Archive for the ‘evaporites’ Tag

Mountain Monday: Telescope Peak & Death Valley   12 comments

Telescope Peak and the Panamint Range from southern Death Valley's Saratoga Springs.

Telescope Peak and the Panamint Range from southern Death Valley’s Saratoga Springs.

Occasionally I like to highlight a mountain I like for Mountain Monday.  Today it’s Telescope Peak, in Death Valley National Park, California.  This has long been one of my favorite national parks.  I started visiting when it was still a national monument.  My first visit was a college seminar and field trip.  My second time was freelancing with friends, and we climbed Telescope Peak.

The top is just over 11,000 feet high, and since it was early spring, we waded through hip-deep snow drifts to get there.  After the all-day climb, we drove back down into the valley, took our sleeping bags, and tumbled out into the sand dunes to sleep under the stars.  What a contrast!  An icy morning at 8000 feet, a snowy climb, then sleeping out in balmy weather at sea level.

Snow-capped Telescope Peak has been lifted by the range-front fault over 11,000 feet above the floor of Death Valley.

Snowy Telescope Peak has been lifted by faulting along the range-front over 11,000 feet above the hot desert floor of Death Valley.

GEOLOGIC INTERLUDE

Telescope is the highest point in the park and crowns the Panamint Range.  The Panamints are an upraised block of the earth’s crust, lifted along the west side of a fault zone that at the same time dropped Death Valley down.  And down a lot!  The floor of the valley is a few hundred feet below sea level.

But the valley is filled with thousands of feet of sediments that were eroded from the Panamints and other ranges as they rose.  The top of the the bedrock that was dropped down by the fault lies some 11,000 feet beneath the valley floor.  This enormous wedge of valley fill is made of gravels, sands and clays.  But overall it’s quite salty.  There are thick sections of salts of various kinds, including good old NaCl, table salt.

These salt flats at Badwater in Death Valley are just the top of thousands of feet of salt and sediments filling the valley.

Geologists call these types of deposits evaporites because they are formed when large bodies of water evaporate away in a drying climate.  In Death Valley’s case it was a large lake called Lake Manly.  From about 2 million to 10,000 years ago, mega ice sheets lay to the north.  Because of this, the climate was quite wet in the now ultra-dry Death Valley region.  Early hunter-gatherers, recently migrated in from Siberia, were able to spread south because of this climate, which supported a diversity of life much greater than today’s desert does.

But when the ice sheets retreated during inter-glacial periods, the climate grew more arid, and Lake Manly shrank.  Because of how fault-block mountains border almost all sides of Death Valley, often there was little or no chance for the lake to drain in the normal way, via rivers.

The old Death Valley Borax Works, with a heavy-duty wagon.  This wheel is six feet high.

The six-foot high wheel of a heavy duty borax wagon.

Evaporation was (and is) the main way that water left the valley.  Salts that were dissolved in the water grew more concentrated as the lake grew smaller.  A brine was the result, and as the lake grew and shrank many times, often down to nothing, the salts were precipitated out.  They built up layers and layers of evaporite deposits.  The famous 20 mule-team wagon trains transported tons of borax from the borates (a type of salt) mined from the valley (image above).

A close-up of Death Valley’s evaporites (salt deposits).

BADWATER SALT FLATS

The current desert climate of Death Valley is one in which standing water from paltry winter rains evaporates rapidly, leaving behind fresh salt.  The salt can take very interesting forms (image above).  The mix of fine muds and salt, combined with repeated wet/dry cycles, can form fantastic polygonal patterns, as the bottom image shows.  Salt is also eroded away occasionally by the Amargosa River when infrequent storms allow it to flow south out of the valley.

The water in the image at the top of the post is really not part of this equation.  It’s fresh not salty, and comes from the amazingly strong Saratoga Springs in southern Death Valley.  I camped nearby one time and captured this view early the next morning.  Saratoga is well off the beaten track and most visitors to the park miss it.  There’s a very cool dune field nearby.

The salt flats in Death Valley form interesting polygonal patterns.

The salt flats in Death Valley form interesting polygonal patterns.  Telescope Peak is just left off the photo.

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Death Valley IV: Geologic Features   Leave a comment

This is the first of three posts on the geology and ecology of Death Valley National Park in California.  Death Valley is Disney Land for geologists, and for anybody interested in earth science.  What isn’t as well appreciated is it’s also a very special place for desert ecologists and botanists.  But first the geology:

A colorful dawn breaks over Death Valley National Park in California.

A colorful dawn breaks over Death Valley National Park in California.

Since it is the driest place in North America, vegetation does not cover geologic features at Death Valley.  And since it lies in a place where there’s been a lot of geological action for an awfully long time, there exist a great variety of rock types and structures.  Regarding the latter, the whole region has been first smashed by mountain building and more recently torn apart by rifting.  Death Valley’s structure (meaning twisted and folded rocks, fault zones, etc.) shows this in dramatic fashion and is one of the major draws for geo-types.

I first visited Death Valley with my first year geology class.  We came down on Spring Break from drippy Oregon and boy was it nice to be in warm sunshine for a week.  We all got 3 credits for it, but it was a lark!  Since my professor was a biologist and avid birder as well as a geologist, he mixed ecology and raptor-spotting in with rocks for a really complete picture of this amazing place.

