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Geology and Geologic Time through Photographs

Archive for the tag “geology”

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

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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)

HugPointCRBGYou can read more about the Columbia River Basalt Group on an earlier post, but basically, it mostly erupted from fissures in northeastern Oregon and southeastern Washington between 17-6 million years ago (although most activity ended by 14 Ma). They’re called “flood basalts” because they completely flooded the landscape. Many of the flows made it all the way to the Pacific Ocean. They followed the ancient Columbia River and also probably one farther south across today’s Coast Range.

Most researchers now agree that on reaching the coastline, some of these flows somehow invaded the existing sediments, now mostly the Astoria Formation, to form dikes and sills in the rock. In places, they also inflated weak zones to form shallow magma chambers. Some of these magma chambers are preserved as thick sills along the northern coast, such as at Neahkahnie Mountain, while some actually fed undersea volcanoes, such as Haystack Rock at Cannon Beach.

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Basalt sea cliffs at Neahkahnie Mountain

Probably the most accessible exposures of the basalt are in the small headlands immediately north and south of the parking lot. If you go south, look for a pair of dikes as well as a sill of the basalt. Just north of the parking lot, you can’t miss the narrow sill that extends diagonally up from a wave-eroded alcove. Walk around the point to see a much larger intrusion that appears to connect with the sill. One of its sides is faulted and has eroded into a small sea cave.

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Overview of Hug Point area, looking north past Cannon Beach to Ecola Point, Oregon. If you click on this image, you can see it in another window at a much larger size.

Hug Point itself forms the second headland north of the parking area, and it protrudes far enough into the surf that you practically need to “hug” the rocks to get around, hence its name. In the late 1800’s travelers carved a road along the cliff base, which makes passage relatively easy when the tide isn’t too high. Sandstone of the Astoria Formation along this stretch is coarse grained, with mostly quartz and feldspar grains. You can also see abundant cross-bedding from deposition by currents, as well as rip-up clasts, where storms eroded the underlying bed and included parts of it within the newly deposited sand.

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Cross-bedding in Miocene Astoria Formation and road along base of Hug Point. Note the rip-up clasts at the base of the light-colored bed. Also note the barnacles –no passage here at high tide!

This coarse part of the Astoria Formation is called the Angora Peak Member, and was deposited on a delta that was continually affected by waves and storms of the Miocene ocean. The rocks dip gently to the northeast, to indicate younger rocks in that direction.

A cliff of highly contorted sandstone lies Immediately north of Hug Point. The irregular nature of the folding suggests that deformation took place while the Astoria Formation was still soft, hence the term “soft-sediment deformation”. Given the abundance of the basaltic intrusions here, geologists interpret the cause of this folding as related to the intrusions. More effects of the intrusions are the breccias you can see another quarter mile to the north. There, highly fragmented sandstone and basalt are mixed together, a likely product of the explosions that resulted when the lava encountered the water-rich sediment.

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Contorted Astoria Formation in sea cliff.

In another mile, you reach Lion Rock and Humbug Point, where the Astoria Formation shows more interaction with the basalt –and another half mile beyond that, Silver Point. There, a cliff exposes shale of the Astoria Formation’s Silver Point member, deposited in a deeper part of the delta complex. As these rocks sit on top the coarser Angora Peak member, they suggest that the delta was subsiding through time. And if you want to keep walking? Cannon Beach’s Haystack Rock is just another two miles away!

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Haystack Rock at Cannon Beach. The vent area of an undersea volcano fed by lavas of the Columbia River Basalt Group!


For more photos of Hug Point, including a closer view of those weird folds, please visit my website, geologypics.com –and type “hug” into the keyword search. All the images there are freely downloadable for noncommercial, personal or instructional uses.

Here’s the reference for the map of the CRBG: Reidel, S.P., Camp, V.E., Tolan, T.L., Martin, B.S. 2013. The Columbia River flood basalt province: stratigraphy, areal extent, volume, and physical volcanology. In The Columbia River Basalt Province, Geological Society of America Special Paper 497, eds. S.P. Reidel, V.E. Camp, M.E. Ross, J.A. Wolff, B.S. Martin, T.L. Tolan, and R.E. Wells, p. 1-44.

