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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17
votes
4answers
14k 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 ...
15
votes
2answers
45k 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
850 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
votes
1answer
219 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
6k 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
456 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
votes
4answers
2k views

What sort of rock is the large polished boulder?

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
288 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
votes
2answers
3k 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
votes
1answer
453 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
763 views

What kind of coal did I find?

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 ...
10
votes
2answers
176 views

Strange scar marks on pebble

My question is not specifically about identification of this piece of rock. Instead I am focusing on the origin of the scar marks on this rock. I have found this stone in the pebbles in the path. This ...
9
votes
3answers
603 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 ...
9
votes
1answer
847 views

Rock ID: a black rock/lava? around another white rock, unknown location

I tried to look for any similar rocks on the Internet but couldn't find anything and I don't know much about geology. I've been given this rock when I was a child and now I'm just curious to know ...
8
votes
1answer
1k views

What is this brown and white rock?

We found this rock in our backyard. Could someone please tell me what it could be?
8
votes
2answers
14k views

How long does it take for a stone to decompose?

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 ...
8
votes
1answer
616 views

We found this rock and we don't know what it is

I found these rocks at a road in Sweden. They are different from the other rocks used for the road. The hardness is higher than 5.5 as the rocks aren't affected by a knife. the size is 2.0-2.5cm ...
8
votes
2answers
607 views

Identification of dark red, metallic mineral

I recently found this mineral crystallised within quartz at a rock crushing plant. Because of this I have no idea where this rock came from as material is transported to the site from a variety of ...
8
votes
2answers
667 views

What type of rock is this paving stone and what is the origin of the red colouration?

This paving slab is part of the floor of the new atrium in Kings Cross Station in London: The paving slabs are mostly grey but some contain areas of this reddish colouration. They are all fine ...
8
votes
1answer
813 views

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

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
630 views

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

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
779 views

What is this Cape Breton rock?

What is this rock? I found it in Cape Breton, Canada
8
votes
2answers
668 views

Rock ID: Large, heterogeneous, magnetic rock in Cascade Range, Washington, USA

I found this rock in a glacial till in north central Washington State, on the east side of the Cascade Range. The dark round crystals are roughly 1 inch (2.54 cm) by 2 inch (5.08 cm) in size. The ...
8
votes
1answer
530 views

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

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
259 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
2answers
2k views

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

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
1answer
499 views

Identification of a rock from Kentucky [closed]

I was hiking the other day in southeastern Kentucky and ran across this rock on a river bank. I've never seen a rock like this before. Can anyone here identify it?
7
votes
3answers
1k 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
332 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
3k views

Rock ID: smooth, black, durable rock from Lake Ontario

What is this black rock? I found it beneath the waters of Lake Ontario near the shoreline, it is a dark greyish-black with small flecks of mineral deposits on it that are visible in the light. The ...
7
votes
2answers
168 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
902 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
737 views

Rock identification: meteorite or lava?

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
1answer
576 views

What are the blue spots in this rock?

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
526 views

What kind of rock is this (from UK)?

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
393 views

Is this iron ore?

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
1k views

Basalt, slag or meteorite? [closed]

I found this rock on the edges of a known Drumlin in North East England. It looks to be a fragment from a larger piece. It attracts a magnet strongly, it weighs 150 grams. It has vesicular holes (on ...
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
66 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
160 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
4answers
574 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 ...
5
votes
5answers
1k views

What type of rock is this that is light colored with greenish crystals?

I found this rock. It is probably a very common rock but could an expert please tell me what rock this is?
5
votes
1answer
308 views

Identify rock with circular features

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
2answers
693 views

Is this iron ore? If so what kind? And is it naturally deposited in texhoma?

It's completely hollow on the inside and looks like a bubble while layered on the outside. Slightly rusted.
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
216 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
98 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 ...