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13 Feb 2024

Levels of Time

I’m hiking up a closed road in Death Valley National Park to see a pile of gravel. I guess that’s one way to look at it. What some folks might view as a waste of precious time in this magnificent place I see as a vehicle for time travel.

*Gravel ridge along the Beatty cut-off road in Death Valley was deposited as a spit near the shoreline of Glacial Lake Manly. The highway cuts right through the center of the spit.* *Two smaller spits are visible on the far right side of the image. (photo 240106-97)*

Just 4 months ago in August, Hurricane Hilary dropped some 2.2 inches of rain on Death Valley—more than what typically falls here over the course of a year. With virtually no soil to absorb it, the water ran off immediately. It gathered in rivulets, confluenced into small channels, then larger channels, and finally streams that flash-flooded down canyons and alluvial fans. The flooding closed every road in the national park. It’s now early January, 2024 and this road up one of the fans isn’t supposed to open to cars for another two weeks.

In just under two miles, I reach my destination, a low ridge extending eastward from the base of a hill. It was deposited by waves near the shoreline of a giant lake called Glacial Lake Manly, sometime between186,000-120,000 years ago. The ridge grew by fits and starts out from the hill as a spit, with waves obliquely slapping its front and moving the gravel out to its tip. You can see wave-rounded cobbles in the roadcut forming curved layers that slope towards the valley. They’re also scattered about on the top of the spit where I sit down and take in the view.

*The highway cuts right through the spit. Death Valley, once filled to a spot above and behind the spit, is in the background.* *(Photo 191101-75)*

In front of me, the highway descends its gentle gradient to where I parked the car, nearly 200’ below sea level. From there, the floor of Death Valley is practically flat, continuing well past Badwater Basin some 25 miles to the south. When this gravel spit formed, Lake Manly, more than 50 miles in length and some 6-8 miles wide, filled the entire scene. At its high stand, I’d be below water because the lake’s highest shoreline reached another mile up the road. The gravel spit formed as the lake receded. Two smaller ridges lie just below where I sit and another very small one lies just above, marking different stages in its retreat.

Just like anybody, I wrestle with the ever-changing and fluid concept of time. Stopped highway traffic that delays my arrival by 15 minutes can seem interminable and I bemoan how quickly a year passes. I’ve heard countless people comment at how Badwater Basin is still flooded by water from Hurricane Hilary but when it all finally evaporates, we’ll probably describe the shallow lake as short-lived. This remnant of a giant lake that existed over 100,000 years ago takes my confusion to a new level. Was that a long time ago?

It seems so, but then I think of the mountains that enclosed the lake. They started rising some 3-3.5 million years ago—more than 20 times the age of the lake. I’ve always considered the mountains to be young, even going so far as to tell park visitors that Death Valley’s present landscape was “only about 3 million years old”. Compared to their rocks, many of which are older than 500 million years, that’s true. Some of the rocks are well over a billion years old.

Those rocks tell stories –about how they formed and about what’s happened to them since. I pick a cobble up off the spit. It’s a beautiful maroon color and made of tiny grains of quartz all mushed together. I suspect it came from the Zabriskie Quartzite, a distinctive rock unit that forms prominent cliffs throughout the region. Its sand was deposited mostly in a shallow ocean and various coastal environments during the Cambrian Period, which lasted from 539-485 million years ago.

Overturned Anticline in Titus Canyon–the Zabriskie Quartzite forms the prominent red cliffs in the right-center of the photo. To the left (west) is overturned Cararra and Bonanza King Formations. At the canyon mouth, the rocks are nearly horizontal, yet upside down. (Photo SrD-10)

I’ve studied geology my entire adult life and I still find it incredible that I can hold, in my own hands, a piece of the Cambrian sea floor. Each of the millions of tiny sand grains that make up this rock originated from some still-older rock and were transported by streams to the Cambrian shoreline. There, they were probably kicked around by coastal waves until getting buried by layers of more sediment, followed by more sediment for who knows how long –until circulating groundwater cemented the compacted grains together as layers of rock. In the Death Valley region, there are more than 10,000 feet of sedimentary rock on top the Zabriskie Quartzite and at least another 10,000 feet of sedimentary rock below. Each bedding plane in that sequence of rock was once the Earth’s surface.

