Geology and Geologic Time through Photographs

Archive for the tag “landscape”

Sampling New Zealand’s (Amazing) Geology

New Zealand’s landscape can make just about anybody appreciate geology. Its glaciated peaks, its coastline –that ranges from ragged cliffs to sandy beaches to glacial fjords– its active volcanoes… they all work together to shout “Earth Science!” With that in mind, here’s some basics of New Zealand’s amazing geology, followed by some geological highlights of my trip of January and early February, 2018.

NZ map--all

Map of New Zealand, showing accreted terranes in colors and cover assemblage in gray.

North and South Island Bedrock  The different colors on this map show New Zealand’s basement rock, named so because it forms the lowest known bedrock foundation of any given area. The basement tells stories of New Zealand’s deep past, from about 500-100 million years ago. Individual colors signify different terranes, accreted (added) one-by-one through plate motions to the edge of what was then the supercontinent Gondwana. They mostly consist of sedimentary and metamorphosed sedimentary rock, although the narrow belt of purple-colored Dun Mountain Ophiolite formed as oceanic lithosphere, and the red-colored areas consist of granitic igneous rock, some of which has been metamorphosed to gneiss.

Gray indicates the younger cover rock, formed after accretion of the terranes. Consisting of a wide range of sedimentary and volcanic rocks, as well as recently deposited sediment, it’s just as interesting and variable as the terranes. Because it includes volcanoes, it’s largely the cover that gives the North Island its distinctive flair. By contrast, the South Island consists largely of uplifted basement rock, much of which has been –and still is—glaciated. All those long deep lakes, such as Lakes Wanaka and Tekapo, were carved by glaciers and are now floored with their deposits of till.

Andesite stratovolcano, New Zealand

Mt. Ngauruhoe, a 7000 year-old andesite stratocone near Ruapehu on the North Island

Those differences exist largely because the North and South Islands occupy different plate tectonic settings. The North Island sits over a subduction zone, so it hosts an active volcanic arc, expressed as the Taupo Volcanic Zone. The North Island also hosts older volcanic arcs –and even an active volcanic field that roughly coincides with the city of Auckland! By contrast, the South Island is transected by the Alpine fault, which connects the subduction zone in the north with the one in the south as a transform fault. Much like the San Andreas fault in California, it and some related faults are responsible for a great deal of uplift—and formation of mountain ranges such as the Southern Alps and Kaikoura ranges –as well as uplift and subsidence along different parts of New Zealand’s coastline. You can see how far the Alpine fault has slipped by matching up the belts of accreted terranes on either side, particularly the Dun Mountain Ophiolite –some 460 km!

My trip: January 5-February 7

A quick note… you should be able to click on any of these images to see them as larger versions in another window!
… The greatest thing about my trip was that I was able to see (and photograph!) most of these elements of New Zealand’s geology. Please take a look at the many other images of New Zealand geology that I posted on my website! I started in Auckland, where I met up with a group of faculty and students from Centre College, a liberal arts school based in Danville, Kentucky. They were studying North Island volcanoes and graciously let me tag along. We spent most of our time near Auckland and in the Taupo Volcanic Zone. From there I went to Wellington and then on to the South Island, where I rented a car and traveled for another two weeks, mostly with my friend Megan.

Auckland area

Here’s a geologic map of the Auckland area, published by the New Zealand GNS (Geological and Nuclear Sciences). The reds and pinks are basalts of the Auckland Volcanic field; the browns are Miocene sedimentary rocks that contain an abundance of volcanic particles, and the blue marks the Waipapa Composite Terrane, which forms the local basement.

The Auckland Volcanic Field consists of more than 50 volcanoes, many of which are small cinder cones that show evidence of explosions related to water-magma interactions. The first eruption occurred 250,000 years ago and the most recent, at Rangitoto, only 600 years ago. Rangitoto is the large red circular island in the northeast corner of the map. It’s a small shield volcano, capped by a cinder cone, which makes for a lovely boat ride and hike! Rangitoto is the only volcano in the field that shows evidence for repeated eruptions. Because of the recency of the volcanism –and the fact that it encompasses Auckland, the GNS is actively monitoring the field and making preparations in the event of an eruption.


