geologictimepics

Geology and Geologic Time through Photographs

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.

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2 thoughts on “Igneous Rocks

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