geologictimepics

Geology and Geologic Time through Photographs

Archive for the category “Geology, Geologic Time”

Today’s hazards, yesterday’s hazards: Earthquake damage, ongoing rock fall, and basalt flow

The M 6.3 February, 2011 Earthquake in Christchurch, New Zealand caused more than considerable damage; 185 people lost their lives and estimates of damage now exceed $40 billion.  When I visited in January, 2014, there was still clear evidence of the destruction, such as this broken house teetering on the edge of a cliff face.  The cliff had apparently given way during the earthquake and taken the entire back yard with it.  Now, rock fall provides an ongoing hazard –hence the stacked shipping containers to keep it off the road.

And then there’s the lava flow –Miocene in age, filling an ancient river channel, as plain as day.  Some 10 or 11 million years ago, this lava flow probably burned everything in its path.

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photo downloaded from marlimillerphoto.com (type “New Zealand” into the search)

Great Unconformity in Montana –and rising seas during the Cambrian

Here’s yet another picture of the Great Unconformity –this time in southwestern Montana.  Once again, Cambrian sandstone overlies Precambrian gneiss.  You can see a thin intrusive body, called a dike, cutting through the gneiss on the right side.  You can also see that the bottom of the sandstone is actually a conglomerate –made of quartzite cobbles derived from some nearby outcrops during the Cambrian.

Great unconformity in SW Montana.

Photo of Cambrian Flathead Sandstone overlying Proterozoic gneiss in SW Montana.

 

And that’s me in the photo.  My left hand is on the sandstone –some 520 million years or so old; my right hand is on the gneiss, some 1.7 BILLION years old.  There’s more than a billion years of missing rock record between my two hands.  Considering that the entire Paleozoic section from the top of the Inner Gorge in the Grand Canyon to the top of the rim represents about 300 million years and is some 3500′ thick… yikes!

And… just like in the Grand Canyon and elsewhere, there is Cambrian age shale and limestone above the sandstone.  This rock sequence reflects rising sea levels during the Cambrian.  It’s called the “Cambrian Transgression”, when the sea moved up onto the continent, eventually inundating almost everywhere.  If you look at the diagram below, you can see how this sequence formed.

Marine transgression

Sequence of rock types expected during a transgression of the sea onto a continent.

If you look at time 1, you can see a coastline in cross-section, with sand being deposited closest to shore, mud a little farther out, and eventually carbonate material even farther out.  As sea levels rise (time 2), the sites of deposition for these materials migrates landward, putting mud deposition on top the earlier sand deposition and so on.  At time 3, the sequence moves even farther landward, resulting in carbonate over mud over sand.  If these materials become preserved and turned into rock, they form the sequence sandstone overlain by shale overlain by limestone –just what we see on top the Great Unconformity.

 

 

 

Igneous Rocks

Here are some samples of different igneous rocks.  The upper photo shows intrusive igneous rocks and the lower photo shows volcanic (extrusive igneous) rocks.

From left to right, these rocks are arranged in order of decreasing silica content: granite, diorite, and gabbro. Click here for more photos of igneous rocks and features.

I can’t claim that these are the most artistic photos, but they do show a couple things about igneous rocks.  First off, to be igneous, a rock needs to have cooled and crystallized from a molten state.   Intrusive rocks, shown in the photo above, are the type of igneous rock that cools and crystallizes within the crust; volcanic rocks, shown in the photo below, cool and crystallize on the Earth’s surface.  Because they form by cooling and crystallizing, crystals in both types  generally have a random orientation and an interlocking texture.  You can see that in the photo above, because intrusive rocks tend to be coarse grained.  It’s much harder to see that feature in volcanic rocks because they tend to be fine grained.

Intrusive igneous rocks are coarse-grained, and volcanic rocks are fine-grained because it takes time to grow crystals –and intrusive rocks take longer to cool and crystallize because they’re insulated by the surrounding rock.

These photos also demonstrate how igneous rocks generally become lighter in color as their silica content increases and their iron content decreases.  By definition, granite (left photo) has more silica than diorite, which has more silica than gabbro.  Iron tends to follow silica in an inversely proportional sort of way –so the gabbro has the most iron.  Same thing with the volcanic rocks.

When it comes to great lengths of geologic time, the intrusive rocks are the most instructive.  They form within the Earth– at depths of several kilometers to several tens of kilometers –but here are some hand samples at the surface?

So the big question is, how long does it take for a rock at a depth of say, 10 km, to make it to the surface of the Earth?  It depends on the rate of uplift and erosion –but really fast uplift rates are on the order of 1 meter/thousand years.  That makes for 10 million years at minimum just to get these little hand samples to the surface!

From left to right, these rocks are arranged in order of decreasing silica content: rhyolite, andesite, basalt. Click here for more photos of volcanic rocks and features.

Cambrian Limestone, Death Valley National Park, California.

Limestone’s a common sedimentary rock –it’s made from calcium carbonate.  The calcium carbonate is precipitated in shallow marine conditions with the help of biological activity, most commonly algae, but also by the many invertebrates that form shells.  This material then settles to the ocean bottom as a lime-rich mud and if the conditions are right, eventually becomes rock.

Compared with many other sedimentary rocks, limestone deposits can accumulate pretty rapidly –about 1 meter per thousand years in many cases –and even two or three times that under optimal circumstances.  These rates are for uncompacted sediment, and a great deal of compaction occurs as the sediment turns into rock.  Additionally, if the deposit is to accumulate to any significant thickness, the crust on which it is deposited must also subside.

Thousands of feet of limestone, deposited during the Cambrian Period, are exposed in the Death Valley region. Click here for a slideshow of Death Valley geology

 

So all this limestone in Death Valley was deposited as a bunch of horizontal layers in a shallow marine setting –not too deep, or light wouldn’t penetrate to the seafloor to allow photosynthesis –key to the ecosystem that produced the calcium carbonate in the first place.  And since it was deposited, it’s been uplifted and tilted and eroded.

It’s about time

It’s about time that I started a blog.  Geology and Geologic time are so visual–they lend themselves beautifully to photography.  And those are things that I feel very passionately about.  So that’s what I plan to do here… post the occasional photo of something geological and discuss geologic time.

Why is an understanding of geologic time important?  Briefly, it’s important because it gives us perspective on resources, environmental degradation, and perhaps most important, what it means to be human.  Resources form at geologic rates, but we humans use them at human rates.  Similarly, environmental degradation will eventually heal… at geologic rates –but we cause the degradation at human rates.  And with respect to our humanity?  Seems to me that to realize human beings have been around for only a tiny fraction of Earth History naturally gives us some humility –something we could all probably use.

I plan to take a visual approach to geologic time… through photographs.  There’s no need to discuss radiometric dating, because one doesn’t radiometric dating to show that the Earth is inconceivably old.  You only need to think about the processes involved in forming a geologic feature and an open mind.  Many of these photos come from my website: marlimillerphoto.com –you can download over a thousand geology images from there for free.

Thousands of feet of sedimentary rock, exposed in the canyons of SE Utah, attest to great lengths of geologic time. This particular canyon is in the Needles District of Canyonlands National Park.

So when you look down a canyon, like the one in this photo, you’re looking at unmistakeable evidence for great lengths of time… it’s not that the canyon itself took so long to form (maybe 100’s of thousands of years?  More?  Less?), but the rock itself, made of layer upon layer of sediment, deposited in different environments —That’s where we can get a glimpse of geologic time.

More later… thanks for looking!
–Marli

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