High Desert & Great Basin. Part 1

18 Aug

 

It’s been awhile since I have posted here. Not for lack of want, but perhaps too much to write about. Until technology develops a flash drive that can be inserted into my brain and record all my thoughts, I will always be behind. Meantime, I must be selective about writing my thoughts as they jumble and tumble through my head, some of them lost after a moment or two of expression via a smile, snort or shake of my head. Then they dissipate, pushed out by the next train.

Since this blog is primarily about deserts and arid lands, there are boundaries which help determine what I write and publish here. Although I am known to blur the boundaries, sometimes push them, this is not too different from deserts, too. Although humans like delineated and tidy boundaries that help them categorize everything, that is not representative of real life, even non-life. Many categorized zones overlap into what is known as transitional zones. But scientists even attempt to draw borders around those zones. They/we take out our pencils and highlighters and mark boundaries of this zone and that. Not as strict and myopic as politicians and governing bodies, but few ecological boundaries are wanted hard, fast and final. If you step over an international border, you could be shot or jailed. If a plant or bird migrates over a border, well, they are completely unaware of  human borders. Perhaps we could learn a thing or two from them in that respect.

That leads us to a question: what is a desert? It depends on who or what you ask that question of. Nor will I address that question in this post (later post). Several technical delineations define a true ‘desert’ depending on the authority and context. The two common denominators are very low rainfall and evaporation that exceed precipitation (the latter depending on old versus more current definitions). Several journal papers exist that will contest exact definitions of a ‘desert’, which would require a lengthy explanation.  Nevertheless, I will introduce you to a desert that exists in one of those blurred boundaries, the High Desert of Eastern Oregon. Where I am now.

The Great Basin

North America has four major deserts: the Great Basin, the Mojave, the Sonoran, and the Chihuahuan, extending into Mexico. The largest of them, and the northern-most, is the Great Basin. It also has the highest elevation and the coldest winters. The exact area it covers is relatively inconsequential because its boundaries change over time. Regardless, the region covers significant parts of Oregon, Idaho, Wyoming, Colorado, Utah, Nevada and Arizona. The region is typified by its arid climate and basin and range geology. The latter is an integral factor in its climate and geography, and, hence, the rich variety of ecosystems and diversity of life. Thus, an abridged introduction to the Basin and Range physiogeography is necessary to appreciate the complexities and beauty.

Basin and Range

If we go back 200 million years to the Triassic Period and look down upon the continents, Oregon did not exist. It was a long time expanse between the end of the Triassic Period and the current topography of what is called the Basin and Range Province.  Plate tectonics (one of my most favorite earth science topics!), volcanism and glaciers created the Basin and Range as well as the Pacific Northwest. (See the video at bottom of post, courtesy of the Basin National Park)

Like a cracked egg shell, the earth’s crust is made up of large pieces, or plates, floating on its inner soft mantle. These are either continental or ocean plates that have been moving after the planet was formed. This gradual movement is called continental drift. When large sections of these plates collide and grind against each other, they often make themselves known, often with ‘earth-shattering’  effects.

Around 200-140 million years ago (mya) the North American continental plate drifted away from Europe and north Africa, moving westward. By a process called subduction, the oceanic plate moved under the continental plate and sunk into the mantle. Regions where this process occurs are called subduction zones. Often associated with this process is mountain-building, which occurs when large pieces of material on the subducting plate (such as island arcs, or literally, arches of island, like Japan) are pressed into the over-riding plate.

Mountain-building also takes place when subhorizontal contraction occurs in the over-riding plate. Both of these process built the many mountain ranges in the Pacific Northwest.  Firstly,  recurring island arcs were incorporated into the coast line (at one time, the coast line was the current Idaho west state line). Later, block faults rose while tension from plate tectonic movement stretched the surface to the breaking point.

The Basin and Range province was created about 20 mya as the North American plate stretched and thinned. Rock doesn’t stretch,  so it broke into about over 400 mountain blocks that partly rotated from their originally horizontal positions. The spaces in between these giant fault blocks are low valleys, lakes and basins. These mountains of late Precambrian and Paleozoic rock continue to erode and fill the intervening valleys with fresh sediment.

Steens Mountain. A large fault block in SE Oregon.

 

Oregon’s Basin and Range

Over millions of years, the topography resulting in the entire Basin and Range province was shaped and changed by periodic episodes of tectonic plate clashes, volcanism and climate change, including glaciers. Many areas of the province were covered with floods of lava during periods of continental drift and subduction.  The province’s most northern area occupies what is now southeast Oregon. And it was here during a fiery era when lava eruptions and blanketing ash covered most of Oregon east of the present Cascade mountains and resulting in basalt lava plains three miles thick in some places.

During the Pliocene period (5-1.8 mya), the climate in the Northwest became wetter. Run-off and precipitation contributed to sculpting many of Oregon’s river valleys and canyons. Continued volcanic activity in the Cascade Mountains began creating the high peaks and a significant future climatic influence to the east: a rainshadow.

Late in the same period, regional tectonic spreading caused huge areas of the basalt lava plains on the Cascades east side to rise and tilt along the north-south fracture zones. Many of these tilting blocks can be seen today as one sided rims with basalt columns, or giant fault blocks mountains, such as Steens Mountain, Hart Mountain and the Abert Rim. Where two faults run in parallel, the land dropped and formed a basin valley called a graben. Locally, the Alvord Basin sits below the towering east face of the Steens Mountain, whose west side gradually slopes to marshland and the Donner und Blitzen River.

And it is here where begins perhaps the most wonderful juxtaposition of the northern Basin and Range: aridity combined with watersheds that contribute to interesting and delightfully varied ecosystems of plant and animal life. How the two seemingly contradictory biomes interplay is what attracts me to this area, and will be the topic of the next post. And how their boundaries blur annually and seasonally into a kaleidoscope of life and death.

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2 Responses to “High Desert & Great Basin. Part 1”

  1. John Grimm August 19, 2014 at 6:59 am #

    Very interesting. I find myself wondering how the Chisos were formed. The little reading I have done is controversial, they are volcanic in origin, or they are not. Pissing Indian has been referred to as a old volcanic vent, then others say it is not.

    • Macrobe August 19, 2014 at 8:37 am #

      The current theory is a long interplay of tectonics and volcanism. It is intriguing that the volcanic activity of Big Bend was influenced by ‘local’ tectonics (the North and South American plates) and the combination of tectonics far to the northwest during the building of the Rocky and Cascade Mountains. All this activity contributed to the local volcanism in Big Bend. I recall reading somewhere about the local geological factors that resulted in building the isolated sky island of the Chisos, but don’t recall the details.

      The link below is an excellent resource if you can wade through the technical jargon 🙂 It’s available for sale at Panther Junction and an abridged version can be downloaded. Barton Warnock Visitor’s Center has an excellent interpretive exhibit on the geological history of southern Big Bend.

      http://pubs.usgs.gov/sim/3140/

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