Questions tagged [petrology]

Petrology is the study of rocks and the processes that lead to their formation and alteration.

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Does fold tightness correlate to deformation intensity?

I'm studying a migmatite with two overprinting fold structures. The first (Dn+1) is tight and the second (Dn+2) is gentle. Would it be fair to assume that, since Dn+1's tightness is, well, tighter, ...
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What are Shoshonites? What conditions make them occur instead of andesites?

What is the significance of shoshonites? Why in some convergent margins, do shoshonites occur instead of andesites?
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XRF analysis - recalculating results

For TAS diagram plot, one need to recalculate Major oxide data (NaO, K2O, SiO2). Do one need to recalculate the ppms of Zr,Nb and Y and % of TiO2 for Zr/TiO2 vs Nb/Y diagram plotting for rock ...
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Are there any differences between the subalkali and alkali rock occuring tectonic environments?

Here question ends. Beyond details exist Do they occur on the same tectonic regime? For a start to distinguish Subalkali rocks froms Alkali rocks, my knowledge says that mantle is the source of ...
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Marks on boulders of volcanic origin

I am working in a catchment in the southern Ecuadorian Andes, about 3700 m.a.sl. Rock outcrops at the upper part of the catchment, as massive blocks varying their size from one meter up to five meters,...
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Why is diabase/dolerite erosion resistant?

Diabase is often considered to be relatively erosion resistant. We can generally find diabase dikes cross-cutting various strata through the geologic landscape, wherein, the diabase dike or sill is ...
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How is the difference in crust and mantle composition explained?

We know that the composition of the crust is SiAl and SiMa and the one of mantle is mostly ophiolites, but why? Which are the geological processes that make this difference exist?
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What kind of rock is this being eaten by a rock-eating worm?

The New York Times article This Creature Eats Stone. Sand Comes Out the Other End. describes the shipworm Lithoredo abatanica, an organism that appears to "eat solid rock". It links to the item in ...
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What determines the scale of columnar jointing?

What physical parameters determine the scale of columnar jointing? What makes the columns thinner or thicker? What makes them taller or shorter? What causes the variation in a given site? What ...
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1answer
46 views

could a rhyolitic magma melt a ferromagnesian mineral like olivine?

if the temperature of a felsic magma is about 800-650 °C and olivine melting point oscillates between end members forsterite and fayalite near 1900-1200 °C, Does this mean that an olivine crystal in ...
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Why don't pyroxenites and peridotites appear in Streckeisen's QAPF diagram?

Wikipedia says for QAPF diagrams: "A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition." Why don't pyroxenites and peridotites ...
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How can I identify “tough” (mixed) granites?

I am trying to parse an old geologic description of stone suitable for use as road metal: Care must be taken to distinguish the difference between the hardness of a stone and its toughness......
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how can a asterisk/star like crack occur on a rock?

how can a asterisk/star like crack occur on a hard pyroclastic rock? Could it caused by a meteroite or by some other mechanism? You may see the shape of the rock on attached photo.
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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 ...
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Mineral identification - please help!

I'm currently writing an essay on a mosaic in the British Museum, and am unsure about which stone the frame is constructed from (pictured). I would be most grateful for help identifying it. Thanks! ...
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479 views

Why do we consider hydrostatic pressure and lithostatic pressure separately?

In petroleum geology, why do we consider hydrostatic pressure and lithostatic pressure separately? Surely the pressure at any point, whether fluid or rock, at depth is simply equal the weight of all ...
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Basic petrophysics

The following question was inspired by Effective pressure or what is the effect of presssure?, which may be accessed here: https://inside.mines.edu/UserFiles/File/CRA/2005/...
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336 views

Difference between thermal and contact aureole

Are therms "contact aureole" and "thermal aureole" synonyms? I found some sentence at A. M. PATWARDHAN (2012): "The peripheral extent up to which the thermal effect of the intrusion is ...
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475 views

Can cooled/hardened lava rock return to a molten state?

