbia Plateau Basalts into the Seven Devils complex.  To put it in terms discussed in the June 2 post, we had entered one of those “exotic terranes” where an island arc crashed into the North American Plate.

Geologists have dated the “crash” at about 90-100 million years ago (the Cretaceous Period, when some of the last dinosaurs, like the duck-bill dinosaurs and T. Rex, were walking around).  However, the rocks of the island arc were as much as 250 million years old (Permian and Triassic-age) and were formed in the warm waters of the South Pacific. The very large, tough, granitic Idaho Batholith was the western edge of the NA Plate, so the “softer” sedimentary rocks of the island arc simply smashed into an “immovable object” and were partly subducted and partly crushed (metamorphosed) into a thick sequence of gneiss and schist.  The partially subducted rocks are called the Seven Devils Complex and the highly metamorphosed rocks are called the Riggins Complex.

The following photo shows the Seven Devils Complex dipping (sloping) down toward the east (as if to dive under the Idaho Batholith granites of the NA Plate).  You can easily see how little vegetation there is in this exotic "terrane,” a sharp contrast to the conifer forest of the area just to the south along US 95.

The photo below shows the southwestward-dipping rocks of the Riggins Complex along the west bank of the Salmon River, just downstream from the whitewater-rafting mecca of Riggins, ID.  The Riggins Complex rocks apparently hit the edge of the Idaho Batholith and were bent upward, while the rocks of the Seven Devils Complex were forced under the Riggins rocks.  Notice the dip-controlled surface topography above the cliff (the sun-baked, smooth, grassy slope at the upper left).  That is Columbia Plateau Basalt overlying the Riggins Complex.  And why is the basalt dipping southwest too?  It is due to “isostasy,” the removal of the overlying weight of the rocks by the erosion by the Salmon River.  This “unloading” results in the underlying fluid mantle pushing the overlying continental crust up as the load is reduced (see my May 31 post about marshmallows and hot cocoa).  The same thing is still happening in the Midwest and Canada due to the removal of the ice load in the 14,000 years since the Wisconsin glaciation ended.

 

Below is a photo of a blocky, fractured outcrop of metamorphic rocks of the Riggins Complex north of the town of Riggins.  It would be very difficult to tell whether these mica schists were originally sedimentary or igneous rocks when they were deposited in the South Pacific Ocean 250 million years ago.

The photo below is a close-up of the same outcrop of Riggins Complex schists.  Note the way the sunshine on the rocks gives them a “sheen” caused by the planar alignment of the flat mica sheets that make up the schist.

After climbing a very long hill on US 95 northwest of White Bird, ID, we encountered another abrupt change from the rocks of the island arc back into the Columbia Plateau Basalts.  It will be interesting to see if the island arc terranes of the Okanagan Valley of central Washington and southern British Columbia have as abrupt contacts with the surrounding rocks as those of Idaho. 

And now, if you happen to be driving home from Alaska via US 95 from Coeur d’Alene to Boise, you will know exactly when you leave North America and enter the “South Pacific”!

Next time: Glaciation in the Coeur d’Alene to Grand Coulee region has immensely modified the Columbia Plateau basalt surface in east-central Washington State.

Craters of the Moon is an otherworldly experience.  For a geologist, walking up a 2,000 year-old volcanic cinder cone with not one plant growing on its sides is like climbing a staircase built last week for a non-geologist.  Time measured in human terms is so recent that all of human existence would literally be the thickness of a hair lying on top of a yardstick standing on end, where the yardstick represented the rest of geologic time.  So, a geologic event that occurred within the oral history of a people that still exist in southern Idaho, like the Shoshone-Bannock tribes, is not yesterday, but last second, in geologic time.

Craters is an area of not just cinder cones, but both pahoehoe and a'a volcanic flows, spatter cones, lava tubes, and tree trunk molds.  These features are not large, like the Grand Canyon, or spectacularly colored, like Bryce or Zion, but resemble features on the Big Island of Hawaii than like other national monuments in the U.S.

