This is the final installment of my series following my father and my cross-country road trip from Tennessee to Oregon so I could start my master’s program at Oregon State University.
Road Trip Part 1: Why are the high plains so flat?!
Road Trip Part 2: Wyoming’s Great Divide Basin
Road Trip Part 3: The Wasatch Range
Day 6: Salt Lake City, through Idaho, to Pendleton, OR. Sorry Idaho, I’m skipping your geology, maybe another blog post…
Day 7: Pendleton, OR to Corvallis, OR!
The last big geologic conundrum of my trip was the giant layer cake of volcanic deposits that came into view along Highway 84 just past Boardman. The Columbia River sliced through it like a knife, revealing stair-stepping steep cliffs.
Highway 84 clings to the side of these cliffs for dear life, and every now and then a spur road would snake up the cliff to a town perched high above.
Welcome to the Columbia River Gorge! The river has cut 4,000 feet down into almost basalt deposits up to 2 miles deep over the past 15 millions years, and the results are amazing.
Tennessee’s only volcanic rocks are thin ash deposits, so this landscape was utterly foreign. My research on the topic was delayed by the first three blog posts and a 20-page paper on the philosophy of geography, but during the week of final exams I found Central Washington University professor Nick Zenter’s engaging video series on YouTube. He gives a wonderful introduction to the geologic world of the Pacific Northwest in a format that’s friendly to both non-geologists and geologists whose brains are too fried by studying to read off-topic academic journals. Manatash Mapping out of Ellensburg, WA made some of the best maps I found of the basalt flows to accompany his lectures: the one below shows the total extent of the Columbia River basalts! The Columbia Gorge is not indicated on these maps, but it defines the OR/WA border from just south of Pasco, WA to the Pacific Ocean.
Brown shading = the sum of the area covered by over 300 basalt flows. 63,320 square miles in all!
This giant pile of 41,985 cubic miles of basalt was belched out by a swarm of “dikes”, or vertical ruptures in the Earth’s crust where lava escaped, between 17 million and 6 millions years ago. 80% of this lava came to the surface between 16.5 and 15.5 million years ago as part of the Grande Ronde Member, which we saw as we drove through the Columbia River Gorge. The Grande Ronde basalts flowed out of the dikes in the area where Washington, Oregon, and Idaho’s borders meet on the map below.
Orange = area covered by Columbia River Basalt Groups, Black lines = approximate locations of dikes
The next map shows the approximate depths of these lava flows, focusing on the Washington-Oregon border. While depths in the Gorge are between 0.5 and 2 miles, the flows are 3 miles thick in south-central Washington!
These flows continued east, following the path of the Columbia all the way to the Pacific. However west of the Cascades, as we approached Portland, the wetter climate hides the sheer cliffs with a carpet of trees.
Looking upriver from an overlook near Hood River, Oregon.
But what caused the Earth’s surface to split open and spew out vast sheets of lava? 16 million years ago in the middle of the Miocene period of geologic time, northern Oregon and southeastern Washington would have looked a lot like the fiery slopes of Mt. Kilauea in Hawaii, or Mt. Bardarbunga in Iceland.
Pendleton, OR in the Mid-Miocene?
Geologists don’t have a definitive answer yet, although many interacting geologic events have been proposed to have contributed to the eruptions.
- The eruptions may be related to the historical path of the Yellowstone Mantle Plume, or “hot spot”. The oldest dikes in southern Oregon opened up just as the Yellowstone hot spot was erupting in what is now northern Nevada, directly south of the dikes.
- As the North American plate moved to the southwest over the hot spot towards its current position, cracks in the crust radiated northward, likely along lines of weakness between accreted terranes (bands of islands and sea floor scraped onto the continent by subducting plates) and the core of the continental shield.
- As the Farallon oceanic plate collided with and sank beneath the North American plate, crumpling the Coast Ranges and creating the stratovolcanoes of the Cascade range, these stresses could have helped open up these dikes. The majority of the dikes are perpendicular to that west-to-east direction of stress, which would be typical, and the eruptions happened directly after the collision.
- It’s possible that after the Farallon Plate slid under North America, parts of it tore open along long north-to-south trending lines. A tear in this subducted plate could allow hot rock to rise up from the upper mantle and punch through weaknesses in the crust.
Luckily for us these dikes have been quiet for the past 6 million years, and don’t show signs of starting back up. Nowadays, only water flows through the Columbia River Gorge. I’m looking forward to going back and exploring the many waterfalls that feed into it this spring as during our road trip in late August 2017 the area was ablaze for a different reason – forest fires!
References:
Blue Marble Earth 