Our group hiking up Rangitoto

Taupo Volcanic Zone

The Taupo Volcanic Zone (TVZ), which first started erupting about 2 million years ago, occupies a rift zone, marked by numerous, recently active normal faults. The rift is superimposed on the modern volcanic arc produced by subduction of the Pacific beneath the Australian plate. The TVZ is dominated by andesitic volcanism towards the northeast and southwest ends of the zone and rhyolitic volcanism in between, all of which is active or recently active. We visited localities throughout the zone.

Active volcanic crater, New Zealand (Pan)

Active crater in White Island. Crater Walls are mostly pyroclastic deposits.

White Island, in the photo above, is an active stratovolcano that rises right out of the Bay of Plenty. The volcano marks the northern end of the zone. Its breached crater hosts a gigantic rock avalanche deposit, crater lake, sites of active sulfur mineralization, crater walls of pyroclastic deposits, and steam, steam, steam!

Andesitic dike, New Zealand

Meads Wall, an andesite dike intruding lavas on Mt. Ruapehu, New Zealand (180114-18)

Ruapehu, another active stratovolcano, marks the southern end of the zone. We spent the last 8 days of the course in and around that area, including hiking the well-known Tongariro Crossing and climbing to Ruapehu’s crater lake at its summit. I, of course, sprained my ankle just before the Tongariro hike and missed the summit hike because I was on a bus to Wellington. Those are altogether different stories! One place I especially like is Meads Wall, shown in the adjacent photo. It’s a large andesite dike that cuts through older andesite lava flows.

The rhyolitic volcanism that dominates the central part of the TVZ gives a more subtle expression to the landscape than the prominent andesitic stratovolcanoes –which is largely because the rhyolitic eruptions were so gigantic as to form calderas and blanket the entire area with ash flow tuff and air fall. The ash flow tuff shown below was deposited by the Taupo eruption only 1800 years ago. It erupted more than 100 km3 of material. An earlier eruption from the same volcano, called the Oruanui eruption, put out an estimated 1000km3 of material and formed the caldera that holds Lake Taupo. Altogether, eruptions in the TVZ have erupted more than 10,000 km3 of magma since they started about 1.6 million years ago. Cole and Spinks (2009) list seven calderas within the zone.


Ash flow tuff. Bluffs near Turangi; close-up showing pumice fragments; Joe Workman, our leader, leaning against a wall of tuff near Taupo.

The thermal springs in and around the town of Rotorua provide a great expression of the current activity. One of my favorite spots was the Waiotapu thermal area, some 30 km southeast of Rotorua. Among other things, it hosts numerous explosion craters filled with hot springs. Below is a photo of the colorful “Champagne Pool”, which gets its name from the CO2 bubbles that rise to the surface. The colors result from a variety of things, including yellows and oranges from the sulfide minerals orpiment (arsenic) and stibnite (antimony), and white from sinter (silica) deposition.

Hydrothermal spring and deposits, New Zealand

Hydrothermal spring and deposits, Waiotapu, New Zealand. The white color is sinter (silica) and orange and yellows from antimony and arsenic precipitation.


Wellington area

In Wellington, my friends Rob and Sue took me to Red Rocks Scientific Preserve, some beautiful coastal exposures of the accreted Torlesse Composite Terrane. While most of the Torlesse around Wellington consists of Jurassic and Cretaceous submarine fan deposited sandstone, these rocks consist of chert, argillite, and basalt of probable Triassic age.


Distracting Fur Seal on the Red Rocks


And Wellington sits right next to the Wellington fault—one of the several fault zones that link the Alpine fault to the subduction zone. The Wellington fault is the closest of many faults that pose major earthquake hazards for New Zealand’s capitol city. According to the GNS, it last ruptured some 300-500 years ago –and has an estimated recurrence interval of 500-1000 years. The Wairarapa fault, only 20 km southeast of Wellington, broke in 1855 to unleash an earthquake estimated to be M8.2-8.3. Even the 2016 Kaikoura Earthquake, which occurred on the South Island, caused more than $500 million in damage to Wellington.

