posted
Small question for some of you more scientifically inclined Hatrackians: What would happen to a large boulder if it was struck by lightning? Would it shatter, explode, just wind up with a burned spot?
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posted
Most of the time lightning striking rock leaves no visible trace.
Lightning strikes Yosemite's Half Dome all the time, and leaves no visible trace. I read one account where a group of five hikers who were caught in a sudden thunderstorm after summiting. They scrambled down a cave -- more of an overhang -- and lay huddled there. Lightning hit the peak, and they were all killed except for one hiker who was crouched rather than prone. There was no physical evidence of the strike other than the dead bodies.
It make sense. Think of all the bald mountains you see; they're hit by lightning all the time, but you don't see scorch marks or shattered boulders lying around.
It doesn't mean lightning *can't* split a boulder. Suppose the boulder had a deep crack and water had wicked up into it; the water flashing to steam could split the rock.
Not far from where I live a woman had a lightning strike that split a tree, excavated a ditch ten meters long and about a meter deep (leaving mounds of dirt on either side), and then demolished her porch. The firefighters who arrived didn't even recognize the ditch as being an act of God; they asked the lady why she was excavating her front yard. All the spectacular damage was no doubt caused by water flashing to steam.
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posted
Hmmm interesting. I am asking because I have a story wherein a mage lobs a lightning bolt at a boulder as a demonstration of offensive magic.
Now yes I realize magical lightning can do whatever, but in most of my stories I try to keep magic that involves physical forces as logical as possible...if a wizard hurls a bolt of electrical energy I want it to behave as such a thing really would. Perhaps a different test subject is in order.
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quote:Originally posted by Merlion-Emrys: Now yes I realize magical lightning can do whatever, but in most of my stories I try to keep magic that involves physical forces as logical as possible...if a wizard hurls a bolt of electrical energy I want it to behave as such a thing really would. Perhaps a different test subject is in order.
C.S. Lewis discusses this in his book *Miracles*, which of course is about divine intervention, but the logic remains the same. He calls this process "naturalization" in analogy to an immigrant becoming a naturalized citizen of his host country. Once a miraculous event occurs, it is *naturalized*; the consequences have to follow mundane rules of cause and effect.
This is good rule for writers of fantasy too. Maybe your wizard smites a rock, and the tree next to it explodes into splinters. Point made: supernatural actions have powerful natural consequences.
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posted
I'm not a geologist, so please take that into consideration, but here is my take.
I think it would depend on the chemical composition of the rock and where the lightning hit and a hundred other variables, but a boulder could explode under the right conditions.
In fact, I found this pretty cool article in which scientists think that mountain tops may have been shaped by exploding rocks by lightning strikes, link here.
Really cool thing if lightning strikes a rock has a lot of silica in it, it can form a rock fulgurite which is kind of crude form of glass. This more frequently happens when lightning strikes sand. It is pretty cool. Here's a link and another one with pictures, but it is of sand.
Also just for fun, here is a cool youtube video of lightning striking rock that I found during my googling. I don't know how to tell if it is authentic or not, but it looks real to me.
posted
I've seen samples of fulgurite -- one was over 3m long. It *is* very cool stuff; like frozen lightning. What's interesting is that that while lightning strikes are pretty common, fulgurite is quite rare; it just shows how unpredictable the effects of a lightning strike are.
I once read of a case where a cyclist was struck by lightning. He had ridden past the entrance to a shopping mall when he was struck, and he literally did not know what hit him. He thought he'd just hit something on the road and gone of the handlebars.
As he got up, a woman jumped out of her car to help him. "Are you OK?" she asks.
"I'm fine," he said. "This happens to me all the time." Then he realizes that his helmet is in the ditch, cloven in two, and steam is rising off his sweaty cycling jersey. The lightning had passed over his skin rather than entering his body. The lesson is that magical lightning may be an impressive, "shock and awe" weapon, it's not quite as reliable as a crossbow bolt to the head.
It's interesting to think about the physics of why a tree that's hit by lightning explodes, but a rock usually does not. If you heat rock, it goes through a phase change where it becomes a viscous liquid (or perhaps a plastic solid), then a thinner liquid. These changes absorb a huge a amount of energy. The water in the tree's tissues undergoes a phase change from liquid to gas, and suddenly it wants to expand but it's trapped in the body of the tree.
If you pour a few grains of black powder onto a surface and light it, you get a flash, not an explosion. Black powder isn't a "high explosive". But if you contain that powder in a gun barrel, giving it no room to expand, you get a supersonic shock wave, aka "an explosion". Same thing with the water in a tree.
So in solid rock, heating in the solid phase causes a little physical expansion. It takes a lot more energy to change the phase of rock to liquid than it does to change water to steam (try melting gravel on your stove), and even if you have that energy the rock wouldn't expand dramatically.
