New "Milepost Alaska Highway Geology" blog

See my new blog at http://fanrgs.blog.com.  I have had to continue my Milepost Alaska Highway Geology blog on a new site because I was not able to post on this site for a week due to technical difficulties and no response from Google to my problem.

Milepost “Central Access Route”:

We’re here!  We have finally arrived on the “Central Access Route” approach to the Alaska Highway.  However, we didn’t get on it at Mile “0” at Ellensburg, but at Omak via WA 155 after visiting Grand Coulee Dam.  So, I can’t really comment much on the geology farther south, but even beginning at Milepost E157.6 and following the access route only to Kamloops at Milepost E386 provides an amazing journey into an “exotic terrane.”

We drove across a granite batholith called the Okanogan Dome from Grand Coulee Dam,  The granite dome is a melted portion of a “subcontinent” (larger than an island arc) that migrated by continental drift and hit the North American Plate just before it melted 50 million years ago.  The photo below shows an pale gray outcrop of the granite of the dome.

E157.6: At Omak, we entered the exotic terrane called the Okanogan Trench, the old subduction zone at the edge of the North American Plate plate (see my June 10 post on Amchitka Island and the Aleutian Trench). 

The photo below was taken from WA 155 just east of Omak, looking west across the Okanogan Valley.  I am standing on the rocks of the Okanogan Dome, looking down into the trench and across to the mountains of the Cascade Subcontinent on the west side of the valley.  Millions of years ago, these three zones of the Okanogan Trench were separated by thousands of miles of Pacific Ocean.

 

E159 - E180: The Okanogan Dome is surrounded by one-time sedimentary rocks of the subcontinent that were metamorphosed by heat from the melted granite below them.  As the dome rose, the metamorphic rocks were tilted upward around it.  In the photo below, the angled rocks are located east of US 97 and dipping (sloping) westward into the Okanogan Trench.

E171 – E181 (north of Riverside, south of Tonasket): Look west of US 97 at the base of the hills.  Do you see a valley (depression) that is lower than the road and looks like it should carry a stream, but there is no stream?  There are a couple of small lakes at the bottom of the deeper parts of the depression—Booher Lake and Crumbacker Lake—but nothing that could have carved such a deep valley.  That valley is an old river channel, now dry, that was carved by glacial meltwater at the end of the last ice age about 14,000 years ago (see my May 27 post).  This channel will probably remain dry until the next ice age!

The photo below shows the dry channel west of US 97/WA 20 and the present course of the Okanogan River east of the highway.  It also shows the short distance across the Okanogan Trench on the U.S. side of the border.

 

E197 - E315: A series of natural glacial lakes filling the bottom of the Okanogan Trench begins at Oroville, WA and ends at Vernon, BC, a road distance of 120 miles.  The lakes, from south to north, are Osoyoos Lake, Vaseux Lake, Skaha Lake, the big one—Okanagan Lake, and Kalamalka Lake.  These remnants of the last ice age provide substantial quantities of irrgation water for the intensive fruit and wine growing industry of the Okanagan Valley.

E231.2: Photo taken at Christie Memorial Provincial Park day-use area on in Okanagan Falls showing Hwy. 97 climbing into the bedrock on the west side of Skaha Lake.  This dark gray, white-banded gneiss is part of a subcontinent that “docked” with the NA Plate about 50 million years ago.

E237.9: Other remnants of the ice age are less obvious than the huge lakes of the Okanagan Valley.  From the overlook of Skaha Lake just before entering the city of Penticton, there is a view of two of those remnants.  In the photo below is a grass-covered, light-colored, nearly vertical bluff rising from the lake.  This bluff is composed of silt and clay deposited in Glacial Lake Okanagan, which once filled the entire valley with significantly more water than exists today in all five lakes. 

Above and to the left of the bluff are several dark gray, rounded rock outcrops.  And, if you look to the right above those three outcrops, you can see a number of other smaller, similarly shaped outcrops.  These rounded features are called “roche mountonnees,” a French term meaning “sheep rocks.”  On all of these outcrops, there is a smooth, gently sloping side on the left and a steeper, more angular side to the right.  These features can be used to determine the direction of movement of a glacier, with the smooth side ground down more by the glacier moving across it from left to right.

 

E290 - E320: Highway 97 barely fits into the bench along the east side of Okanagan Lake.  The slopes above the road are steep because the road is built almost on top of the Okagangan Valley Fault, a system of normal faults that run the entire length of the Okanagan Valley.  Near Vaseux Lake in the southern end of the valley, the faults dip (slopes) only about 10 degrees to the west.  North of Kelowna, however, the faults become much steeper before veering off to the northeast north of Vernon.  The photo shows the northern end of the fault along Kalamalka Lake just before it leaves Highway 97.