The first photo shows a small spatter cone, a feature built up by small blobs of red-hot lava being ejected from a nearly stationary flow by escaping gas or steam.  This cone is formed from a’a lava, the jagged, broken type of lava flow that is nearly impossible to walk across.

 

 The second photo shows a sloping pahoehoe flow with no vegetation after 2,000 years.  “Pahoehoe” is a Hawaiian word for a “rope-like” flow that forms a smooth skin as it cools, sort of like the skin that forms on a bowl of chocolate pudding after it has cooled.  The smooth skin only forms on cooling lava that contains a considerable amount of gas or water vapor. 

 

The photo below shows the Indian Tunnel lava tube that formed when molten pahoehoe lava continued to flow under a cooling skin until the source finally stopped producing lava and the cooling tube emptied.  This tube is 30 feet high, 50 feet wide, and 800 feet long, but several portions of the roof have collapsed, as can be seen by the “skylights” and rubble in the photo.  This tube got its name because of evidence that Native Americans used the tube for temporary habitation.

 

As it travels farther from its source and begins cooling, the gases begin to be lost and the lava becomes thicker and more brittle.  Instead of stream of flowing lava, the front of the flow becomes a tumbling mass of jagged pieces of lava containing many tiny voids where gas bubbles get trapped.  This is called “a’a” lava in Hawaiian.  The photo below is of an a’a flow that is just beginning to support plant life after 2,000 years!  The weathering and erosion process for lava flows in land-locked, semi-arid southern Idaho is very different from the same geologic processes on the humid Big Island of Hawaii surrounded by the Pacific Ocean.

 

Next time: Driving down a valley cut into an island arc that smashed into the Naorth American Plate 200 million years ago.

We’re finally on the road to Alaska!  We spent the first night at Flaming Gorge Reservoir in northeastern Utah.  The Uinta Mountains are one of the very few east-west trending mountain ranges in the Lower 48.  Geologist-explorer John Wesley Powell led the first expedition through this gorge in May of 1869, the same month the first transcontinental railroad was completed at Promontory Point, Utah.  Powell named this gorge for the “flaming” red rock formations towering above him as he floated down the Green River (see photo).  He also developed his theory of "superposition" to understand why the Green River would flow across the hard Precambrian rocks in the core of the Unitas instead of around the range.  Today Utah provides great road signs along US 191 that identify each of these formations as you are driving up into the mountains, but down in geologic time.  From south to north, the highway runs from the Eocene Green River Formation (50 million years old) to the PreCambrian Uinta Formation (800 million years old).

 

 

From the Uinta Mountains, we crossed the desolate Green River Basin to the rugged Wind River Mountains.  Near the town of Pinedale is a 14,000 year-old terminal moraine that is famous to North American glacial geologists.  This moraine marks the greatest extent of the last advance of mountain glaciers in the Rocky Mountains, so this glacial time period is named the “Pinedale.”  In the U.S. Midwest and Canada, this same glacial period is called the “Wisconsin.”

Northwest of the Wind River Mountains is another northwest-trending range called the Gros Ventre Range.  The Gros Ventres are at the eastermnost front of the Wyoming Overthrust Belt, a north-south trending series of “thrust faults.”  Unlike strike-slip faults, thrust faults have nearly flat-lying or low-angle fault planes that resemble a new deck of playing cards.  If you split the deck in half and turn the two halves face up, you can see the cards in each half.  Place one half against a wall and push the other half against the first half.  Some of the slick second-half cards slide across the top of the stationary half and others slide between the stationary cards.  This analogy is that a thrust fault pushes one geologic formation over another.  Most of the overthrust belts in the Rocky Mountains, such as the foothills west of Calgary on the road to Banff, were pushed eastward by lhe island arcs crashing into the western margin of the North American Plate.  These forces pushed older rocks over younger rocks, the reverse of normal geologic sequencing.  Near our campground on Granite Creek, a tributary of the Hoback River, 150 million year-old Nugget Sandstones was pushed up over 60 million year-old Hoback Formation shales and sandstones (see photo of Flying Buttress Mountain taken from our campsite).  Great camping and interesting geology combined into a single visit to the mountains just outside Jackson, Wyoming.  You can’t beat that if you are a geologist!

