When You're Doing Geology, You've Got To Break a Few Rocks

Had you been walking along the trail at Chesterfield Gorge in New Hampshire last Sunday, you might have come across this scene:

Dr. Evelyn Mervine doing the geologist Hulk smash thing.

That is my dear friend, fellow Geokittehs blogger, Georneys author, and PhD survivor, Dr. Evelyn Mervine, demonstrating the lengths a geologist will go to when both geologists on the hike had to leave their rock hammers back home. She couldn?t schlep hers from South Africa, and the TSA wouldn?t let me carry mine on the plane. Some government entities have no appreciation for geology. Sigh.

So why? Why is the Doctor throwing one rock at another, rather larger, one, whilst her companion shoots the proceedings in burst mode and curses the low light? Geologists take this sort of thing for granted. Non-geologists think we?re a bit mad. But there?s method to the madness.

I used to be a bit horrified by geologists? propensity for attacking rocks with hammers (or, in a pinch, other rocks). You?ve got this lovely weathered bit of stone, but instead of admiring its beauty, you?re doing violence to it. Why? Why break rocks? I?m sad to say it took me a bit of time to appreciate the necessity of fresh surfaces, but Lockwood once drove the point home beautifully by cracking open a cobble. He took a plain ol? gray hunk of rock and revealed something really fascinating.

Porphyritic Basalt, from Cape Perpetua, Oregon.

If you look at the rock to the left, you see what the external appearance was: just a bland gray blob. But when Lockwood cracked the thing open with his hammer, it sprang out with vivid color and white speckles. There was no way to tell from the weathered surface what it was, no indication at all that this was porphyritic basalt. This is the moment that I learned, beyond geodes, what delicious secrets rocks can hide within their severely plain exteriors.

Geologists are very big on obtaining a fresh surface for just this reason.

Our Hulk Smash rock, showing both weathered and fresh surfaces.

Take the rocks in Chesterfield Gorge. They are, as Evelyn described them, mossy brown rocks. That?s all you can tell from the exterior. Weathering and biological hangers-on have obscured their true nature. Everything outside becomes a rather dull, homogeneous mass. You can safely say it?s a rock, but that?s about it ? until you have a look inside. Then you discover its secrets. In this case, we discovered we were looking at slightly metamorphosed granitic rocks, on their way to being gneiss.

That's not quite the schist, but it's almost gneiss. Ah-ha-ha.

A different view of the same rock. Notice the texture looks different from this angle: I'll have an explanation in a future post.

We packed some out of the Gorge and brought them back to our lakeside geologist lair for further investigation.

Samples all ready for the breaking. Kids: do not ever try this with a regular hammer unless you are a geologist and desperate.

There?s our gathering of plain ol? rocks. Also, a hammer. Note that it is not a rock hammer. We were desperate. When you are engaging in the good science of rock-breaking by breaking some rocks, it?s best and safest to use a certified geological hammer. They?re specially tempered so that they don?t explode into awful metal shrapnel. Our rocks were, for the most part, soft enough to risk a regular hammer, but this is absolutely not recommended. This is just an example of the lengths geologists will go to in order to obtain a fresh surface.

Doctor Evelyn applying some tough love to a hand sample.

You should also wear long pants, incidentally. Evelyn?s done this often enough she didn?t pepper herself with rock shards, but trust one who has managed to lacerate herself playing weekend geologist: protective clothing is an excellent idea. You will note the wrap-around sunglasses employed as eye protection. Eye protection is key. Never ever, no matter how desperate you are, attempt to obtain a fresh surface without protecting your eyes. Skin heals. Eyes don?t.

With that public safety reminder accomplished, let?s get to the fresh surfaces.

Evelyn brings the hammer down, and our rock shatters in a cloud of dust.

Those rocks didn?t stand a chance. And here we have our fresh surface.

Moi holding our fresh surface up in triumph.

Have a look at that beauty! From dull brown rock, we have achieved super-sparkly goodness. Look at those round black patches of biotite! See how the minerals have begun to reorganize themselves! This granitic rock has gone through some changes. It was once a fairly even salt-and-pepper, but now the quartz and feldspar and biotite have been subjected to enough heat and pressure to begin the journey toward gneiss. Evelyn and I are classifying this as a type of metagranite, but perhaps someone more expert in metamorphism of terrestrial rocks will chime in and give us a better idea of what we?re looking at.

Close view of metagranite samples

Macro of metagranite.

See what a fresh surface can reveal? This is why I?ve taken up the rock hammer (except when the TSA won?t let me) and wield it now with glee, and why you will see geologists going to (still safely constrained) extremes to break through weathered exteriors to the beauty within.

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