Thursday, August 7, 2008

Some of these rocks are not like the others...

I've been out scouting field trip sites for my intro class, and I want to test some observations. Non-geologists, I need your help! I've got four pictures here. The last two are of the same rock - the second is just a close-up picture taken a few inches from the first.

I'm curious what you see when you look at these pictures (aside from my office keys, which were the handiest thing available to use for scale). How are the rocks similar? How are they different? Any speculation on why they're different?

Rock #1:

Rock #2:

Rock #3:

Rock #3 up close:

(I'm asking because I'm thinking about how to get my students to look at the rocks like geologists do. My eyes have been trained to see certain differences, but I've found that my students don't see them. Maybe if I know what other people see, I'll be able to ask better questions in my lab.)

(Oh, and Mom: no, I didn't leave the keys on the outcrop. They are... ummm... oh, yeah, they're here on my desk.)

PS: I'm more interested in what you see than what you would call each rock. (The students won't have learned the terminology yet. I'm curious what they might see before they get caught up in memorizing new words.)


Anonymous said...

Ok. So, I am a geophysicist with no geology training (I am coming from physics) but with a sufficient number of years in Earth Sciences to be a little bit biased. But not so much. Here is what I see.

My first reaction was that the rocks look the same. They all seem to have "some pink" in them. But then I saw that the grain size appears different, larger in rock #1 than rock #2. Rock #3 seems to have a larger number of different "stuff" in it and the grain size is more variable. While they all have "some pink", #1 is distinct because it has a clearly black element.
mmm... now I feel a bit stupid not to know more about rocks but I am not a geologist after all. I leave that to my spouse :-)

Jude said...

I took intro to geology at, of all places, FLC back in the 1970s (btw, you have the best blog title *ever*). Besides grain size, it seems as though rock 3/4 is a conglomerate, so maybe that makes it sedimentary (see how long it's been since intro to geology?) Rock #1 looks as though it was metamorphosed because the grains are smooth. I don't have any opinions about rock #2--well, maybe it looks more jagged than rock #1. I will be amazed if this is at all useful.

Anonymous said...

I have taken intro geo..., Why do I want to use the word pyroclastic? I have no ides what that means really, guess I'll go google it now.

same materials, just cooled differently?

Anonymous said...

k has white, black and pink parts, with the white outnumbering the others. The pink piesces are more irregular and smaller than the pink pieces in the last two pictures. The last 2 picutres show a more granular rock with larger pink parts and fewer white parts. As should be obvious, I'm not a geologist, nor have I had any geological training.

Mike Clinch said...

Good luck getting your students to see the differences. By any chance, can you find a location where rock 3/4 is in contact with rocks 1 and 2?

I AM a geologist, and can clearly see that the angular, interlocking crystals in rocks 1 and 2 are different from the sub-rounded particles in rock 3/4. There's also a difference in crystal/clast size, large in 1, smaller in 2, and a mixture of large and small in 3/4.

The thing to point out to your students is that the overall composition of the different rocks (granite vs. arkosic conglomerate) isn't as significant as the other features that show different histories of the rocks.

Kim said...

Mike - I think the contact is in a little drainage without much exposure, unfortunately. There are other places in the area (with a different rock as the basement) where the relationship is very nicely exposed. (Actually, I posted pictures of one of them last spring.) I want to take them to this spot instead for a number of reasons: 1) they'll be coming back to this spot to study a river, and I want to keep coming back to the same area in hopes that it will seem familiar later on, 2) I think rock 1 is one of the prettiest rocks in the area, and 3) I want a spot where they can identify minerals in different kinds of rock, and the grains in the arkosic conglomerate are big and are different colors, which makes them easier to tell apart for students.

I'm thinking of making them make labeled sketches at each stop. (And then sketching what I see on a big white board at the end of the stop, so they get immediate feedback about what geologists notice.) We'll see if that helps them see the differences.

And I would say that the texture is more important than the minerals for the big picture of how the rocks formed, but the question of "why are the minerals the same?" is interesting as well.

RBH said...

Total non-geologist here.

Well, the first two obviously differ in grain size, which I reckon would suggest different cooling regimes if they're igneous, which I'd guess they are. If they're metamorphic the difference might be associated with peak temperature they reached as well as cooling rate. Also, the grain differentiation -- contrast -- is sharper in the first than in the second, which I guess is consistent with either the cooling rate or the peak temperature difference, too.

In the third the pink inclusions strike me first, though in the close-up it's the crystal-looking inclusion that's more striking, along with the contrasts -- black, red, crystal, grey, even some amber.

Old Bogus said...

I'm not a geologist but have enough knowledge to be dangerous to all rocks!

The first two pictures look much like varieties of the Pikes Peak Granite upon which our house sits.

But the other stuff looks like some kinda of granitic melt in which previously formed crystals got trapped.

sarah said...

They look like something that you'd see on a kitchen bench top or the side of a building ... but I am a geologist so I notice the sides of buildings!

Hank Roberts said...

The first one looks like it cooled and parts separated out -- the dark stuff is fairly evenly spread through the lighter stuff in bits all about the same size. (igneous?)

The second one looks like it collected a lot of bits from various different sources then got squoze into a solid rock -- each little chunk has a discrete edge, some of them look rounded (metamorphic?)

Hank Roberts said...

P.S., let me recommend:
Mine's the one with the ruler.