Next time: Lunar camping—a day (and night) at Craters of the Moon National Monument.

It's Not My Fault! Strike-Slip Movements and Big Earthquakes

“Southeast Alaska escaped damage or casualties from a tsunami triggered by a magnitude 7.7 earthquake Saturday night in Haida Gwaii, the Canadian archipelago formerly known as the Queen Charlotte Islands, that lies south of the Dixon Entrance.

But the Haida Gwaii temblor occurred along the same Fairweather—Queen Charlotte transform fault system that lies beneath the Alaskan Panhandle, and seismologist Natasha Ruppert said this week that there is no way to tell when or where the fault’s next big quake might be coming.” 

This quote was published in a Juneau Empire article on October 31, 2012.  Two months later. a magnitude 7.5 earthquake occurred around midnight on January 4, 2013, on the southwest side of Prince of Wales Island, which bounds the entrance to the Inside Passage in Southeastern Alaska.  This one was felt by most people on the island, as well in British Columbia and as far south as Washington State.

The 1989 Loma Prieta Earthquake that famously rocked San Francisco during a World Series Game, collapsed portions of the Oakland Bay Bridge and the elevated I-880 in Oakland, and burned buildings in the Marina District was a “mere” 7.1 magnitude earthquake.  So, if the 2012 Haida Gwaii quake and the 2013 Prince of Wales Island quake were of greater magnitude, why have few people in the U.S. even heard of them?  The answer is that they occurred in a relatively lightly populated area, not California, and their epicenters were located in the Pacific Ocean on the Queen Charlotte-Fairweather Fault system, not on the well-known San Andreas Fault.

Just like the San Andreas Fault, the Queen Charlotte-Fairweather Fault system is a north-northwest-trending, “transform” fault that bounds the western margin of the North American Plate.  The San Andreas is 810 miles long, running from the Gulf of California to the Pacific Ocean north of San Francisco in the Point Arena-Manchester area.  The Queen Charlotte-Fairweather Fault System is 950 miles long, running from the southern end of the Queen Charlotte Islands to a junction with the St. Elias Fault in Wrangell-St. Elias National Park.

A transform boundary forms between two tectonic plates that are moving horizontally past each other and not moving with one plate subducting beneath the other. The fault associated with a transform boundary is called a “strike-slip fault.”  This means that, if you are looking along the fault, one side of the fault is moving to the right and the other side is moving to the left.

The red line on the map above shows the trace of the Queen Charlotte-Fairweather fault system (map from: http://earthquake.usgs.gov/hazards/products/ak/1999/documentation/).  The Pacific Plate is moving northwest along the fault and the North American Plate is moving to the southeast, so the fault system is a “right-lateral” strike-slip fault.  This designation means that, if you are standing on the fault and looking along its length, the right side is moving toward you and the left side is moving away from you.

The map also shows a long red line within the NA Plate that runs northwest from the Haines area to Mt. McKinley.  This is the Denali Fault System, which forms the northern boundary of both the St. Elias Range and the Alaska Range.  In fact, this fault extends in a great curve from Southeastern Alaska around the northern side of Mt. McKinley, then southwest all the way to the Alaska Peninsula near Bristol Bay.  If you have driven the Alaska Highway from Border City (MP 1222) to Northway Junction (MP 1264), you were driving on top of the Denali Fault, which forms the valley of the Tanana River along this route. 

So, why are these extremely long, transform faults important?  During the relatively short historical record of the Pacific Northwest, the Queen Charlotte-Fairweather Fault System has generated some of the largest earthquakes ever experienced.  Seven major earthquakes have occurred along this fault system in slightly more than a century: 8.2 magnitude near Yakutat in 1899; 7.1 on northern Chichagof Island in 1927; 8.1 in the Queen Charlotte Islands in 1949; 7.7 near Lituya Bay in 1958; 7.1 near Sitka in 1972; 7.7 in Haida Gwaii (the Queen Charlotte Islands) in 2012; and 7.5 near Prince of Wales Island in 2013.