The soaring dunes at Mesquite Flat in Death Valley National Park, California.

The soaring dunes at Mesquite Flat in Death Valley National Park, California.

GEOGRAPHY AND CLIMATE

Death Valley is an enormous trench.  The vertical relief from Badwater at -283 feet elevation to the top of Telescope Peak is about 11,300 feet (almost 3500 meters)!  This giant steep-walled valley is called by geologists a graben (German for grave).  Steep fault zones, called “normal” faults, force the bordering mountain ranges up while the valley drops and fills with sediments.  This sort of faulting is repeated across the Basin and Range Province of Nevada and bordering states.

The steep mountains left by the normal faults to stand high above valley floors block moisture coming in from the Pacific and cause an extreme form of the “rainshadow effect”.  The Sierra Mountain Range, which tops out at over 14,000 feet at Mount Whitney, gets most of the rain and snow.  The Panamint Range, which borders Death Valley to the west, also gets its share.  This leaves almost no moisture for Death Valley.  That is why years can pass without any rainfall.  It is extremely arid, and this of course causes the plant and animal life to be sparse.  But the fascinating adaptations that have evolved in the life forms at Death Valley more than makes up for the paucity of biomass.

Basin and Range structure has led to two types of features.  These features, both of which are displayed at Death Valley, determine much of what goes on geographically, ecologically and even with human history here.

The extensive salt flats near Badwater in Death Valley National Park, California.

The extensive salt flats near Badwater in Death Valley National Park, California.

PLAYAS

 First thing you’ll notice are the playas (or pans), which are dried up lake beds.  These flat surfaces, which can be floored in white salts or a tan clay surface, are caused by internal drainage.  Because of the normal faulting described above, water that washes from the ranges into the basins of the Basin and Range often never makes it out along a river course. Instead, the water collects in large, shallow lakes.

When the water evaporates, salts (chlorides and sulfates of sodium, calcium, phosphorous, etc.) are left behind in the lakes.  These so-called evaporites are too heavy to be lifted into the air with the water vapor.  (This is why rainwater is fresh and why the oceans are salty.)  The salts come from weathering of the minerals in rocks of the surrounding mountains.

The full moon sets just as morning light hits the cracked salt flats near Badwater, North America's lowest point, in Death Valley, California.

The full moon sets just as morning light hits the cracked salt flats near Badwater, North America’s lowest point, in Death Valley, California.

The evaporite minerals are inevitably concentrated into the shrinking pools of water, where they crystallize into fascinating patterns.  This happens during most seasons (winters are wet and summers very dry), and so salt layers build up.  Gypsum and borax are also formed in this way.  Death Valley’s human history includes the charismatic 20 Mule Team borax story.  Near Badwater in Death Valley proper, a huge salt pan is spectacularly developed.  Take the West Side road for the best access.

 Go over to Panamint Valley in the western part of the park to see and walk on a great playa.  It was formed when fine sediment was deposited instead of pure salt.  Certainly Death Valley’s best-known example of this is Racetrack Playa, where stones appear to have skated across the playa, leaving behind their tracks.  It’s still uncertain how they move, but winds and a thin layer of ice probably have something to do with it.  Note that to visit the Racetrack in the far northern part of the park requires driving a long, long washboard gravel road.  And to make things worse, the road bed is made of especially sharp gravel, so you’ll need very good tires (and two spares).

A close view of the ridges that form the salt polygons at the Badwater salt flats, Death Valley N.P., CA.

A close view of the ridges that form the salt polygons at the Badwater salt flats, Death Valley N.P., CA.

ALLUVIAL FANS

But mostly what you’ll see in Death Valley are the other feature that result from Basin and Range faulting.  As you drive through the park, one thing you’ll notice is that this is a rocky desert, not so much a sandy one.  As you look across the valley, you’ll notice large semi-circular (fan-shaped) gravel features that narrow to a point at the canyon mouths.  These are alluvial fans, and they form everywhere that rapid uplift of mountains overwhelms the ability of rivers to transport the debris out of there.

Try walking up an alluvial fan and you will get a feel for their deceptive steepness and difficult, loose surface of cobbles.  But it’s a great education on how they form.  You’ll also see desert varnish, a dark, sort of rust that forms on the rocks when they sit undisturbed for a long time.  I rarely link to Wikipedia, but heck, go ahead and check out desert varnish.  It’s an  interesting, part living feature of the Mojave.

A black and white rendition of the simple beauty of Death Valley's sand dunes.

A black and white rendition of the simple beauty of Death Valley’s sand dunes.

When alluvial fans merge into a wedge of debris that flanks the entire range of mountains, it is called a bajada.  Eventually the mountains disappear and all that’s left is a gravel plain.  Namibia has extensive ancient gravel plains, but the American West is really much younger.  Large outcrops that stick up island-like out of alluvial fans or bajadas are called inselbergs.  Great words in geology!

I’ll get to the “rest of the story” in my next post.  I miss Paul Harvey!

The pre-dawn hours in Death Valley's sand dunes promises a beautiful sunrise.

The pre-dawn hours in Death Valley’s sand dunes promises a beautiful sunrise.

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