Ferry from Wellington to Picton (South Island)

I had a beautiful day for this beautiful ride, three and a half hours from Wellington across Cook Strait to Picton, a small town tucked away inside the countless inlets and bays of the South Island’s north coast. The intricate coastal landscape results from gradual subsidence of the land and invasion of the sea up pre-existing river valleys, many of which follow fault zones. Two of these faults are sketched in on the map.Ferry map

And here’s a map of the South Island!
S Island

Kaikoura area

Recently uplifted coastline, New Zealand

wave-cut bench, stranded beaches, and uplifted terrace at Kaikoura

Unlike the subsidence of the northern coast, the story around Kaikoura is one of coastal uplift. In fact, the 2016 Kaikoura Earthquake caused more than 5 meters of uplift in places. 5 meters! Near Kaikoura itself, that value was more like 1 meter, but still, you could see its effects. There’s an easy trail that crosses the Kaikoura Peninsula and returns via the beach. You can see the wave-cut bench, now largely above the intertidal zone, and stranded beach ridges. Then there’s the flat top of the peninsula. Formerly a wave-cut bench, it’s been uplifted by multiple earthquakes through thousands of years!

And the earthquake spawned countless landslides, many of which affected the coastal highway. My favorite slide was a few miles west of the highway up the Clarence Valley, shown below. I still haven’t determined if this is the same slide, but three cows got stranded on a slide block to become the subject of my now-favorite children’s book: “Moo and Moo and the Little Calf Too” by Millton and Hinde… it’s wonderful! Together, the landslides and ground ruptures during the earthquake closed the highway for over a year. It reopened in December, 2017—and they’re still working on it.GeologyPics-180124-93fMoo



Mouth of Waimakariri River, New Zealand

Waimakariri River and alluvial plain near Christchurch

Christchurch is really easy to find on a map because it sits on the north side of the Banks Peninsula, an almost circular protrusion into the Pacific. The peninsula marks a complex of shield volcanoes, active some 11-8 million years ago but now deeply eroded. Christchurch itself is pretty flat. It sits on the alluvial plain of the Waimakariri River, which rises in the southern Alps and enters the sea just north of the city. I picked my friend Megan up at their lovely airport and we went for a long beach walk –very few other people—just miles of sand and clouds and water and dunes…

Southern Alps (Mt. Cook –Aoraki)

Rocks are cool, but glaciers… wow! The glaciers, their erosional features, and their deposits are absolutely spectacular in this area! And besides, the bedrock around Mt. Cook/Aoraki is mostly this poorly bedded greywacke of the Torlesse Composite Terrane –so my camera saw mostly ice and landscape carved from the rock.


Mt. Cook Village, which has a couple hotels and a youth hostel, sits on a small alluvial fan on the edge of the glacial outwash plain. The mountains positively soar overhead. Megan and I hiked up to the Mueller Hut, situated on the long high ridge just south of the village. Everywhere, we saw the effects of ice. We could even hear the erosion, as rock and snow avalanches broke loose frequently on the glaciers across the valley. No wonder the area hosts incredible U-shaped, glacially eroded valleys and no wonder that the upper reaches of the valleys form bowl-shaped cirques, many of which are still occupied by glaciers. No wonder that steep headwalls rise behind the cirques. No wonder the peaks and ridges are so jagged. At the same time, it fills you with wonder!

For me, the most instructive view was up the Hooker Valley towards Mt. Cook. There, you see the lake below the Hooker Glacier, dammed in by one of the glacier’s moraines –and in the foreground, you see another lake in another valley dammed in the same way. Turning your gaze downstream, you can see the magnificent outwash plain, merging with the outwash plain of the Tasman Glacier –and then emptying into long Lake Pukaki—which fills a glacial valley that’s dammed by a moraine!

Glacial erosion and deposition, New Zealand

Moraines of till and moraine-dammed lakes.Mt. Cook -Aoraki- in background


Milford Sound

Glacial fjord. Milford Sound, New Zealand.

Gneiss and waterfall, New ZealandAs if Mt. Cook/Aoraki wasn’t enough, the experience of seeing the glacial fjord of Milford Sound kind of blew our minds. I was anxious beforehand because a storm was blowing in and I thought we’d get all wet and cold and be tossing about on the boat, but it turned out that the rain, sprouting thousands of waterfalls, made the place magical—and we were very comfortable on the boat. The rocks were pretty amazing too. Described by the GNS as orthogneisses, they originated as part of the Median Batholith during the Paleozoic but were metamorphosed and deformed during later mountain-building events, natural results of terrane accretion.