The most spectacular of these quakes wasn’t the largest one, however.  On July 9, 1958, the Fairweather Fault generated a magnitude 7.7 quake that was felt as far south as Seattle and east as far as Whitehorse.  Lituya Bay is a part of the Glacier Bay National Park and Preserve.  The earthquake along the fault loosened about 40 million cubic yards of rock that plunged 3,280 feet into Lituya Bay.  The splash caused 1,300 feet of ice along the entire front of the Lituya Glacier to jump up such that an eyewitness survivor saw it rise into the air and move forward.

 

After the glacier disappeared from view, a wall of water that has been measured at over 1,720 feet (the Empire State Building is 1,470 feet high) washed away everything—6-foot diameter trees, boulders, even the soil right down to the bedrock.  In the 1958 USGS photo above, the rockfall is at the right and the 1,720-foot high wave scar is at the left. 

 

This giant wave swept down the bay and, by the time it swept over Cenotaph Island, it was moving at 600 miles per hour (nearly the speed of sound!).  The 1958 USGS photo below shows Lituya Bay, Cenotaph Island, and the sand spit at the mouth of the bay.  Three fishing boats were anchored near that spit when the wave hit.  Although one boat was destroyed, killing the two-man crew, the other boats rode the wave like surfboards.  The wave carried the two boats 80 feet above the spit and out into the Gulf of Alaska, but the two crews survived.  That 1958 event is still the largest landslide-generated wave ever documented worldwide. 

 

 

However, such earthquake-induced rockfall and landslide events are not uncommon in Alaska, where similar wave scars have been found in Glacier Bay, along the Lynn Canal, near Yakutat, and in Prince William Sound.  And these days, a cruise ship might not "surf" a 600-MPH wave out of Glacier Bay quite as easily as two small fishing boats exited Lituya Bay in 1958.  Instead of a death toll of two, we could see a death toll of 2,000.  And that makes the Queen Charlotte-Fairweather Fault System important, not just to Alaskans, but to everyone who visits the state.

Next time:  We are finally on the road to Alaska!  When I find some interesting geology, I'll post again.

Rat Islands and Atomic Bombs

Amchitka is one of the Rat Islands--35-miles long and only 3-miles wide.  It is located near Kiska and very near the epicenters of those 1957 and 1986 earthquakes of magnitude 8.0 or larger.  The Google Earth image below shows the relative positions of Amchitka and Kiska, the two large islands in center of the image; the Aleutian Trench, part of which is called James Canyon along the bottom of the image; and Buldir Island, where the last remnant of the Aleutian Canada Goose population was found in 1962, in the upper left-hand corner of the image.  Of course, we now know that the very deep Aleutian Trench is actually there because the Pacific Plate is being subducted beneath the North American Plate, on which all these islands are located.  Can you see that when you look at James Canyon in the photo?  And, from the image, you can easily understand why so many of those earthquake epicenters shown along the Aleutians in the last post seem to be located just south of the islands.

Just before the Japanese abandoned Kiska in 1943, Amchitka became a major U.S. military base, but it closed after the war.  In the early 1960's, the U. S. Atomic Energy Commission created a nuclear testing facility on this island.  The WWII-vintage airfield and base camp were reused by crews preparing for underground nuclear tests. 

The first test was an 80-kiloton bomb known as "Long Shot," located 2,400 feet below the surface and detonated on October 29, 1965.  The second, "Milrow," was a one-megaton device located 4,000 feet below the surface and exploded on October 2, 1969.  The third, "Cannikin," was a 5-megaton bomb detonated 6,000 feet below ground level on November 6, 1971.  However, the morning before the Cannikin blast, the test site had hard rain and wind gusts up to 124 miles per hour.  Man may be able to destroy whole cities, but can't control Aleutian weather! 