And so much has happened to them since! Besides today’s mountain-building, driven because the earth’s crust in the region is extending, they’ve all experienced an earlier period of mountain-building by crustal compression. At the mouth of Titus Canyon just 20 miles northwest of here, those events folded the rock to where the sequence is completely upside down. Elsewhere, the rocks were intruded by granitic magma, while others were carried to depths of 15 miles or more and partially melted. And now, as today’s mountains erode, they shed rocks of all ages and types and sizes into their canyons, which get washed out onto the alluvial fans during floods.

From my perch on the gravel spit, I’m just a few feet above the alluvial fan. It’s unmoving and silent. The road will reopen soon, and tourists will once again drive past this spot without a second thought. But the myriad channels and wild assortment of rocks of the fan speak to a process that never stops. It will flood again. I see the whole fan in motion, with gravel streaming over the road, tearing up the asphalt, eventually burying or eroding the gravel spit. Today, this year, my existence—they all seem to diminish into the infinitesimal. I close my eyes and start walking downhill, deeper into the lake.

This essay came about from researching my forthcoming book: Death Valley Rocks! Forty amazing geologic sites in America’s hottest National Park, to be published by Mountain Press. (Sept, 2024)

Each photo (and >5000 more) is available for free download from my photography site, geologypics.com –just type the description or stock number into the search.

Posted in Death Valley, essay, Geologic Time, Geologic Time, Geology, geophotography, science and tagged California, Death Valley, Earth Science, geologic time, geology, hiking, nature, personal essay

25 Oct 2022

6 Comments

Mountain in Transition

inside the crater of Mt. St. Helens
(click on images to see them at full size)

*Terminus of Crater Glacier, Mt. St. Helens, Washington*

There’s a glacier in the crater of Mt. St. Helens—and unlike most other glaciers, it’s growing. Shaded by soaring crater walls and continually replenished by snow avalanches and rockfalls, it oozes forward more than 100 feet per year. At the sound of a loud crack, I look up at its steep front to see a bunch of rocks breaking free of the ice and tumbling downwards to rejoin their compatriots at the bottom. Soon, the ice will advance over them and they’ll be a part of the glacier again.

I’ve long been attracted to these landscapes, practically devoid of organic life but alive just the same. They evoke a sense of timelessness, a connection to Earth’s past, long before mammals and reptiles, let alone humans, colonized its surface. Besides this glacier, near-continuous rock fall and rising dust plumes, there’s a 1000’ tall lava dome that still puts out steam. Younger than many teenagers, the dome formed as sticky silica-rich lava extruded from a vent in the crater floor. It rose to within 800 feet of the highest part of the rim over a period of 5 years, from 2004-2008. It now towers over an older dome, which grew from 1980 until 1986, scarcely middle-aged by human standards.

*Crater domes and fumarole emissions. The dark ledge across the bottom of the photo marks the terminus of Crater Glacier.*

My group is taking its lunch break. One person’s off shooting photos, a few are talking quietly, another’s inspecting some of the pumice and broken rocks that blanket this part of the crater. Our guides are conferring over our route. I feel lucky to be here, having been invited a few months ago to accompany the group to talk about the geology. The Mt. St. Helens Institute, the non-profit that organized the trip, does so under a special use permit from the Gifford Pinchot National Forest.

Looking northward to Spirit Lake and Johnston Ridge, I see a landscape of somber browns, yellows, and grays, decorated with greens. Most of those colors come from rock, either ancient bedrock or eruptive products from the big eruption in 1980. The greens though come from vegetation that is rapidly covering the lower elevations. On the way here, we walked over ashy fields spotted with prairie lupine, paintbrush and a host of other wildflowers I couldn’t identify, small stands of juvenile Douglas Fir and through the occasional jungle of alder.

*View northward from the crater entrance to Spirit Lake and the eastern edge of Johnston Ridge*.

Almost nothing in my view was spared the effects of the 1980 eruption. At 8:32am, May 18, a magnitude 5.1 earthquake caused the bulging north side of the mountain to fail as a gigantic avalanche, releasing pressure on the magma within the volcano, which naturally expanded northwards at supersonic speeds. Entire forests blew over in the scalding hot gas. 10 miles from the crater and forty years later, you can still see many of these trees lying down in formation, just like toothpicks. The failed north side of the mountain, all .67 cubic miles of it, ranks as the world’s largest debris avalanche in recorded history. I can see its path, marked by giant piles of rock called hummocks. Even here in the crater are deposits of the debris avalanche, forming towers of conical pointed hills more than 100 feet high.