Glacial Fjord. Milford Sound, New ZealandFjords are narrow arms of the ocean that cut into the land. They have steep steep walls that rise 1000’s of feet on either side and drop steeply to great depths below water line. Fjords occupy former glacial valleys, deepened and steepened by the ice as it flowed down the valley towards the sea. When sea levels rose after the ice retreated, the sea invaded the former valleys. During rainy periods, the water, with little area to collect, runs off almost instantly as thousands of waterfalls. The next day, they’re mostly gone.

Bombing up the West Coast

We didn’t give enough time to the west coast –driving all the way from Wanaka to Hokitika in one day. It turns out that the storm we encountered at Milford Sound was much worse to the north, temporarily blocking the coast highway with fallen trees and landslides. And it washed out the access road to the Fox Glacier and the trail to Franz Josef Glacier, two glaciers that flow out of the Southern Alps to reach low, accessible elevations. I got to see the Fox Glacier 4 years ago –and this year, we did get a nice view of the braided river below the Franz Josef Glacier.


Fox Glacier and metamorphosed sedimentary rock–from my trip in 2014.

One of the things I remember about the Fox Glacier was the awesome bedrock! There, the greywacke of the Torlesse Terrane is metamorphosed to schist. This metamorphism seems to concentrate along the Alpine fault and die out away from it. The ridge we hiked to near Mt. Cook/Aoraki, for example, lies only 25 km to the south and consists of unmetamorphosed rock. These differences show how the Alpine fault is causing uplift of the range, the highest rates being right along the fault. We missed seeing the exposure of the fault at Gaunt Creek, just north of Franz Josef. Bummer.

Beach cobbles and heavy minerals

Beach near Hokitika

Hokitika was great. Turns out that it’s the prime place for New Zealand jade. The jade is nephrite, which is a microcrystalline variety of amphibole of the actinolite-tremolite series: the closer its chemistry to the iron-rich actinolite, the greener its color; the closer it’s to the magnesium-rich tremolite, the whiter its color. The nephrite forms in the southern Alps as a replacement mineral in lenses of serpentinite, enclosed by schist of the Torlesse Terrane. Most people find it as boulders and cobbles on the local beaches or rivers that drain this part of the southern Alps. We went looking of course –and came back with a lot of pretty rocks—but I’m skeptical we found any jade.

We also visited the Pancake Rocks near Punakaiki—made of 30 million year old limestone, which is part of New Zealand’s post-accretion cover sequence. The rocks have eroded along vertical fracture surfaces and show this really cool enhanced bedding, likely the result of dissolution along original bedding planes during compaction of the sediment. And it’s a beautiful headland, with sea stacks, a sea cave, blowhole, and a fabulous rock-fall deposit. And there are a zillion tourists—one guy practically pushed Megan over the edge to take his picture! Sigh…


Pancake Rocks near Punakaiki. Photo on right shows close up of enhanced bedding.

The highway across Arthur’s Pass back to Christchurch goes through some awesome scenery. In my mind, the best part was the eastward descent along the Waimakariri River—which at higher elevations makes beautiful braided channels in an outwash-choked floodplain. And the river terraces along the Broken River, each higher level marking an older floodplain were positively amazing. But we had a plane to catch.

Braided river and gravel bars, New Zealand

Braided channels and gravel bars on Waimakariri River.

To see more geological images, of here or New Zealand, please check out my website!

A few useful references:
Taupo Volcanic Zone: Cole, J.W. and Spinks, K.D., 2009, Caldera volcanism and rift structure in the Taupo Volcanic Zone, New Zealand, in Murphy, Kepple, and Hynes, eds., Ancient Orogens and Modern Analogues, Geological Society of London Special Publication 327.

Wellington fault:

General NZ geology: Graham, Ian J. editor, 2015, A Continent on the Move, New Zealand Geoscience Revealed 2ed., Geoscience Society of New Zealand, 397p.

And of course: Millton, J., and Hinde, D. (illustrator), 2017, Moo and Moo and the Little Calf Too. Allen and Unwin, Auckland.







Mauna Loa Volcano, Hawai’i –Earth’s largest active volcano

To get an idea of the immensity of Mauna Loa Volcano, take a look at the photo below. That rounded shape continues from its summit area at 13,678 feet above sea level to about 18,000 feet below sea level –and then another 25,000 feet or so below that because the mountain has sunk into the oceanic crust. It’s unquestionably the world’s largest active volcano.