Cannikin did come as close to being more powerful than nature than anything else man has ever built.  It was the largest underground nuclear explosion in U.S. history.  Some Alaskans, still rebuilding 7 years after the Good Friday Earthquake, were concerned that the explosion could trigger another “Big One.”  In fact, two days after the explosion, a crater more than a mile wide and 40 feet deep did form above the blast site.  But Cannikin generated no 1964-sized earthquakes on the mainland despite producing a shock wave registering magnitude 7.0.  And no Aleutian volcanoes immediately began erupting.

Although I had no role in these tests, I was working in the Denver office of the Engineering Geology Branch of the USGS during the last two tests.  Several other geologists and technicians in my branch were working on all three tests—interpreting post-blast geologic data from the 1965 test and pre-blast data for the second two.  Several of my co-workers also spent months each summer during 1968-1971 performing fieldwork for these tests.  They drilled into the rocks of the test sites to obtain a set of rock “cores” that could be used for geologic interpretation of the blast zone.  These cores and simultaneous geophysical tests were also used to locate faults on which movement might be triggered by the blasts.  But the topics that they most often talked about when they returned to Denver were the poorly-maintained, WWII-vintage “deuce-and-a-half” (2-1/2 ton) trucks they had to use on Amchitka, the difficulty of getting supplies and spare parts for the drill rigs, and—now here’s a surprise—that they had to work outside every day in horrible Aleutian weather!  Of course they couldn’t really talk about the tests themselves because they were classified.

The Amchitka nuclear test facility closed in 1994.  But a government clean-up of the residual radioactive, chemical, and other hazardous waste left on the island continued for years after that.  Today, the buildings, roads, and airfields are gone and Amchitka's World War II role is just a memory, although their sites can still be picked out on Google Earth.  And, apparently, a small plaque commemorating the Cannikin blast is the only reminder of the island’s role in the Cold War.  Well, that and the fact that the island is still off-limits to visitors.  Fortunately, I didn’t have a role in the Amchitka cleanup.

Next time: We are getting anxious to get on the road—only another week to go!  But first, a brief discussion of faults—not the “it’s your fault!” kind, but the geologic “break,” “fracture,” “slip” kind that sometimes generate earthquakes.  The San Andreas Fault in California may be the one we most often hear about because it is responsible for earthquakes in heavily populated Los Angeles and San Francisco.  This post will introduce you to the Fairweather Fault, which is similar to the San Andreas in the way it moves, but much longer and with the potential to generate even larger earthquakes! 

Crazy Foxes, Lousy Weather, Earthquakes, and Atomic Bombs!

Attu and Kiska--during World War II, the Japanese invaded North America at only those two locations.  And where are those strange-sounding places?  Well, not on any road that we can reach on our trip up the Alaska Highway, that’s for sure.  And not even by the Alaska Marine Highway Ferry System.  They are two of the 200 islands that make up the Aleutian Island chain.  Attu is located at the far western end of the chain, about 1,500 air miles from Anchorage and Kiska is located about 250 miles east of Attu.  Both islands are so far west that the International Date Line would lie to their east if the line hadn’t been bent far to the west around them.  So, instead of being on a different day than the rest of Alaska, the Aleutians are just in a different time zone—Aleutian time—one hour behind the rest of Alaska.  Even Unalaska-Dutch Harbor, the only Aleutian town of consequence, the only one on Alaska time, and the only one that can be reached by regularly scheduled ferries or commercial airlines, is still located 800 miles southwest of Anchorage and 1,000 miles east of Attu! 

The Aleutian Islands contain four island groups: the Fox, Andreanof, Rat, and Near Islands (“near” Russia, which named them).  Interestingly, despite these names, there are no native foxes or rats (in fact, no native land mammals at all nor any biting insects) on the Aleutians.  Perhaps the mosquitoes were all blown out to sea by the constant windstorms, known as “williwaws,” that frequent the islands.  When I landed on Shemya in 1992, the aircrew pointed to the wind gauge outside the airfield terminal.  Instead of a windsock, it was a driftwood log suspended by a logging chain from a long arm (driftwood because there are no trees in the Aleutians).  Their tongue-in-cheek warning was “If the log is at 45 degrees, stay on the plane!” 