*Hikers and landslide hummocks in the crater of Mt. St. Helens, Washington.*

I stoop down to look at a small plant. It’s all by itself, growing between loose, rounded pieces of pumice and undoubtedly rooted in more of the same. An early colonizer. If the crater floor were to remain undisturbed, more vegetation would creep in and eventually, this place would become lightly forested like other places nearby at this elevation. Of course, that’s unlikely here, with an advancing glacier and all the rocks falling from the rim.

Or more eruptions! Mt. St. Helens is, without question, the most active volcano in the Cascade Range, with more than a dozen smaller eruptions since the big one on May 18. And long before 1980 there were hundreds. The US Geological Survey closely monitors Mt. St. Helens’s ongoing activity as well as its geologic past to understand the volcano’s behavior through time. They define its history in terms of four distinct eruptive “Stages” that stretch back some 275,000 years. The most recent of these, the Spirit Lake Stage, started about 3900 years ago and is further divided into six eruptive “Periods”, each marked by multiple eruptions and separated by times of dormancy. One of these eruptions, between 3500-3300 years ago during the Smith Creek Period, dwarfed the 1980 eruption. And in the early 1480s, two eruptions of comparable size to the one in 1980 occurred just two years apart.

Shading the sun from my eyes, I look southward to the volcano’s rim, and I can see a whitish layer produced by these two eruptions. It’s called the “W Tephra”. More than 250 feet of rock now sits above it –all formed after 1480. And there used to be 1300 feet of rock above that, until it was blasted off in 1980—which means that in 500 years, Mt. St. Helens grew more than a quarter mile in height. Just before its cataclysmic eruption, its summit stood 9,677 feet above sea level.

*Dacite domes and Crater Glacier in crater of Mt. St. Helens, Washington. The W Tephra forms the light-colored band at the end of the arrow.*

It’s the myriad smaller eruptions that cause a volcano to grow, and in the case of Mt. St. Helens, they mostly produce lava domes instead of lava flows. As a college student, I had the hardest time visualizing a lava dome because the photographs I saw just looked like gigantic piles of broken rock. But that’s basically what they are, gigantic, dome-shaped, piles of rock. Imagine lava that is so viscous it can hardly flow, oozing out of a volcanic vent, and pushing aside or piling on top of earlier extruded lava, and cooling and breaking up in the process. By its very nature, silica-rich lava like the dacite of Mt. St. Helens is highly viscous. Silica molecules bond with each other to form interconnected networks, so the more silica in a lava, the more viscous it tends to be. Lower silica lavas like andesites contain fewer of those networks so they’re more likely to produce lava flows. Basaltic lavas contain even less silica and flow even easier.

Feeling a gentle breeze, I close my eyes and imagine this place sometime in the future. Two hundred years? Five hundred years? I picture the crater walls, slightly lower now, having steadily eroded through time, their debris gradually filling the crater. A couple large landslides have removed large fractions of one of the walls and piled their debris up against today’s modern domes. During this time, the mountain was largely dormant, but now a pulse of basaltic magma is working its way towards the surface, intruding the gray dacite domes along fracture surfaces as black basaltic dikes. Some of these dikes break through the top of the domes and spill out as basaltic lava flows, which flow a short way down and out of the crater. Mt. St. Helens awakens again, but this time, the lava is basalt so it awakens quietly.

I remind myself that this volcano hasn’t produced much basalt during its 275,000-year history. It doesn’t tend to “awaken quietly”, but more like some grumpy person who you’ve roused with a splash of ice water. Besides the numerous domes, its products typically depict violence: deposits of fallen ash or pumice, or pyroclastic flows, which consist of fast-moving ash, pumice and rock fragments mixed with hot gases. The only basalt lavas from Mt. St. Helens erupted during the end of the Castle Creek Period, which stretched from about 2500 to 1900 years ago. Ape Cave, the lava tube on the volcano’s south side, formed in these lavas, as did the steep rocky slopes we climbed on our hike into the crater. And there, low on the east crater wall, I can see the drama: black Castle Creek dikes slicing through light-colored rock of an older dacite dome. They’re overlain by dark-colored Castle Creek lava flows likely fed by the dikes below, just like my daydream.

*Basaltic dikes of the Castle Creek Period cut an older dacite dome. Pyroclastic flow deposits form the layered materials near the bottom of the photo in shadow.*

As we start our descent, I reflect on the ongoing Modern eruptive period. Before the first earthquakes on March 16, nearly everyone in the Pacific Northwest viewed Mt. St. Helens as a perfectly symmetrical and docile ice cream cone of a mountain. My college friend Michael longed to return home to Portland for long enough to climb it, partly because it offered such a wonderful glissade after summiting. Then less than two weeks after those first earthquakes, the first steam eruptions began. Then more earthquakes. Bulging of the northern edifice. Steam. Earthquakes. May 18.