And another really good reference for the invasive basalts: Wells, R.E., Niem, A.R., Evarts, R.C., and Hagstrum, J.T. 2009. The Columbia River Basalt Group—From the gorge to the sea. In Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacific Northwest, Geological Society of America Field Guide 15, eds. J.E. O’Connor, R.J. Dorsey, and I.P. Madin, p. 737-774.

–This vignette is a draft version of an entry for my latest book project: Oregon Rocks! A guide to the geology of the Beaver State. To be published (probably not until 2021) by Mountain Press in Missoula.

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.

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

 

180629-58ceThe 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…

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.

Lake Billy Chinook, Oregon

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.

CovePalisades2From 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.

Read more…

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!

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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.

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

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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.

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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!

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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.

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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.

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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!

Bandelier Tuff, Los Alamos, New Mexico

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).

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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.

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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.

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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.

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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!

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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!

 

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…

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

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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.

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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.

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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.

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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!

Landscape and Rock–4 favorite photos from 2015

Landscape and bedrock… seems we seldom connect the two. We all like beautiful landscapes, but most of us don’t ask how they formed –and even fewer of us think about the story told by the rocks that lie beneath it all. Those make two time scales, the faster one of landscape evolution and the much slower one of the rock record. Considering that we live in our present-day human time scale, it’s no wonder there’s a disconnect!

Take this photo of Mt. Shuksan in northern Washington. My daughter Meg and I drove up to the parking lot at Heather Meadows and went for a quick hike to stretch our legs and take some pictures just before sunset.We had about a half hour before the light faded –and all I could think about was taking a photo of this amazing mountain. But the geology? What??

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1. Mt. Shuksan and moonrise, northern Washington Cascades.

Thankfully, I’d been there in September scoping out a possible field project with a new grad student, and had the time to reflect… on time. From the ridge we hiked, shown as the dark area in the lower left corner of the left-hand photo below, we could almost feel Shuksan’s glaciers sculpting the mountain into its present shape. Certainly, that process is imperceptibly slow by human standards.

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Mt. Shuksan: its glaciated NW side, summit, and outcrop of the Bell Pass Melange.

But the glaciers are sculpting bedrock –and that bedrock reveals its own story, grounded in a much longer time scale.

It turns out that the rock of Mt. Shuksan formed over tens of millions of years on three separate fragments of Earth’s lithosphere, called terranes. These terranes came together along faults that were then accreted to North America sometime during the Cretaceous. At the top of the peak you can find rock of the Easton Terrane. The Easton Terrane contains blueschist, a metamorphic rock that forms under conditions of high pressures and relatively low temperatures, such as deep in a subduction zone. Below that lies the Bell Pass Melange (right photo) –unmetamorphosed rock that is wonderfully messed up. And below that lies volcanic and sedimentary rock of the Chilliwack Group.

Here’s another of my favorites from 2015: the Keystone Thrust! It’s an easy picture to take –you just need to fly into the Las Vegas airport from the north or south, and you fly right over it. It’s the contact between the gray ledgey (ledgy? ledgeee?) rock on the left and the tan cliffs that go up the middle of the photo.

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2. Keystone Thrust fault, Nevada–gray Cambrian ridges over tan Jurassic cliffs.

The gray rock is part of the Cambrian Bonanza King Formation, which is mostly limestone, and the tan cliffs consist of  Jurassic Aztec Sandstone. Cambrian, being the time period from about 540-485 million years, is a lot older than the Jurassic, which spanned the time 200-145 million years ago. Older rock over younger rock like that requires a thrust fault.

Talk about geologic history… the thrust fault formed during a period of mountain building during the Cretaceous Period, some 100-70 million years ago, long before the present mountains. And the rocks? The limestone formed in a shallow marine environment and the sandstone in a sand “sea” of the same scale as today’s Sahara Desert. We know it was that large because the Aztec Sandstone is the same rock as the Navajo Sandstone in Zion and Arches national parks.

Cambrian-Jurassic

left: Limestone of the Cambrian Bonanza King Formation near Death Valley; right: Cross-bedded sandstone of the Jurassic Navajo Sandstone in Zion NP, Utah.

So… the photo shows cliffs and ledges made of rocks that tell a story of different landscapes that spans 100s of millions of years. But today’s cliffs and ledges are young, having formed by erosion of the much older rock.  Then I flew over it in about 30 seconds.