Part of the reason fulgurite is relatively rare is that it's not always easy to find; it is under the surface and may be obscured by desert vegetation. But the conditions of the soil have to be right; too much moisture and you lower the resistivity of the soil too far to fuse silica. Other conditions get you a furrow blasted by steam. I think most of the time you get no obvious effect from a strike.
When I was little I saw lightning strike the street in front of my house; it didn't leave a mark. There's no telling why; there's too many possible factors in the path the lightning takes and the materials it passes through.
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posted
I've seen lightning split a tree---left a big char mark on what was left of it---but I've never seen lightning split a rock.
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posted
MAP, your post reminded me of SWEET HOME ALABAMA, where (if I remember correctly) Josh Lucas' character would stick rebar into the beach before a thunderstorm and then sell the resulting fulgurite as sculptures.
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posted
Natural lightning as understood today may not affect a boulder. Magic lightning, though may do whatever the "rules" of the milieu support. North American neolithic cultures coming into first contact with firearms thought the thunder lances where a gift from Michi Ginebig given to the Strangers who came from under the world wearing the pale faces of death.
Michi Ginebig is a New World sky spirit dragon analog, rampant (upright, erect), with forked horns, forked tongue, forked body, forked tail. Tailwalking across the land, invisible--only when the brilliant gem in its forehead flashed lightning and the forked tongue rumbled thunder could the great horned serpent be located. The Strangers from under the world carried lances that erupted with brilliant light and rumbled thunder, and invisibly struck whom the lightning touched with a deadly bite. Pale faces speak with forked tongue.
quote:It's interesting to think about the physics of why a tree that's hit by lightning explodes, but a rock usually does not. If you heat rock, it goes through a phase change where it becomes a viscous liquid (or perhaps a plastic solid), then a thinner liquid. These changes absorb a huge a amount of energy. The water in the tree's tissues undergoes a phase change from liquid to gas, and suddenly it wants to expand but it's trapped in the body of the tree.
If you pour a few grains of black powder onto a surface and light it, you get a flash, not an explosion. Black powder isn't a "high explosive". But if you contain that powder in a gun barrel, giving it no room to expand, you get a supersonic shock wave, aka "an explosion". Same thing with the water in a tree.
So in solid rock, heating in the solid phase causes a little physical expansion. It takes a lot more energy to change the phase of rock to liquid than it does to change water to steam (try melting gravel on your stove), and even if you have that energy the rock wouldn't expand dramatically.
This is very logical and a good hypothesis, but one thing I've learned is that when you don't know the results, it is easy to use a little knowledge to reason and speculate either way. But there could be many other factors that you aren't taking in to account. You don't really know if the energy of the lightning strike is all absorbed by the phase change (or how much energy is delivered) or how much the melting rock process would expand the rock or if that expansion no matter how small won't be strong enough to crack rock. You don't know if there is another way altogether for exploding the rock. So while it is a good hypothesis, it needs evidence to back it up. Surely the experts already have these answers.
Personal experiences and anecdotes (or sometimes a lack of them) are not evidence. Just because no one here has heard stories of lightning exploding rock doesn't mean it doesn't happen. Sure everyone has at least heard of few stories of lightning breaking trees, but humans are more likely to live near tall trees than near mountain tops or towering rock structures. So the probability of someone witnessing a tree being struck by lightning is just much much higher than someone witnessing lightning striking rock; thus, the lack of rock exploding anecdotes in no way indicates that these events are less probable than tree splitting.
Truthfully, I have no idea how likely it is. Maybe it is extremely rare, or maybe it is comparable to lightning damaging trees (which is still fairly rare considering all the lightning storms). But lightning can crack and explode rock, and from that article I linked too in my last post, scientists are starting to believe it plays a much larger role in shaping the landscape than previously believed. Clearly that article is good, solid evidence supporting my position.
But here are some more interesting articles I found both discussing rocks exploding from lightning strikes and evidence that is left afterwards.
This article specifically addresses how rocks explode from being struck by lightning. Here is an interesting quote.
quote:Physical impacts of lightning strikes on exposed bedrock result from the lightning's high temperature which can reach 30,000oC (54,000oF). Any moisture on the bedrock surface or within bedrock cracks is immediately superheated and evaporated. Air within the bedrock cracks is also superheated and expands in volume, forcing cracks to widen very rapidly and resulting in the rock exploding apart (Appel et al., 2006; Knight, 2007). The most obvious expression of a lightning impact is therefore 'exploded rock'. This can be identified by the presence of freshly-fractured rock surfaces that may not correspond to bedrock joints, and angular bedrock-derived debris that has been scattered away from a central impact point (Figure 2). Boulders of considerable size can be detached in this manner.
I find this fascinating. First of all, man that is crazy high temperature, that is a scary amount of energy. Second of all, superheated air expanding in cracks is all that is needed to make a rock explode. So Merlion, if your mage wants to do this, he should pick a boulder with some obvious cracks in it.
It is an interesting article all around and well worth a read.