We’ve learned so much since then! We’ve watched the Modern Period unfold with our own eyes as well as with a vast array of ever-evolving instruments. We can see the rich details of the volcano’s behavior to the point that researchers divide the last 40 years into two subperiods, defined by the formation of each lava dome. We’ve watched Crater Glacier evolve from a snowfield to a full-on glacier that itself has experienced major changes in its shape and behavior. We’ve watched the crater rim erode. We’ve watched vegetation recolonize a landscape that was suddenly rendered devoid of nearly all organic life.

*Loowit Falls and debris avalanche deposits*.

Our group stops for a quick break at Forsyth Spring to absorb the flourishing willows, alders, and sounds of running water. I contemplate the earlier phases of Mt. St. Helens, reaching back to its origin as a series of lava domes 275,000 years ago. Those phases must have been as interesting and diverse as today, but we can experience them only through whatever preserved evidence we can find in the volcano’s deposits. We can’t taste the water or feel the freshness of new growth. Still, we can imagine it.

For this reason, I don’t think of today’s Mt. St. Helens as being “reborn” as it’s so often described. Sure, the vegetation is returning, but I can’t help but see it as incidental to the overall story of a volcano that’s always in transition. The 1980 eruption wasn’t an end point or a beginning –just an event that happened during a long eventful history. We’re experiencing just another moment, another transition, in this volcano’s cycle of life.

All photos freely available from geologypics.com. Click here for more photos of Mt. St. Helens and its geology. Type “Mt. St. Helens” into the search to see them all!

Posted in Geology, Geologic Time, science, Uncategorized, volcanoes and tagged essay, geology, geophotography, Mount St. Helens, Mt. St. Helens, volcano, volcanology

25 Jul 2022

7 Comments

In Transit

This little black pebble, now sitting on my desk, traveled a lot today. After picking it up beneath a cliff face in southwestern Montana, I carried it in my pocket for a few hours and then drove some 20 miles to this college dorm where I’m staying. It’s the most this pebble has moved for millions of years.

Some time ago–this morning, a week, a year, 10 years, 100 years—the pebble weathered out of a much larger rock and fell to the ground. Its worn, rounded edges tell me that before it became part of that larger rock, it traveled down a stream bed –and its size tells me that its source probably wasn’t too far away. As is typical of stream gravel, its movement was irregular, marked by short bursts of movements during floods separated by longer periods of rest on a gravel bar or in the channel itself. Somewhere along the line, the pebble became buried by more sediment, probably because the land subsided or the river channel switched to another position. Eventually, the pebble and the rest of its surrounding sediment turned into rock.

Smith Rock State Park –great geology at the edge of Oregon’s largest caldera

Smith Rock, the Crooked River, and modern Cascade volcanoes from Misery Ridge.

The view from outside the small visitor center at Smith Rock State Park offers a landscape of contrasts. The parking lot, and nearby camping and picnic areas, are flat, underlain by the edge of a basaltic lava flow that drops off in a series of steps to a narrow canyon, some 120 feet (37 m) below. The Crooked River, which rises about 100 miles (162 km) away in the High Lava Plains, fills much of the canyon bottom. Across the canyon, tan cliffs and spires of tuff, another volcanic rock, soar overhead. Smith Rock itself forms a peninsula of this rock, enclosed by a hairpin bend of the Crooked River. The tuff erupted 29.5 million years ago in the largest volcanic eruption to occur entirely within Oregon. Read more…

Posted in Geology, science, volcanoes and tagged caldera, geology, Oregon, Oregon geology, rock climbing, Smith Rock, tuff, volcanic

03 Feb 2019

5 Comments

Seeing some cool properties of water through the lens of its molecular structure

We all know the importance of water—our bodies are mostly water, we need it to survive, it’s the second most important ingredient in coffee… Geologically, it facilitates almost everything we know, from erosion to magma formation to rock fracture. I’m often struck by how so many of water’s unusual properties are determined by its chemistry and molecular structure –and in a very understandable way.

waterfalls in Fjordland, South Island, New Zealand.

