Questions tagged [rocks]

Questions about rock types, formation, chemistry, and physics. If you just want help identifying a rock please use the tag "identification-request".

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19
votes
4answers
15k views

What was the first rock in the rock cycle?

I am an undergraduate student but I am a tutor at a High School, and one student asked me. Attempting to explain the rock cycle "if one rock turns into the other then which came first" my gut is ...
16
votes
2answers
48k views

Are the processes of the rock cycle currently in dynamic equilibrium?

The standard representation of the rock cycle usually looks like this: Does each stage of the rock cycle currently balance? That is, are the various processes in dynamic equilibrium, so that the ...
14
votes
2answers
888 views

How to learn the different types of rock

I would like to be able to point to a piece of rock and say that is rock type such and such. I have tried learning but it literally seems like an endless list of types and sub types and the pictures ...
14
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1answer
224 views

How are the patterns in solid rock born

I took this photo on my vacation in Finland showing white stripes in the rock going different ways. How are these patterns born?
12
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1answer
7k views

How are giant sinkholes filled up?

There are a few impressive photographs of giant sinkholes that appeared in cities, such as this one: However, followup information (e.g. Guatemalan sinkhole) or photographs on how these kind of ...
12
votes
1answer
485 views

Formation of limestone without living organisms?

Limestones are formed mostly from skeletal fragments of marine organisms. However, calcium carbonate dissolves naturally in water, so the living organisms are not the only source of it in water. Is ...
11
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4answers
3k views

What sort of rock is the large polished boulder? [closed]

This large polished boulder sits outside one entrance the University College Hospital in London: Here is a close up: What sort of rock is it and how would it have formed?
11
votes
2answers
2k views

Zeolites, odor control, and the witchcraft of sunlight

Various companies offer "volcanic crystals" or "volcanic rock" available in small mesh bags for odor control. The best description I have found is that these are made up largely of Zeolites. All of ...
11
votes
2answers
290 views

Can rocks have pressure signatures?

Since rocks can have magnetic signatures, as is the case with paleomagnetism, can they store past pressures also? For example, when researchers are trying to determine past ocean levels, they ...
11
votes
3answers
1k views

Identifying sedimentary rocks in hand specimen

I'm currently learning how to identify rocks in hand specimen. The initial classification into igneous, metamorphic, and sedimentary is mostly ok, but I'm having a problem with further identifying ...
11
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2answers
4k views

What is the oldest sediment?

Sedimentary rocks in Akilia or at Hudson Bay have been proposed as the oldest in the world. My question is not related to rocks, but rather to sediment while still forming sedimentary deposits but ...
10
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2answers
16k views

How long does it take for a stone to alter?

We see stones and they appear to be the same throughout our life time. Mountains also appear to not significantly change. So do mountains and stones decompose? What is the life span of a stone? After ...
10
votes
1answer
476 views

Are “monoliths” giant, isolated rocks sitting in dirt, or are they continuous with bedrock

I am at a loss for the precise, correct terminology, but I have read in Wikipedia that Uluru is not a monolith, but Savandurga is, and that I probably shouldn't be using the word monolith to begin ...
10
votes
1answer
829 views

What kind of coal did I find? [closed]

I found a piece of (what I believe is) coal, on Cape Cod last week. Its density is 1.43 grams per milliliter. With this information is it possible to know whether it's Anthracite, Bituminous or ...
9
votes
3answers
645 views

What do you call boulders of non sedimentary rock that were lithified into sandstone?

I'm convinced there is a word for this. I was in the Hoodoos at Writing on Stone this weekend and kept noticing what looked like reddish quartzite boulders laying around in the sand, or sometimes ...
8
votes
1answer
906 views

Rock ID: brittle black/white w/ crystals & mica-like cleavage layers, Raleigh, NC, USA [closed]

Where it was Found: I found the rock laying on the sidewalk near some landscaping at my apartment complex in Raleigh, NC. Size: The largest intact portion of the rock is about 2.2 cm in diameter. ...
8
votes
2answers
650 views

What kind of a rock is this (from Finland)? [closed]

I found this rock at my friend's summer cottage in Finland. I have never seen a rock like this in my life. It resembles a lava stone, but Finland isn't a volcanic country. I am no expert in geology ...
8
votes
1answer
578 views

What kind of rock is this gray and brown weathered cylinder-shaped rock I found near a New York lake? [closed]

I found this rock on the beach of a freshwater lake in the Finger Lakes region of New York state and am interested in knowing more about it. It is about 2" tall and 2" wide at widest point. Weighs 14....
8
votes
1answer
292 views

What is the global distribution of oil filled geodes?