Mauna Loa Shield Volcano

Profile of Mauna Loa Shield Volcano from… Mauna Loa Shield Volcano! (Geologypics: (170919s-15))

Briefly, Mauna Loa’s made of basalt. Basaltic lava flows, being comparatively low in silica, have low viscosities and so cannot maintain steep slopes, resulting in broad, relatively low gradient volcanoes called shields. With just a little imagination, you can see how Mauna Loa’s shape resembles that back side of some shield one of King Arthur’s Knights might carry into battle.

Read more…

Summarizing Washington State’s Geology –in 19 photo out-takes

Washington State displays such an incredible array of geologic processes and features that it makes me gasp –which is one reason why writing “Roadside Geology of Washington” was such a wonderful experience. I also got to do it with my long-time friend and colleague (and former thesis advisor at the University of Washington) Darrel Cowan. The book should be on bookshelves in mid-September –and I can’t think of a better way to celebrate than by summarizing Washington’s amazing geology with a bunch of out-take photos –ones that didn’t made it into the book or even to my editor. Like the photo below:

Mount Baker, Washington (150916-4)

Mt. Baker, a glaciated stratovolcano in northern Washington State.

Mount Baker’s a stratovolcano that erupted its way through the metamorphic rock of the North Cascades. I took the photo from the parking lot at a spot called Artist’s Point –at the end of WA 542 –and my editor nixed it because I already had enough snow-capped volcanoes in the book.

On the cross-section below–which includes elements of Oregon as well as Washington, Mt. Baker is represented by the pink volcano-shaped thing labelled “High Cascades”. The following 15 or so photos illustrate most of the other features on the cross-section –so together, they illustrate much of the geology and geologic history of the state!

Cross-section across PNW

Generalized cross-section across Washington and Oregon.

Washington State and geologic provinces

Washington State and geologic provinces.

A quick note about organization: I’m separating the images according to their  physiographic province. There are six in Washington: Coast Range, Puget Lowland, North Cascades, South Cascades, Okanogan Highlands, and Columbia Basin.


Coast Range:
As you can see in the cross-section, the Coast Range borders the Cascadia Subduction Zone and consists of three main elements: the Hoh Accretion Assemblage in yellow, Siletzia (called the “Crescent Formation” in Washington) in purple, and the post-accretion sedimentary rock in brown. Siletzia is the oldest. It was thrust over the Hoh Accretion Assemblage, which is still being accreted at the subduction zone. The post-Accretion sedimentary rocks were deposited over the top of Siletzia after it was accreted about 50 million years ago.

And here are some photos! Siletzia formed as an oceanic plateau and so is characterized Read more…

Washington’s waterfalls–behind each one is a rock!

Of all the many reasons why waterfalls are great, here’s another: they expose bedrock! And that bedrock tells a story extending back in time long long before the waterfall. This posting describes 9 waterfalls that together paint a partial picture of Washington’s geologic history. The photos and diagrams will all appear in my forthcoming book Roadside Geology of Washington (Mountain Press) that I wrote with Darrel Cowan of the University of Washington.


Rainbow falls along WA 6 in the Coast Range


And waterfalls in heavily forested areas are especially great because they may give the only view of bedrock for miles around! Take Rainbow Falls, for example–the small waterfall on the left. It’s in Washington’s Coast Range along State Highway 6–a place where a roadside geologist could otherwise fall into total despair for lack of good rock exposure. But this beautiful waterfall exposes a lava flow of the Grande Ronde Basalt, which belongs to the Columbia River Basalt Group. Significant? Yes!

This lava erupted in southeastern Washington and northeastern Oregon between about 16 and 15.6 million years ago and completely flooded the landscape of northern Oregon and southern Washington. We know how extensive these flows are because we can see them–and they cover the whole region. The photo below shows them at Palouse Falls in the eastern part of Washington. Take a look at my earlier blog post about the Columbia River Basalt Group? (includes 15 photos and a map).

Read more…

Oregon’s rocky headlands: geologic recycling through erosion and uplift and erosion…


Crashing waves at Heceta Head, Oregon

You can’t avoid thinking about erosion while standing on one of Oregon’s rocky headlands. The waves keep coming, one after another, each crashing repeatedly against the same rock. Impossibly, the rock appears unmoved and unchanged. How can it not erode?