At Beach 2 near Shi Shi Beach in Washington State are some incredible sea stacks, left standing (temporarily) as the sea erodes the headlands. The sea stack and arch in the photo below illustrates the continuous nature of this erosion. Once the arch fails, the seaward side of the headland will be isolated as another sea stack, larger, but really no different than the sea stack to its left. And so it goes.

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3. Sea arch and headland at Beach 2, Olympic Coast, Washington.

And of course, the headland’s made of rock that tells its own story –of  deposition offshore and getting scrunched up while getting added to the edge of the continent.

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Bedrock at Beach 2 consists mostly of sandstone and breccia. The white fragment is limestone mixed with sandstone fragments.

And finally, my last “favorite”. It’s of an unnamed glacial valley in SE Alaska. My daughter and I flew by it in a small plane en route to Haines, Alaska to visit my cousin and his wife. More amazing landscape–carved by glaciers a long time ago. But as you can expect, the rock that makes it up is even older and tells it’s own story.

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4. Glacial Valley cutting into Chilkat Mountains, SE Alaska.

Of course, this message of three time scales, the human, the landscape, and the rock-record time scale applies everywhere we go. Ironically, we’re usually in a hurry. I wish I kept it in mind more often, as it might slow me down a little.

Here’s to 2015 –and to 2016.

To see or download these four images at higher resolutions, please visit my webpage: favorite 10 geology photos of 2015.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Conglomerate!

A trip to Death Valley over Thanksgiving two weeks ago reignited all sorts of things in my brain, one of which being my love of conglomerate. Honestly, conglomerate HAS to be the coolest rock!

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Tilted conglomerate in Furnace Creek Wash, Death Valley.

Just look at this stuff! Just like any good clastic sedimentary rock, it consists of particles of older rock–but with conglomerate, you can easily see those particles. Each of those particles opens a different door to experiencing deep geologic time.

As an example, look at the conglomerate below, from the Kootenai Formation of SW Montana. It contains many different cobbles of light gray and dark gray quartzite and pebbles of black chert. The quartzite came the Quadrant Formation and chert from the Phosphoria Formation. So just at first glance, you can see that this conglomerate in the Kootenai contains actual pieces of two other older rock units.

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Conglomerate of the Kootenai Formation, SW Montana.

But consider this: The Quadrant formed as coastal sand dunes during the Pennsylvanian Period, between about 320-300 million years ago and the Phosphoria chert accumulated in a deep marine environment during the Permian, from about 300-250 million years ago. The Kootenai formed as river deposits during the early part of the Cretaceous Period, about 120 million years ago. All those are now together as one.

Similar to the modern river below (except for the glaciers), the Kootenai rivers transported gravel away from highlands –the highlands being made of much older rock that was uplifted and exposed to erosion. That older rock speaks to long gone periods of Earth history while the gravel speaks to the day it’s deposited.

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Athabasca River in Jasper National Park, Alberta

But this is where my head starts to spin: the modern gravel is made of rounded fragments of old rock –so when you look at a conglomerate, you glimpse at least two time periods at once: you see the conglomerate, which reflects a river or alluvial fan –or any environment near a bedrock source– and you also see the particles, which formed in even older environments.

And it gets worse –or better. What happens when you see a conglomerate eroding? The conglomerate is breaking up into modern sediment, which consists of pieces of older sediment –that at one time was modern sediment that used to be older sediment?  Look at the pebbles below. I keep them in a rusty metal camping cup on a table in my office.

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“Recycled” pebbles of the Kootenai Formation.

These stream pebbles eroded out of the Kootenai conglomerate. So… they’re simultaneously modern stream pebbles and ancient ones –AND… they originated as the Quadrant and Phosphoria Formations. Four periods of time, spanning 300 million years, all come together at once.

And if that’s not enough, those conglomerates in Death Valley? They  contain particles of… conglomerate! Look! The arrow in the left photo points to the boulder of conglomerate on the right. If you click on the photos, you can see them enlarged.

All those particles, which are now eroding and becoming modern sediment, were yesterday’s sediment. And the conglomerate boulder? It too is becoming “modern sediment” and it too was “yesterday’s sediment” when it was deposited on an alluvial fan with the rest of the material. However, it goes a step further: its pebbles and cobbles were both “modern” and “yesterday’s” sediment at a still older time. And before that? Those pebbles and cobbles eroded from even older rock units, some of which date from the Cambrian, about 500 million years ago.