In a general 40 mile radius of Keokuk, Iowa many sedimentary geodes can be found. They are abundant in the Warsaw Formation with the type section at Geode Glenn in Warsaw, Illinois. In one specific ...
7
votes
4answers
744 views

How is this crumbly crystal-rich rock formed?

I found this crumbly rock embedded in the channel walls of a small creek in Everett, WA, USA. (North Creek that flows through McCollum Park.) If possible, I'd like to know how the rock formed... what ...
7
votes
2answers
2k views

Rock ID: Geode, soft, dull black exterior with crystal inside, from Snæfellsnes, Iceland [closed]

My fiancée found this on a recent trip to Iceland, on Snæfellsnes peninsula - she spotted the sparkle of the crystals among the abundant black, grey, and dark red rocks. There is a roadside vista spot ...
7
votes
3answers
2k views

How was this egg shaped rock made and what is it? [closed]

The following small egg-like rock was found laying on the ground in Tampere, a city in southern Finland. It weights about 950 grams, has a volume of about 360 cm3 and thus a density of about 2600 kg/...
7
votes
3answers
355 views

Could one say that granites are metamorphic rocks, or not at all?

In my old textbooks, granites used to be part of the metamorphic rocks. Is this still true or not ? If not fully true, to which extent ? Could someone point me towards clear examples or definitions ...
7
votes
2answers
174 views

Recreating the rock cycle

I want to recreate the rock cycle using some item that can behave as a rock. The purpose of this is to get a point across to my students about how really there is a reason why the rock cycle is called ...
6
votes
1answer
905 views

How can a non-geologist identify a rock? [closed]

I found a rock on a beach in north-east Evia, Greece. Half is light gray, and half is dark gray. There are white (or very light gray) thin lines that form swirls throughout, crossing the light/dark ...
6
votes
1answer
788 views

Rock identification: meteorite or lava? [closed]

I was told by a crystal quartz store owner it was a type of lava rock but I have high doubts. hardness: diamond size: 4'' $\times$ 4'' weight: 1.5 lb location: unknown smell: sulfur trait: ...
6
votes
2answers
70 views

Curious natural patterns on the surface of basalt blocks that make up the sidewalk

During one of my walks through the city streets, I noticed that some basalt blocks that make up the sidewalk have in their surface some very curious natural patterns: The photos above was taken at ...
6
votes
1answer
660 views

What are the blue spots in this rock? [closed]

The rock is 4 inches (~10 cm) long. The vendor claims this rock is from K2. Are the blue spots real or painted on?
6
votes
1answer
545 views

What kind of rock is this (from UK)? [closed]

What kind of rock is this? It was found in Threemilestone, near Truro in Cornwall, UK. The rock has a hardness of over 5.5, as a knife doesn't scratch most parts of it. It is 10cm across the top, ...
6
votes
1answer
444 views

Is this iron ore? [closed]

Is this iron ore? I found it in the back yard in Western Australia few years back; it has a roughish surface. It was sticking out of my grass. It has sand grains stuck on it. It is reasonably hard, ...
6
votes
2answers
2k views

Why are most minerals so rare?

There are 4,660 known minerals, so why are most of them so rare? Is it to do with the structure of minerals?
6
votes
1answer
1k views

How to identify a predator's coprolite from an herbivore's?

I have a coprolite that is 8" by 5" and weighs 8 lbs. The shape is similar to a lumpy brain, the colors below outer crust are deep red and the crust is pastel green and grey. It has a strong density ...
6
votes
2answers
113 views

Is there a widely accepted reason for the formation of tafoni?