This article has some nice anecdotal stories in it about geologists either witnessing lightning striking rock or surveying the evidence in the aftermath. It also has some more solid evidence supporting rocks exploding by lightning strikes as well. But I want to share a quote from it that gives a reason for why we don't have more personal accounts of exploding rock from lightning storms.
quote:Despite its spectacular effects, lightning's role in destroying mountains is likely limited to bare rock settings, such as high mountaintops. At lower elevations, trees and soils (or sheep) absorb the energy.
So it seems that the presence of trees helps to prevent rock explosions by lightning strikes by absorbing some of the energy. Since humans live near trees, this is another logical reason that there aren't a lot of visual accounts of lightning exploding rock.
In Merlion's situation, these lightning strikes wouldn't be random like we see in nature. If a mage wields the power to throw lightning, he could up the odds, by picking the right rock in the right environmental conditions and hitting it in the right spot with the right amount of power. I have no problem at all believing this mage could use lightning to make a boulder explode.
ETA: @ KDW, I've never seen that movie, but I'd love to own a fulgurite sculpture.
posted
Thanks for all that info, MAP. I haven't read all the articles yet but that is useful info to have access to. The particular scene I mentioned may wind up getting cut anyway, but I tend to use a fair amount of lightning so everything I can learn about lightning and electricity tends to come in handy sooner or later.
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quote:Originally posted by MAP: Personal experiences and anecdotes (or sometimes a lack of them) are not evidence.
I pretty much agree with everything you wrote. I'm an obsessive researcher. I have, for example, consulted historical weather records and nautical ephemeris data when writing a late afternoon scene that takes places on a certain place at a certain date. Researching literature on the geological effects of lightning strikes is exactly the kind of thing I'd do if I were in Merlion-Emrys's position. I'm not sure if that's always a good thing; perhaps in moderation is best.
I will disagree very slightly with what you've written above, in that personal experience and reliably documented anecdotes are quite valuable sources of information for a writer. They can tell you what *might* happen. For example, big lightning storms when I was a kid were preceded with a kind of tang in the air; that's a sensory detail I can use. But now that I'm older it doesn't happen. That could be because I live in the suburbs instead of the city; perhaps it had something to do with pollution. Or maybe I don't run around outside in thunderstorms. Or maybe my nose is getting feeble with age. But the phrase "lightning-strike tang in the air" appears in my writing when the focus character smells ozone.
What personal experience can't tell you is what can *never* happen. So if you've seen lightning strike a boulder with no visible effects, that doesn't mean that a boulder can *never* explode during a lightning strike.
As extrinsic and Merlion-Emrys himself pointed out, magical lightning doesn't even have to behave like ordinary lightning. I think the value in researching ordinary lightning is that it gives you options; colors to have on your descriptive palette. You can choose to have your magical lightning to act just like ordinary lightning, or totally differently. You can let those differences pass without comment, or with comment. Whatever serves your purposes.
posted
Personal experiences and anecdotes have meaningful, creative, personal color, flavor, emotional expression. Empirical scientific data is lifeless for many sensibilities. A contemporary legend with scientific or even fantastical inconsistencies may not in and of itself express evidentiary facts, except for the one most intriguing one; that is, this is what the observer meaningfully perceived, interpreted, and expressed.
For example, does lightning cause violent behavior in rocks? What kind of rock? A porous rock absorbs water, may contain other compounds that react violently when superheated, and will explode. Porous rock is uncommon on suitably electrically grounded promonitories because it erodes comparatively rapidly. Harder rock, resistant to weathering, and most common on high promonitories, isn't as porous. The hardest and least porous rock is even rarer on promonitories, mostly occurring in sea beds.
Though a boulder may not explode, it may exhibit other violent reactions to a lightning strike. Lightning and rainfall may not always occur together, though a large fraction of the time they do. A wet rock will conduct electricity along its surface, following the path of least resistance. Along the way, the heat of resistance may flash melt surface rock. Fulgurites are a variety of lechatelierite in quartz soil that's been partially melted, mostly the silicates that become glass-like. Some vein-like markings seen on promonitory rocks are silicate glazing from lightning strikes: lechatelierite. Tektites are also lechatelierite, though formed when meteorites are superheated by atmospheric friction or collision impact.
So envision a high, wet boulder struck by lightning. Violent steam. Filaments of hot, glowing glass streak to the ground web-like. The wet ground's sandy soil blasts upward. Odds of the superheating event causing the boulder to crack aren't high but not impossible, not if the boulder has weak fracture lines. But this is imagination, not personal experience or even an expressed as yet anecdote. Possible though. And ripe for fiction if not creative nonfiction. Science? For those who dare to hypothesize and prove scientifically beyond a shadow of doubt.
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Merlion, I hope it is helpful. I was happy to look into it, really interesting.
quote:I will disagree very slightly with what you've written above, in that personal experience and reliably documented anecdotes are quite valuable sources of information for a writer.
I never said nor meant to imply that personal experience and anecdotes don't have value. They certainly do. I only said that they aren't evidence.
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