For fun, here’s a photo of another conglomerate boulder.

Conglomerate clast in conglomerate

Conglomerate boulder in conglomerate of the Furnace Creek Formation, Death Valley, CA.

 

I can’t help but wonder how Young Earth Creationists would deal with these rocks. Given their story of the Grand Canyon, in which the Paleozoic section was deposited during early stages of “The Flood” and the canyon was carved during the later stages (they really do say that too!), they’d probably roll out that same blanket answer: The Flood. End of discussion. No questioning, no wondering.

In my opinion, one of the beautiful things about geology is that we’re always questioning and wondering.

 

 

for more geology photos, please visit my website.

 

 

 

 

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!

Rockin’ countertops–geologic time in our kitchens and bathrooms!

I stopped by a “granite” supplier yesterday –the kind of place that sells “granite” and “marble” slabs for countertops.  Besides the fact that almost none of the slabs were actually granite or marble, they were spectacular rocks that showed wonderful wonderful detail. I nearly gushed at the idea of taking a geology field trip there.  It’s local, and you seldom find exposures like this anywhere else!

slabs of polished rock at a "granite" warehouse --not sure if any of this is actually granite, but it all reflects geologic time.

slabs of polished rock at a “granite” warehouse –most of it’s not actually granite, but it all reflects geologic time.

Generally speaking, “granite” in countertop language means “igneous” or “metamorphic” –crystalline rocks that form miles beneath Earth’s surface and so require great lengths of time to reach the surface where they can be quarried.  When I first started this blog, geologic time with respect to igneous and metamorphic rocks were some of the first things I wrote about –it’s such pervasive and important stuff.

So the main point is that your friend’s kitchen with “granite” countertops surrounds you with geologic time every time you walk in there!

But check out that green polka-dotted rock on the right side of the photo.  Full of rounded cobbles –it’s a conglomerate, originating by sedimentary processes on Earth’s surface. Does it indicate great lengths of geologic time? A Young Earth Creationist might say it were a deposit of “the Flood” and end-of-story.

Here’s a closer look:

Polished conglomerate --individual cobbles are metamorphic rocks. The green color comes from the mineral chlorite.

Polished conglomerate –individual cobbles are metamorphic rocks. The green color of the background material comes from the mineral chlorite. That’s a penny (on the left) for scale.

The conglomerate is made of beautifully rounded cobbles and small boulders that are almost entirely metamorphic in origin.  Most of them are gneisses, which form at especially high grades of metamorphism, typical of depths greater than 8 or 10 miles!  After a (long) period of uplift and erosion, the rock was exposed to erosion, gradually breaking into fragments, which eventually became these rounded cobbles, and ended up in the bottom of a big stream channel or on a gravel bar somewhere.

But that’s not the end of the story, because this deposit of rounded cobbles itself became metamorphosed –so it had to get buried again. We know that because the rock is pervaded by the mineral chlorite, which gives the rock its green color.  Chlorite requires metamorphism to form.  Granted, the rock isn’t highly metamorphosed –there’s no metamorphic layering and chlorite forms at low metamorphic temperatures– but it’s metamorphic nonetheless, typical of depths of a few miles beneath the surface.

And if you look even closer, you can see some of the effects of the reburial pressures: the edges of some of the cobbles poke into some of the other ones. This impingement is a result of the stress concentrations that naturally occur along points of contact.  The high stress causes the less soluble rocks to slowly dissolve into the other, more soluble rock.

cobbles, impinging into each other. Stars on right photo show locations.

cobbles, impinging into each other. Stars on right photo show locations.

I’m already jealous of the person who’s going to buy this slab of rock. It tells a story that begins with 1) metamorphic rock forming deep in the crust, then 2) a long period of uplift and erosion to expose the rocks, then 3) erosion, rounding, and deposition of the metamorphic cobbles, 4) reburial to the somewhat shallow depths of a mile or two–maybe more, 5) more uplift and erosion to expose the meta-sedimentary deposit, 6) Erosion by human beings.

And me? Personally, I’d like to make a shower stall or a bathtub out of this rock –can you imagine???


Some links you might like:
a blog I like that’s about science and creationism
another blog about an ancient Earth and deep time
my original song “Don’t take it for Granite“. (adds some levity?)
Geology photos for free download.

 

 

 

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