To relate only one example of typical Aleutian weather, when our plane left Anchorage, it stopped at Adak Island to refuel.  That was not because it needed fuel to reach Shemya (which is 30 miles east of Attu), but because, if necessary, they had to be able to circle Shemya for 3 hours.  This was so the crew could await a diminishing of high crosswinds or a fog bank or find a hole in a 10,000-foot high cloud bank commonly shrouding the island.  If not successful at landing in that length of time, they still had to have enough fuel to get back to Adak. 

Weather in the Western Aleutians is much the same year round—wind, fog, wind, rain, wind, snow--with temperatures ranging from 11 to 60 degrees F through the year.  Annual total precipitation is only around 21 inches.  However, snow generally “falls” horizontally due to winds off the Bering Sea, instead of more normal vertical snowfall!  If you want to learn more about Aleutian weather and its effects on humans, read “The Thousand Mile War” (see the reading list in my profile).

Three mammal species were introduced to the Aleutians during the historic period.  Two rodents, roof rats and deer mice, were accidentally brought to the islands in ship cargo.  In the late 1800's, the native sea otters disappeared due to over-hunting for furs for export to China.  So, in order to stay in business, the fur trappers purposely introduced Arctic foxes, a different color on different islands to prevent crossbreeding, and the foxes soon became feral.  The Aleutian Canada goose was then thought to have become extinct due to foxes consuming both young birds and eggs.  However, a surviving population of the big birds was discovered on Buldir Island in 1962.  To protect the rare bird, the government eliminated the foxes.  Today they are present only on Shemya, where they have been allowed to stay as a way to deter birds from nesting and interfering with the operation of military aircraft. 

The foxes on Shemya are the blue color-phase of the Arctic Fox, but do not have a white coat in winter like their cousins in the high arctic.  The tattered looking coat that occurs when these foxes shed their winter pelt has given them the nickname “scruffy.”  They are completely unafraid of humans due to being land-locked with us on a 4-mile by 2-mile island for 70 years.  For the same reason, they are now so inbred that some are nearly blind, others walk with a peculiar sideways gait, and I seriously doubt many can still hunt due to being fed handouts by personnel assigned to the island.  When I got out of the truck that brought me from the Shemya airfield to the dormitory where I stayed, one of the scruffies walked up to me, lifted its leg, and peed on my shoe.  I guess he knew I was new “territory” and he was just marking me as his territory! 

But enough about the animals and weather; it’s on to the reasons that the geology of the Aleutians is important.  The Aleutian Islands are the tops of submerged volcanic mountains belonging to a range stretching more than 1200 miles into the Pacific Ocean from the Alaskan Peninsula.  This partially submerged continuation of the Alaska Range separates the North American Plate from the Pacific Plate.  At this plate boundary, the Pacific Plate is subducting beneath the NA Plate, creating the deep Aleutian Trench on the Pacific side of the islands.  Due to the heat generated by friction between the plates, the Aleutians contain more than 25 active volcanoes—13 over 5,000 feet high.  The USGS photo below shows Mt. Kanaga on Adak Island during a 1994 eruption.

Some of the largest earthquakes in the world occur in the Aleutians because they are located on a plate boundary, a subduction zone, and part of that Pacific “Ring of Fire” discussed in an earlier post.  A 1965 earthquake in the Western Aleutians had an 8.7 Richter Magnitude, 6^th greatest on record worldwide (the 1964 Good Friday Quake, centered in Prince William Sound, was a magnitude 9.2, 2^nd largest ever recorded).  The 1965 Aleutian event not only caused extensive ground shaking, but also produced a tsunami that measured 35 feet on Shemya Island.  The tectonic setting of Shemya in the western end of the Aleutian Islands results from an oblique-angle collision of the Pacific Plate and the North American Plate.  In 1975, Shemya experienced a 7.5 magnitude earthquake that actually did more damage to its military facilities than the larger one 10 years earlier.  In 1986, a magnitude 8.0 earthquake was centered near Kiska Island, in the same general area as an 8.6 magnitude quake in 1957.  Finally, on June 19 of last year, Shemya experienced a magnitude 6.0 earthquake, but no tsunami.  A U.S. Geological Survey map showing earthquake epicenters and magnitudes in the Aleutians can be viewed below.