Expansion of tafoni seems to be based on weathering (seems reasonable enough.) But what creates them in the first place? There are a number of explanations online (Wikipedia lists eight plausible ...
6
votes
0answers
164 views

Basalt vs. columnar basalt

I recently visited a cliff on the south coast of Ulva in the Inner Hebrides where there are ~10m thick flows of columnar basalt. The columnar basalt forming most of the cliff is sandwiched between ...
5
votes
1answer
333 views

What are the circular features in this rock? [closed]

I found this rock by the shore of Lake Ontario. It's light weight and it was grouped with some other rocks with the same patterns.
5
votes
4answers
6k views

Why is coal classified as a sedimentary rock?

Something just clicked in my head today. Rocks, by the definition I was taught, are composed of minerals which in turn are by definition not organic, so they are or were not alive. However. Coal, ...
5
votes
3answers
242 views

Making a local geological field trip an interesting and educational event for children?

I have no expertise in geology or earth science, but I would like to do a "field trip" with my home-schooled young children (under age 10) where I could teach them about geology from a hands-on ...
5
votes
1answer
104 views

There are cloud identification guides, are there also (guided) rock identification websites?

Seeing all these rock identification questions, got me wondering, is there no straightforward guided rock identification website? In Intro to Geology lab, I remember going through a bit of a process ...
5
votes
1answer
125 views

Strange coating in old lime mine shaft

Near Prague,Czech Republic is old lime mine. Most of it is closed but one short transport tunnel under one hill is publicly accessible. Tunnel has profile around 2,5x2,5m and it's slightly bended and ...
5
votes
1answer
118 views

Identification of limestone type

These are images of a stone found and used extensively in the Jammu & Kashmir region between India and Pakistan. These are called devir or devri in the local parlance. Following is the ...
5
votes
1answer
373 views

What is this yellow stone [closed]

I found this yellow stone in a creek.It was easy to break by slamming it against another stone and has evenly spaced crystal clear grains.They are very clear that under the sun I thought that water ...
5
votes
1answer
383 views

I'm trying to find the name of this rock [closed]

I found it upstate New York... Can someone please help me identify this rock. I added photos with different lighting. It's a very dark purple with a mix color showing more in the center. Its very soft ...
5
votes
1answer
152 views

Identification: mystery rock with golden features and shimmer [closed]

I found this rock in our backyard in Germany. As our garden is filled with bushes it was very oddly placed and I have carried this rock around with me ever since, wondering what sort of rock it might ...
5
votes
1answer
803 views

What type of sparking stone could this be? [closed]

A few years ago, I went to a beach in the south part of Chile, specifically Lake Lanalhue, it was filled with special rocks that when you picked one up and threw it against another that you would see ...
5
votes
1answer
219 views

What could have formed the pattern inside this piece of siltstone?

I have what i think is a piece of siltstone with an interesting pattern on the face. It looks like a lichen or lightning bolt, as shown in the picture. The interior of the pattern is the same color as ...
4
votes
1answer
75 views

Unidentified mineral found in France [closed]

(I am not a native english speaker, so I'm not really aware of the specific terminology.) A few years back, I found an unidentified rock on a beach in Normandie (France). It looks and feels (texture-...
4
votes
2answers
823 views

Can you confirm if this rock is sandstone - it is this from Cape Cod, MA, USA [closed]

A few years ago I found this unusual looking rock while walking around the beach of Cape Code, MA, USA. My guess is that it is sandstone, but I didn't see any others like it on my stay. It is has ...
4
votes
2answers
866 views

How this sedimentary rock is made?

Apologies in advance for a beginners question. Different rock types make me curious and recently went to an area where interesting sedimentary rock is all over. As the area is on the coast, sea has ...
4
votes
1answer
424 views

What rock is this? [closed]

I've had this little guy for a while now and have been trying to identify it for some time with no luck. It looks like its oxidized with tiny little crystals, but it's hard to tell for sure. Any ...
4
votes
2answers
191 views

Help me to identify this rock, please, [closed]

Can anyone tell me what this rock is? It was found on the rocky beach at 'Glen Boreraig' on the Isle of Skye, Scottland. Definitely water-worn. It is 9.5 cm x 4.5 cm with inner darker part 4.5 cm x ...