Water molecules are polar
Many of water’s properties stem directly from its polar nature –and its polar nature comes right from its molecular structure. Here’s how. Read more…

Posted in Geology, science, water and tagged ice crystals, salt crystals, surface tension, water, water and geology, water chemistry, water properties, water science

25 Aug 2018

4 Comments

Hug Point State Park, Oregon, USA –sea cliffs expose a Miocene delta invaded by lava flows

Alcove and tidepool at Hug Point

Imagine, some 15 million years ago, basaltic lava flows pouring down a river valley to the coast –and then somehow invading downwards into the sandy sediments of its delta. Today, you can see evidence for these events in the sea cliffs near Hug Point in Oregon. There, numerous basalt dikes and sills invade awesome sandstone exposures of the Astoria Formation, some of which exhibit highly contorted bedding, likely caused by the invading lava. It’s also really beautiful, with numerous alcoves and small sea caves to explore. And at low to medium-low tides, you can walk miles along the sandy beach!

(Click on any of the images to see them at a larger size)

Devil’s Punchbowl –Awesome geology on a beautiful Oregon beach

You could teach a geology course at Devil’s Punchbowl, a state park just north of Newport, Oregon. Along this half-mile stretch of beach and rocky tidepools, you see tilted sedimentary rocks, normal faults, an angular unconformity beneath an uplifted marine terrace, invasive lava flows, and of course amazing erosional features typical of Oregon’s spectacular coastline. And every one of these features tells a story. You can click on any of the images below to see them at a larger size.

View southward from Cape Foulweather to the Devil’s Punchbowl.

The rocks. They’re mostly shallow marine sandstones of the Astoria Formation, deposited in the early part of the Miocene, between about 16.5 to 22 million years ago. The rocks are tilted so you can walk horizontally into younger ones, which tend to be finer grained and more thinly bedded than the rocks below. This change in grain size suggests a gradual deepening of the water level through time. In many places, you can find small deposits of broken clam shells, likely stirred up and scattered during storms –and on the southern edge of the first headland north of the Punchbowl, you can find some spectacular soft-sediment deformation, probably brought on by submarine slumping. Later rock alteration from circulating hot groundwater caused iron sulfide minerals to crystallize within some of the sandstone. Read more…

Posted in Geology, geophotography, Oregon Coast, photography, science, Unconformities and tagged Devil's Punchbowl State Park, geology, geology pictures, geophotography, Oregon, Oregon beaches, Oregon Coast, Oregon geology, Oregon State Parks

10 Dec 2017

6 Comments

Cove Palisades, Oregon: a tidy short story in the vastness of time

If I were a water skier, I’d go to Lake Billy Chinook at Cove Palisades where I could ski and see amazing geology at the same time. On the other hand, I’d probably keep crashing because the geology is so dramatic! Maybe a canoe would be better.

View across the Crooked River Arm of Lake Billy Chinook to some of the 1.2 million year old canyon-filling basalt (right) and Deschutes Fm (left). The cliff on the far left of the photo is also part of the 1.2 million year basalt.

The lake itself fills canyons of the Crooked, Deschutes and Metolius Rivers. It backs up behind Round Butte Dam, which blocks the river channel just down from where the rivers merge. The rocks here tell a story of earlier river canyons that occupied the same places as today’s Crooked and Deschutes Rivers. These older canyons were filled by basaltic lava flows that now line some of the walls of today’s canyons.

CovePalisades2 From the geologic map, modified from Bishop and Smith, 1990, you can see how the brown-colored canyon-filling basalt, (called the “Intracanyon Basalt”) forms narrow outcrops within today’s Crooked and Deschutes canyon areas. It erupted about 1.2 million years ago and flowed from a vent about 60 miles to the south. You can also see that most of the bedrock (in shades of green) consists of the Deschutes Formation, and that there are a lot of landslides along the canyon sides.

The cross-section at the bottom of the map shows the view along a west-to-east line. Multiple flows of the intracanyon basalt filled the canyon 1.2 million years ago –and since then the river has re-established its channel pretty much in the old canyon. While the map and cross-section views suggest the flows moved down narrow valleys or canyons, you can actually see the canyon edges, several of which are visible right from the road.

Rocks! –a brief illustrated primer

click on any image to see a larger version

Seems like most people I know like rocks. They bring home unusual rocks from vacations; they admire beautiful facing stones on buildings; they frequently ask “What is this rock”? Considering that the type of rock you’re looking at reflects the processes that caused it to form, some basic rock identification skills can go a long way to understanding our planet!

