geologictimepics

Geology and Geologic Time through Photographs

Archive for the tag “science”

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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