E323 - E360: Highway 97 follows a valley that formed along another fault system called the Bolean Creek Fault.  This fault divides two very different geologic provinces—with metamorphic rocks to the northeast of the highway and sedimentary and volcanic rocks to the southwest.  Interestingly, both rock types weather into rounded hills and long ridges and both are nearly completely covered by conifer forest, so little geology is visible to a 55-mph traveler through this area.  The valley, however, is also a geologic feature because it follows the weak zones created by the fault.  Movement of faults form "shear zones" of broken-up rock rubble that are more easily eroded by streams than the surrounding rocks.  So, the streams and rivers follow those shear zones, sometimes in a very straight line, and geologists use stream patterns to locate those faults.  They also use stream patterns to identify certain rock types and geologic structures.

E370 - E386: Highway 97 joins Trans-Canada Highway 1 at E369.5 and follows the South Thompson River Valley into Kamloops.  The most noticeable geologic feature of this portion of the highway is a light-colored bench running the entire distance (photo below).  This is “lucustrine silt,” the rock flour created by the glaciers in the mountains to the north and east washed into Glacial Lake Thompson.  The ice dam that created this lake was located in the area of today’s Kamloops Lake on the Thompson River east of Kamloops.  The silt settled to the bottom of this huge lake after each year’s spring runoff, forming layers called “varves.”  The thickness of this silt is estimated at nearly 1,000 feet and, some geologists believe, may have been deposited in no more than 100 years!

 

Mile-High Ice Dams, Glacial Superlakes, Prehistoric Megafloods, and "Grand Coulees"

What happens to an ice cube when you drop it into a glass of water?  That’s easy!  It floats to the top, right?  What happens when you pond several rivers behind an ice dam half a mile high?  If the water gets deep enough behind the dam, the ice will float, right? 

Now you have a picture of what happened in eastern Washington many times during the Pleistocene ice ages.  Geologists have estimated at least 41 separate gigantic floods, the world’s largest ever, rushed across the Columbia Plateau Basalts of eastern Washington when the ice dam broke.  The continental glacier in British Columbia was called the Cordilleran Ice Sheet, which continued to push lobes of ice across Washington, Idaho, and Montana throughout this period.  These lobes pushed the Columbia River south to its present channel.  When the river channels were blocked, the lake backed up behind each ice lobe was called Lake Missoula.  This lake filled the entire valleys of Coeur d’Alene Lake, Lake Pend Oreille, and Flathead Lake.  The floods that occurred each time the ice lobe floated were called the Spokane Floods.

When each dam broke, these immense floods were apparently as much as 600 feet deep at places like Spokane and Lewiston, Idaho, for days at a time.  The amount of erosion that can be caused by a 600-foot high wall of water is simply unfathomable in today’s world.  Where the floods were confined, they dug deep channels, called “coulees,” into the basalt that are now dry.  My June 25 photo below shows Grand Coulee, which now carries the Columbia River, but which was originally carved into the Columbia Plateau Basalts and underlying granite by the Spokane Floods. 

 

Where the flood waters spread out, they scoured all of the soil, loess (windblown silt), loose rock, and vegetation off vast areas of the basalt lava flow, leaving hils and valleys of exposed basalt.  These stripped areas can be seen along US 2 just east of the town of Creston and along I-90 between Sprague and Four Lakes.  The following satellite image shows the huge scoured areas that are now called the “channeled scablands.”

 

However, these floods were not merely destructive, they were also constructive.  When the flood waters receeded, they left behind enormous volumes of gravel, sand, and silt.  The next photo shows some nearly level sand and gravel terraces as much as 200-feet high formed by receeding Spokane floodwater in Grand Coulee.  Each terrace represents one recession event, with successive floods partially eroding the last terrace and redpositing that material downstream.

 

Another type of depostion in northern Idaho and eastern Washington during the ice ages was not a product of the Spokane Floods, but of winds.  The amount of basalt rock ground up into rock flour by the glaciers during the ice ages was immense.  As the glaciers receeded, this silt-sizes material was picked up by the wind and deposited in “dust dunes.”  After the ice ages, these gently rolling hills were vegetated by grasses, but few trees that we now call the “Palouse.”  Today, these hills form some of the richest wheat-farming regions of the world.  The photo below shows an example of Palouse terrain along US 2 near Creston, WA.

Next time: We finally meet our Milepost  “Central Approach Route” to the Alaska Highway with a discussion of the geology of the “exotic terrane” along US/BC 97 in the Okanogan-Okanagan valley.

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.