Rock (left, igneous-granite) and minerals (right, quartz and kyanite). Notice that the granite is made of a variety of minerals.

Of course there are thousands of different rock types —But! they ALL fit into one of three categories: igneous, sedimentary, or metamorphic. Here’s a brief, illustrated summary of each.

Igneous rocks are those that form by cooling and crystallization from a molten state. Consequently, they consist of crystals of various minerals that form an interlocking mosaic like the rock in the photo to the right. Igneous rocks are further classified as “intrusive” or “extrusive”, depending if they form beneath Earth’s surface (intrusive) or on Earth’s surface (extrusive). Extrusive rocks are more commonly called volcanic rocks. Generally speaking, intrusive rocks are coarsely crystalline whereas volcanic ones are finely crystalline. Check out this gallery of igneous rock photos.

Sedimentary rocks are made of particles (“sediment”) of pre-existing rock that are deposited as layers on Earth’s surface and then become cemented together. Individual layers of sedimentary rock are called “beds”. Bedding is best observed from a distance; most individual sedimentary rocks come from within a bed and so may appear homogeneous. Check out this gallery of sedimentary rock photos.

Metamorphic rocks are pre-existing rocks that change (“metamorphose”) because they are subject to high temperatures and/or pressures. This change involves the growth of new crystals in the rock. Because this growth typically occurs under conditions of high pressure as well as temperature, the new minerals tend to grow in a preferred orientation, leading to a fine-scale layering in the rock. This layering is called foliation. Unlike bedding in sedimentary rock, foliation tends to be irregular and marked by differently colored zones of different minerals. Check out this gallery of metamorphic rock photos.

Sedimentary (sandstone, L), Igneous (granite, Ctr), and Metamorphic (gneiss, R) specimens

simplified key to recognizing main rock types

Telling Igneous, Sedimentary, and Metamorphic rocks apart is usually pretty easy. First, decide if the rock consists of crystals or rounded grains. If it consists of crystals, then it is igneous or metamorphic; if it consists of grains, then it is sedimentary. If the crystals are arranged into layers or bands, the rock is metamorphic; if they are randomly arranged, then it is igneous. Igneous rocks with large crystals generally indicate slow cooling within the earth (intrusive). Conversely, igneous rocks with small crystals generally indicate rapid cooling on Earth’s surface (volcanic).

Igneous Rock –more details

Intrusive and volcanic rocks are further classified based on their chemistry and texture according to the chart below. This is one place where mineral identification becomes very important because minerals reflect the rock’s chemistry. Importantly, rocks with high silica content, such as rhyolite and granite, typically have fairly low iron contents, and so tend to have minerals that are light in color, such as K-feldspar, sodium-rich plagioclase, and quartz. Conversely, rocks with low silica content, such as basalt and gabbro, typically have high iron contents, and so have minerals that tend to be dark in color.

Principal igneous rock types. Their classification depends on texture and composition. Fine-grained rocks are extrusive (upper row), whereas coarse-grained rocks are intrusive (lower row). Silica content then determines the specific rock name: gabbro and basalt <50-57%, SiO2; diorite and andesite, 57-67%; granite and rhyolite, 67+%. Notice that rocks tend to be darker, denser, and more iron rich towards the lower silica end of the spectrum.

More on Volcanic Rock
Being igneous, volcanic rocks are made of crystals –but they’re so fine grained, you often can’t see that without a microscope. Thankfully, many volcanic rocks contain phenocrysts, larger crystals surrounded by the finer grained matrix. If you look closely at the photos of basalt and andesite above, you can see phenocrysts of plagioclase feldspar as the small white things.

Below are more photos, showing a more enlarged view of a rock with phenocrysts. Note how fine grained the surrounding matrix is –you can’t really see anything at all. If you look at the microscopic view though you can see that the whole rock is crystalline, even the super-fine matrix. The point here is that, unless the rock contains glass (see next section), the whole rock is crystalline!

Porphyritic volcanic rock in hand sample (left) and microscopically (right). Note how microscopic view

Volcanic: Glass
One of the more ubiquitous volcanic products, volcanic glass is just that –glass–so it lacks a crystal structure. Glass can form when the lava is so dry as to inhibit crystal growth, as in obsidian, or when lava cools so quickly as to prevent crystal growth, such as with volcanic ash and pumice.

The photos below show pumice, which is frothy volcanic glass. It gets that texture because it forms during violent eruptions –explosively expanding gases in the lava shatter the fast-cooling material so that the rock consists of air bubbles (called vesicles) separated by glassy sidewalls.

