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Circles Through Rocks
Name: Tim
Status: Educator
Grade: 6-8
Location: WA
Country: United States
Date: Fall 2009
Question:
I was talking with my students about being curious as a
scientist. On the beach in Washington state, I found a number of
rocks which had definite circles around and through them. They
appear to be made of different minerals than the original rock? What
causes this to occur?
Replies:
Without actually seeing the samples, it is difficult to ascertain what you may
have. There are several possibilities, ranging from a weathering phenomenon
to volcanic origins. However, since you specifically mention circles rather
than circular or near-circular structures, I will wager a guess that you may
be looking at fossils, perhaps crinoids plates and stems. I think if you search
for crinoids, you may find some pictures to compare to your samples and find out
more about them. If they are crinoids, you likely have limestone, a sedimentary
rock.
If you have a university with a geology department or a museum with a collection,
you may want to contact them and show them your find to discover more about it.
You can have fun with the mystery! You and your students can make this a science
exploration project, coming up with possible scenarios, while you try to get "the
real answer." This is how geologists figured all of this out---what came first,
what came second, what sort of environment, when, etc. Since you found the samples
on a beach, you do not have the benefit of origin and location, but you can still
do a lot of exploring.
Good luck with your mystery!
Patricia Rowe
Without seeing a photo of the specimens it is a bit difficult to say. But, , these
could be fossil shells. They could also be layers of different minerals which cut
the rocks and now look to be circular. Lastly, if the rock is sandstone and the
circles are a darker color, they could be deposits of iron oxide within the
sandstone.
R. W. "Bob" Avakian
Instructor
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology
Tim
Depends on the texture of the rock.
I suspect the rock you found on the beach has a sandy or granular character.
And if you cut through the rock you will see the rings on the outside are a
reflection of a rock made of different layers. The layers are a sign of a
sedimentary rock.
Sedimentary rock starts out as layers of sand or fine earth that are laid
down on the bottom of a lake or sea bed.
Then as more sediment layers build up on them over many years, the sand is
compressed into rock.
Then the rock gets broken up, like from wave erosion on a sea coast. And as
the broken rock rolls around in the surf it becomes round in shape like
river rocks that roll down streams over great distances.
That is the best estimate I can give, given the information in your
question.
Sincere regards,
Mike Stewart
OK, I will give it a good guess based on the pictures. You may still want to
contact someone who can actually look at the samples and ascertain the rock type.
You are NOT looking at crinoids plates, or likely any other fossil.
I think you are looking at an interesting weathering phenomenon. This is what I
think:
Rock formed as a rock bed. I cannot tell from the picture with certainty what
your rocks are, nor the type (igneous, metamorphic, or sedimentary). However,
the process I am describing here is the same.
At some point, the rock bed developed cracks.
Minerals in solution (perhaps from dissolution of other rocks, or parts of the
same rock (I think this is likely---see the next paragraph), or related to
volcanic activity) percolated through the cracks, and recrystallized over time.
I decided this was likely because the rock type is the same on both sides of the
crack. The rock had to exist first, with the vein coming later.
As the rock bed weathered and eroded, the smaller and smaller pieces tumbled,
probably down mountain streams and later in ocean waves (you found them on a
beach). They happened to weather across the vein. If the vein were a completely
different type of mineral, it would likely weather differently (either etched out
or sticking up out of the rock matrix). Since it seems to be weathering at the
same rate, I think you have a mineral that crystallized out of the matrix which
contains the same mineral.
If my educated guess is correct, the circles are an interesting coincidence.
There are probably MANY more rocks without the circles, but you were drawn to
these because they looked more interesting (you could have a discussion about
sampling procedures. In fact, concerns about bias in collecting samples almost
kept NASA from sending a geologist to the moon).
Hope this helps. Enjoy your rocks!
Patricia Rowe
Without actually seeing the samples, it is difficult to ascertain
what you may have. There are several possibilities, ranging from a
weathering phenomenon to volcanic origins. However, since you
specifically mention circles rather than circular or near-circular
structures, I will wager a guess that you may be looking at fossils,
perhaps crinoids plates and stems. I think if you search for
crinoids, you may find some pictures to compare to your samples and
find out more about them. If they are crinoids, you likely have
limestone, a sedimentary rock.
If you have a university with a geology department or a museum with
a collection, you may want to contact them and show them your find
to discover more about it.
You can have fun with the mystery! You and your students can make
this a science exploration project, coming up with possible
scenarios, while you try to get "the real answer." This is how
geologists figured all of this out---what came first, what came
second, what sort of environment, when, etc. Since you found the
samples on a beach, you do not have the benefit of origin and
location, but you can still do a lot of exploring.
Good luck with your mystery!
Patricia Rowe
From
http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry13.html
There are three types of rocks:
Igneous rocks are crystalline solids which form directly from the cooling of
magma. This is an exothermic process (it loses heat) and involves a phase
change from the liquid to the solid state. The earth is made of igneous rock
- at least at the surface where our planet is exposed to the coldness of
space. Igneous rocks are given names based upon two things: composition
(what they are made of) and texture (how big the crystals are).
Sedimentary In most places on the surface, the igneous rocks which make up
the majority of the crust are covered by a thin veneer of loose sediment,
and the rock which is made as layers of this debris get compacted and
cemented together. Sedimentary rocks are called secondary, because they are
often the result of the accumulation of small pieces broken off of
pre-existing rocks.
The metamorphics get their name from "meta" (change) and "morph" (form). Any
rock can become a metamorphic rock. All that is required is for the rock to
be moved into an environment in which the minerals which make up the rock
become unstable and out of equilibrium with the new environmental
conditions. In most cases, this involves burial which leads to a rise in
temperature and pressure. The metamorphic changes in the minerals always
move in a direction designed to restore equilibrium. Common metamorphic
rocks include slate, schist, gneiss, and marble.
What I believe you have here are three sedimentary rock pieces that rolled
around in the surf smoothing their shape into an approximately oval shape.
The white rock is made from white sand. The orange line through it is a
sedimentary boundary where one layer ends and another layer begins.
The three gray rocks are from a sand that has a higher clay and mud content.
The ring around the rock on the left looks like a fracture rock rather than
a sedimentary boundary. The bottom gray rock also looks like a fracture,
but it could be a sedimentary boundary. The top gray rock could be either.
Only way to tell for sure is to take a hammer and crack the rocks open.
Sedimentary rocks will crack along the straight and smooth boundary. The
fractures will be more of an irregular shape than a plane.
Washington state has an on-line geology portal that looks like a lot of fun.
It can be found at:
http://www.dnr.wa.gov/ResearchScience/Topics/GeosciencesData/Pages/geology_p
ortal.aspx
Maybe you can hook up with some of these folks and learn more about the
geology of your state.
I would also like to share this site with you, which hosts images of
different rock types.
http://images.google.com/images?client=firefox-a&rls=org.mozilla:en-US:offic
ial&channel=s&hl=en&source=hp&q=rock+types&um=1&ie=UTF-8&ei=JLu_SoKSH8Sa8Abx
18ygAQ&sa=X&oi=image_result_group&ct=title&resnum=4
Sincere regards,
Mike Stewart
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