The answer, of course, is that headlands do erode, quickly, but on a geologic time scale. We just miss out because we live on the much shorter human time scale. And the erosion belongs to a cycle in which coastal uplift causes eroded and flattened headlands to rise and become headlands once again, all subject to more ongoing erosion and uplift.

Wave energy is most intense at headlands because the incoming wave typically feels the ocean bottom near the headland first, which causes the wave to refract. As shown in the aerial photo below, this refraction focuses the wave energy on the headland.


Wave refraction causes wave energy to focus on the headland. Arrows are perpendicular to wave fronts.

As you can see in the next few images, headlands don’t erode evenly. They erode irregularly, as the waves exploit any kind of weakness in the rocks such as faults and fractures, or if they’re sedimentary, bedding surfaces. The products of this erosion are as beautiful as they are interesting: sea stacks, sea arches, sea caves… The list goes on and on.

Headland and lighthouse, Heceta Head, Oregon

Aerial view of Heceta Head, Oregon.

From the above photo, you can see that sea stacks are simply the leftover remains of a headland as it retreats from erosion. That’s a critical point, because some sea stacks, especially the one with the arch in the photo below, are a long way from today’s coastline.

Sea stacks and sea arch, southern Oregon

Sea stacks and sea arch, southern Oregon

Those rocks, 1/4 to a 1/2 mile away used to be a part of the coastline? The land used to be way out there? YES!!! For me, that’s one of the very coolest things about sea stacks –they so demonstrate the constant change taking place through erosion.

Taken to its extreme, erosion renders headlands into wave-cut platforms, such as the one below at Sunset Bay. Being in the intertidal zone, these platforms make great places for tide-pooling–and ironically, for people-watching too. Geologically, they form important markers because they’re both flat and form at sea level. When found at higher elevations, they indicate uplift.

Wave-cut bench, Sunset Bay, Oregon

Wave-cut bench at Sunset Bay, Oregon

In fact, looking carefully at the photo above, you can see a flat surface on the other side of the bay. It’s an uplifted wave-cut platform! Called a marine terrace, it’s covered by gravel and sand originally deposited in the intertidal zone. Those deposits rest on bedrock that, at an earlier time, was also flattened by the waves. The photo below shows a better view of this terrace from the other side.


Breaking wave at Shore Acres State Park, Oregon. Tree-covered flat surface in the background is an uplifted marine terrace.

These uplifted marine terraces can be found up and down Oregon’s coastline. Researchers recognize several different levels, the oldest being those uplifted to highest elevations. The one in the photo above at Shore Acres State Park is called the Whiskey Run Terrace and formed about 80,000 years ago. You can see a similar-aged terrace below as the flat surface beneath the lighthouse at Cape Blanco, Oregon’s westernmost point. An older, higher terrace forms the grass-covered flat area on the right side of the photo.

Cape Blanco, Oregon

Cape Blanco, Oregon looking NE. The flat surface beneath the lighthouse is the ~80,000 year-old Cape Blanco Terrace, probably equivalent to the Whiskey Run Terrace at Shore Acres; the flat area on the right side of the photo is the higher Pioneer Terrace,  formed ~105,000 years ago.

Researchers take the approximate ages of the terraces and their elevations to calculate approximate rates of uplift. In this area, Kelsey (1990) estimated a rate of between 4-12 inches of uplift every 1000 years. That might seem slow, but over hundreds of thousands of years, it can accomplish a great deal.

And look! The uplifted terraces? They’re on headlands! Of course, because they’ve been uplifted! And the headlands are now eroding into sea stacks and then platforms –to be uplifted in the future and preserved as marine terraces that sit on top headlands. And on and on, as long as the coastline continues rising.


Blowhole near Yachats, Oregon. Incoming wave funnels up a channel eroded along a fracture and explodes upwards on reaching the end.

Some links and references:
Kelsey, H.M., 1990, Late Quaternary deformation of marine terraces of the Cascadia Subduction Zone near Cape Blanco, Oregon: Tectonics, v. 9, p. 983-1014. (Detailed study of Cape Blanco, including uplift rates).

Miller, M., 2014, Roadside Geology of Oregon, Mountain Press, Missoula, 386p. (General reference which details the concepts and includes several of the photos used here).

Earth Science Photographs–free downloads for Instructors or anybody: my webpage!

Death Valley National Park– Geology Overload!