Pumice: frothy volcanic glass from instantaneous cooling. Left-hand image shows close-up view of glass threads. Paper clip for scale.

Volcanic: Pyroclastic Material and Rocks
Pyroclastic materials (also called “tephra”) form during explosive eruptions and so consist of rock fragments and glass ejected violently from the volcano. We classify it according to its size: large fragments are called blocks or bombs; small particles, between about 2mm – 64mm, are called “lapilli”; tiny particles, smaller than 2mm, are called “ash“. Pumice is also pyroclastic, but it’s considered its own rock type –and it can be of any size. Pyroclastic falls can result from any explosive eruption in which pyroclastic materials fall out from the atmosphere; pyroclastic flows are those that flow out over the ground surface.

Most tuff contains fragments of pumice in a matrix of ash

Now the rocks. The most common pyroclastic rock is undoubtedly “tuff”, which is composed largely of ash and pumice fragments, erupted mostly during rhyolitic eruptions. Air fall tuff forms from ash that accumulates in layers as it settles from the atmosphere; Ash flow tuff forms from bodies of ash that flow rapidly along the ground, typically incinerating everything in their paths. Because the material flows, it typically does not form layers. Many ash flow tuffs are welded (called “welded tuff”) because of the high temperatures. These highly welded tuffs are sometimes called “ignimbrites”. To identify tuff, look for pieces of pumice floating around in the ashy matrix.

Below’s a view of the Bandelier Tuff in northern New Mexico. It’s a series of ash flow tuffs formed during huge eruptions 1.6 and 1.25 million years ago in the Jemez Mountains. These eruptions formed the Valles-Toledo Caldera (generally just called the “Valles Caldera”). You can get an idea as to the size of the eruptions based on the size of the flows: they’re thick!

Cliffs of Bandelier Tuff, erupted from Valles Caldera, New Mexico.

New Zealand’s Taupo Volcanic Zone hosts the most frequent recent rhyolitic eruptions than anywhere else in the world, all active in the last 2 million years. The most recent big eruptions, 26,500 and 1800 years ago, were centered on Lake Taupo, near the middle of the North Island. Below is a map showing the distribution of airfall and ignimbrite (welded ash flow) deposits formed during the eruption at AD 186, just over 1800 years ago. The estimated volume of all eruptive products during this eruption exceeds 105 km3 (Wilson, and Walker, 1985). By comparison, the older “Oruanui” eruption, 26,500 years ago? It likely erupted more than 1000 km3! (Wilson, 2001).

Taupo vent (red triangle) and distribution of airfall and ashflow deposits from AD186 eruption. Inset shows Taupo Volcanic Zone on New Zealand’s North Island. From Wilson and Walker, 1985.

references for Taupo eruptions:
Wilson, C.J.N., and Walker, G.P.L., 1985, The Taupo eruption, New Zealand i. General Aspects, Philosophical Transactions of the Royal Society of London, v. 314, p. 199-228.

Wilson, C.J.N., 2001, The 26.5 Oruanui eruption, New Zealand: an introduction and overview, Journal of Volcanology and Geothermal Research, v. 112, p. 133-174).

Sedimentary Rock –More details

Sedimentary rock may be clastic, biogenic, or chemical, depending on how the particles formed. Clastic sedimentary rocks contain actual pieces of the pre-existing rock that have been transported from the original source. During this transportation, the particle breaks into smaller grains and typically becomes rounded. Clastic sedimentary rocks are further classified according to grain size: shale contains clay-sized grains; siltstone contains silt-sized grains; sandstone contains sand-sized grains; conglomerate contains grains that are pebble to boulder-sized.

Clastic sedimentary rocks: shale (left), sandstone (center), and conglomerate (right).

Biogenic sedimentary rocks are those that form through biological activity. By far the most common example is limestone, which forms by the production of calcium carbonate by algae and invertebrate animals for shells. Other examples include dolomite, which forms by the same process as limestone, and chert, which forms by the accumulation of silica-producing organisms on the sea floor.

Chemical sedimentary rocks form by non-biologically induced precipation of minerals. Examples include sinter and travertine, which consist of silica and calcium carbonate respectively, precipitated from hot water at thermal springs. Another important example is bedded salt, which forms today by evaporation in closed desert basins.

Tilted sedimentary rocks –started out horizontally.