The Air Force closed Eareckson Air Station on Shemya in 1995 and the Navy closed its 6,000-person naval base at Adak, also built during WWII, in 1997, several years after I was on both islands.  The military facilities remaining on Shemya are now operated by DoD contractors.  The base at Adak was completely deactivated, cleaned up, turned over to a native corporation, and resettled by several hundred Native Alaskans.  I worked on the cleanup of hazardous waste on both of those islands.  In 2010, the Coast Guard closed its station on Attu.  This last military presence in the Aleutians ended 68 years after the Japanese landed on that island.

Next time: I mentioned atomic bombs in the Aleutians, but spent too much time on weather and animals.  So, the next post will be about those bombs.

Are you driving through “terrain” or “terrane”?  If you are looking at a topographic map, the answer is “terrain.”  If you are looking at a geologic map of western BC, southwestern Yukon, or southern Alaska, the answer may well be “terrane.”  Confused yet?  “Terrain” is the “lay of the land”—e.g., hilly terrain, mountainous terrain, etc.—and generally refers to differences in elevation or “topographic relief.” 

“Terrane” is a more complicated geologic term for a specific area of land that contains rocks formed by a particular geologic process, as in a volcanic terrane or a limestone terrane.  However, since the acceptance of plate tectonic theory in geology 30 years ago, “terrane” has taken on a narrower definition of broken-off pieces of continental plates, island arcs, or even oceanic crust that have crashed into (called “accreting” by geologists) a drifting continental plate instead of being “subducted” beneath plate.  These terranes then become an “exotic” part of the plate.  Dr. David G. Howell of the U.S. Geological Survey (USGS) in Menlo Park, CA, a specialist in terrane analysis, compares the drifting of continents to the motion of geologic “bumper cars"!

The western edge of the North American (NA) Plate has accumulated many long, narrow, flattened-out exotic terranes, some as big as Japan, or even California, over the past 200 million years.  The figure below, from the USGS website http://pubs.usgs.gov/gip/dynamic/Pangaea.html, shows a few of those terranes.  But this is greatly simplified from the newest geologic terrane maps of the West Coast of BC and Alaska. 

Of interest during our Alaska Highway trip is a particular terrane that is still in the process of bumping into the NA Plate.  Although in a remote area not near any highway, the Yakutat Block terrane covers a large area—360 miles long and 120 miles wide.  It is located on the coast between Glacier Bay and Cordova.  This Block has been moving onto the NA Plate at a rate of about 2.5 inches per year, or 4 miles during the 100,000 years since it first contacted the NA Plate.  The Yakutat Block can be seen on the map above as the light-colored "submarine deposits" area on the west (left) side of the long fault attached to the east end of the "Aleutian Trench" (just left of the ". . llia" in "Wrangellia Terrane").

Based on the rock types and fossils found in parts of the Yakutat Block, it apparently broke off the NA Plate at about the location of Prince Rupert and moved 330 miles north along one fault system at the same time 540 miles of relative movement occurred between the Block and the Pacific Plate, making it 870 miles northwest of its starting point after only 25 million years.  So, the NA Plate is now in the process of readopting one of its own children!

Once we get on the road and reach Washington State, we will be driving north along and across several of these accreted terranes on our way through Kamloops and Prince George, then up the Cassiar Highway (I hope, since it is now closed due to flooding!).  So, in the way of a hint about future posts, the north-south trending Okanogan-Okanagan valley is one of those “exotic terranes.”

Next time: Like crazy foxes, lousy weather, earthquakes, and atomic bombs?  Then you would love the Aleutian Islands!