Death Valley… I can’t wait! Tomorrow this time, I’ll be walking on the salt pan with my structural geology students, gawking at the incredible mountain front –and soon after that, we’ll be immersed in fault zones, fractures, and fabrics!

Death Valley salt pan at sunrise.

Death Valley salt pan at sunrise.

Death Valley presents incredible opportunities for all sorts of geology, especially geologic time; you can look just about anywhere to see and feel it.  Take the salt pan.  It really is salt –you can sprinkle it on your sandwich if you want.  It’s there because the valley floor periodically floods with rainwater.  As the rainwater evaporates, dissolved salt in the water precipitates.  And some 10,000 years ago, Death Valley was filled by a 600′ deep lake, which evaporated, leaving behind more salt. Before that, more shallow flooding and more lakes.

Aerial view of faulted front of the Black Mountains.

Aerial view of faulted front of the Black Mountains.

But the basin is more than 4 miles deep in some places! It’s not all salt, because there are a lot of gravel and sand deposits, but a lot of it is salt.  That depth speaks to geologically fast accumulation rates, because it all had to accumulate since Death Valley formed –probably in the last 2 or 3 million years.  But still, 2 or 3 million years is way past our realm of experience.

Hiker in the Funeral Mountains of Death Valley.

Hiker in the Funeral Mountains of Death Valley.

To really go back in geologic time though, you need to look at the mountains. Most of the mountains contain Upper Precambrian through Paleozoic sedimentary rock, most of which accumulated in shallow marine environments.  There’s a thickness of more than 30,000 feet of sedimentary rock exposed in Death Valley! Deposited layer after layer, you can only imagine how long that took.

We can measure the thickness of the rock because it’s no longer in its original horizontal position.  The ones in the photo above were tilted by faulting –which occurred during the period of crustal extension that formed Death Valley today.  The rocks in the photo below were folded –by a period of crustal shortening that took place long before the modern extension.  The folding occurred during the Mesozoic Era –more than 65 million years ago.

Aerial view of Titus Canyon Anticline.

Aerial view of Titus Canyon Anticline.

Above the Upper Precambrian to Paleozoic rock are thousands of feet of volcanic and sedimentary rock, tilted and faulted, but not folded. They reveal many of the details of the crustal extension that eventually formed today’s landscape.  For example, the photo below shows Ryan Mesa in upper Furnace Creek Wash.  In this place, the main period of extensional faulting predates the formation of modern Death Valley.  Look at the photo to see that faulting must have stopped before eruption of the dark-colored basalt flows.  Notice that there has to be a fault underneath the talus cones that separates the Artist Dr. Formation on the left from the Furnace Creek Formation on the right.  Because the fault does not cut the basalt though, it has to be older.  Those basalts are 4 million years old, older than modern Death Valley.  –And that’s the old mining camp of Ryan perched on the talus.

Angular unconformity at Ryan Mesa: 4 Ma basalt flows overlying faulted Artist Drive (left) and Furnace Creek (right) formations.

Angular unconformity at Ryan Mesa: 4 Ma basalt flows overlying faulted Artist Drive (left) and Furnace Creek (right) formations.

And beneath it all? Still older rock!  There’s some 5,000 feet of even older Precambrian sedimentary rock, called the “Pahrump Group” beneath the 30,000 feet of Upper Precambrian and Paleozoic rock–and below that, Precambrian metamorphic rock.  It’s called the “basement complex” because it’s the lowest stuff.  Here’s a photo.

pegmatite dike and sill intruding mylonitic gneiss

pegmatite dike and sill intruding gneiss

The pegmatite (the light-colored intrusive rock) is actually quite young–I think our U-Pb age was 55 Ma –but the gneiss is much older, with a U-Pb age of 1.7 billion years.  Billion!  Forget about the U-Pb age though.  These rocks form miles beneath Earth’s surface –and here they are, at the surface for us to see. Without knowing their age, you’re looking at deep geologic time because of the long period of uplift and erosion required to bring them to the surface.  And it happened before all those other events that described earlier.

THIS is why, when visiting Death Valley, you need to explore the canyons and mountains –not to mention the incredible views, silence, stillness…

Some links:
Geologic map of Death Valley for free download
Slideshow of Death Valley geology photos

–or better yet, type “Death Valley” into the geology photo search function on my website!

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