You can’t see the bedding in the rock samples shown above, but if you were to stand back from an outcrop of sedimentary rocks, you probably could see the bedding. That’s because most individual samples don’t go across bedding but instead come from individual beds.

Metamorphic Rock –more details

Most metamorphic rocks are classified according to their grain size and the resulting nature of their foliation. Slates are the finest grained metamorphic rock, followed by phyllite, schist, and gneiss, being the coarsest grained. Gneiss is especially distinctive because most of its crystals are readily visible and its foliation is marked by bands of different minerals. In general, crystal size corresponds to the metamorphic grade, or intensity, with the most coarsely crystalline rocks being of the highest grades.

Metamorphic rocks. From left to right: slate, phyllite, schist, gneiss. Note that each rock has layering (foliation) that is caused by a parallel arrangement of platy minerals within the rock.

And then there are metamorphic rocks that form just because of high temperatures, typically because they were heated by the intrusion of a nearby igneous body. This type of metamorphism, called “contact metamorphism” is a common origin for non-foliated marbles and quartzites. Marble forms by contact metamorphism of limestone and dolomite; quartzite forms by contact metamorphism of sandstone.

The photo on the below shows the igneous rock diorite intruding the sedimentary Helena Dolomite in Glacier National Park, Montana. You can see how contact metamorphism has turned the dolomite next to the intrusion into a white marble. Ooooh!

Intrusive “sill” of diorite and the resulting contact metamorphism of adjacent gray dolomite to white marble in Glacier National Park, MT.

For more, higher resolution photos of each feature or rock type, try doing a geology keyword search for any of the rock types or features described here. Some useful keywords are “igneous, intrusive, volcanic, metamorphic, sedimentary, phenocryst, tuff, pumice, or volcanic glass” –or any others you can think of. Enjoy!

Posted in Geology, science and tagged geology, geology hand samples, igneous rocks, ignimbrite, metamorphic rocks, rocks, sedimentary rocks, Taupo Volcano, volcanic rocks, welded tuff

06 Oct 2017

Geologypics.com– A new (and free) resource for geological photographs

What better way to kick off my new website than to write about it on my blog? To see it, you just need to click on the word “home” in the space above. Or you can click the link: geologypics.com.

Here’s part of the front page:
home3

As it says, the site offers free downloads for instructors –and for anybody who’s craving a good geology photograph. It’s my way of contributing to geology education –showing off some of our landscape’s amazing stories and providing resources for other folks who want to do the same.

I think the best part of the whole site is that red button in the middle of the home page. It says “Image Search by Keyword”.

Right now, there are more than 2200 images you can search for — all of which are downloadable at resolutions that generally work for powerpoint. If you search for “sea stack” for example, you’ll get 38 hits –and the page will look like this:

First page of sea stacks when you search on the term.

Notice that ALL the photos are presented as squares–which works for most photos, but not all. To help mitigate that, the photos with vertical or panorama formats say so in their title, so you know to click on them to see the whole image. Take the photo in the upper center, for example –it’s got a vertical format. Here it is:

A more detailed caption below the photo, along with its ID number appears at the bottom of the pic. This particular image is the chapter opener to the Coast Range in my new book “Roadside Geology of Washington“, which I wrote with Darrel Cowan of University of Washington.

There are also galleries –a chance to browse a variety of images without having to think of keywords. Similar to the search, they’re presented as squares so you need to click on the photo to see the whole thing.

Here’s what the photo gallery page looks like (on the left), followed by part of the “glaciation” page you’d see if you clicked on “glaciation”. Woohoo!

part of Galleries page (left) and part of Glacial page (right)

Then there’s the “About” page, which gives some information about me and details my policies regarding use of the images (basically, you can download freely for your personal, non-commercial use if you give me credit; if you want to use the image in a commercial publication you need to contact me to negotiate fees). There’s also a “News” page, that gives updates on the website. There’s a contact page from which you can send me emails. And the blog? It goes right back to here!

And finally, if you’re looking for a great web designer? Try Kathleen Istudor at Wildwood SEO –she created the site and spent hours coaching me on how to manage it.

Enjoy the site!

Posted in aerial geology, aerial photography, climate change, Coastal uplift, geologic hazards, Geologic Time, Geology, Geology, Geologic Time, geophotography, mountains, national parks, photography, science, volcanoes and tagged Earth Science, free geology photos, geological photography, geology photos, geology teaching resource, geophotography, nature, photography, rocks, science, teaching resource | Leave a comment

geologictimepics

Geology and Geologic Time through Photographs