Can lava that has cooled to a hardened state be returned to a molten state by, say, a new river of molten lava flowing over it - like re-smelting iron? If you threw a lava rock into the mouth of a ...
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In an Earth-like planet with no history of carbon-based life forms, would there be limestone?

The extent of my understanding of limestone is about what Wikipedia says at the very top of the page: Limestone is a sedimentary rock, composed mainly of skeletal fragments of marine organisms such ...
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What's a reasonable flow rate/mass flux for a fumarole?

Obviously, there is going to be a large amount of variability, but what are some realistic values?
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Employment of Volcanologists and Petrologists

What industries or government departments hire volcanologists and igneous petrologists/geochemists in the United States?
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186 views

Differentiate a rock and mineral

How to differentiate between a rock and a mineral by merely looking at it as a thumb rule?
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634 views

What is meant by intraplate volcanism?

I am having massive confusion on whether intraplate volcanism refers to volcanism occurring at plate boundaries or away from plate boundaries (such as formation of ocean island basalts), or perhaps ...
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How can I tell the difference between granite and syenite?

(I'm not a geologist, just helping my kids collect rocks).
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Please help in understanding redox buffers

I'm trying to gain a firmer conceptual understanding of how redox buffers work in petrology, specifically how oxygen fugacity influences mineral crystallization. While I understand the mathematical ...
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861 views

How do quartz veins form?

Is there an association with thrust fault or dormant volcanos and quartz veins? Where are quartz veins likely to occur in nature and how are the formed? Is it possible to identify these veins from ...
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Effect of reducing conditions on melting point

I've been reading a paper(*) looking at the deep carbon cycle and it mentions a key process that the authors refer to as "redox melting". According to the paper, reducing conditions increase the ...
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How did the terms “acidic” and “basic” come to be associated with $SiO_2$ in igneous rocks?

Students of geology are introduced to in their petrology course, (or used to be), to the terms "acidic", "basic", and the associated term "intermediate" in relation to %$SiО­­­_{2}$ in igneous rocks, ...
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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 ...
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Where can I get mineral testing done? [closed]

Where can I send a sample of material to be tested to see exactly what it is? Do you have an address where items can be sent for testing and evaluation? I have a softball size rock that was given to ...
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1answer
523 views

Crystallization path of diopside-wollastonite-silica systems

Can someone please explain to me (or give a source) how the crystallization diagram of the diopside-wollastonite-silica system looks. I am a new learner of geology and trying to learn ternary systems.
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661 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?
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What causes a change in composition from basanite to nephelinite?

Can the change in composition from basanite to nephelinite (i.e. increasing alkalis with decreasing SiO2) be explained in terms of fractional crystallisation, or only by a decreasing degree of partial ...
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431 views

What are the differences in terms of appearance between sandstone and limestone?

On the field, I find it very difficult to differentiate the two types of rocks just by looking at them.I know that you should do some analysis of the sample before any conclusions. But, for example in ...
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Is a controlled fractional crystallization process feasible on the Moon?

I am looking for a way to separate metal oxides on the Moon. Please bear with me - it's for an ultra-hard science fiction project. Processes typical on Earth are not effective there. Heat is cheap on ...
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Does ground-to-cloud lightning create crystals as well?

I've read that cloud-to-ground lightning can create fulgurites. Does ground-to-cloud lightning also create crystals within the earth as it surges through the minerals?
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What's the diference between claystone ,silt,mud, mudstone ,mudrock, clay ,shale and siltstone?

Here in Perú , we call those rocks ; lutita = shale , siltstone = limonita , but as far as I know we don't use the terms claystone and mudstone in spanish terms.
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Do you have any informations about fossil-rich cherts from Scandinavia?

Me and some of my friends have found many rocks similar to these in central Europe - we believe that it is fossil-rich chert that originally comes from Scandinavia. There are many interesting fossils ...
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What is solidus and liquidus temperature of granite?

My understanding is that because a rock is composed of variety of minerals, so it does not have fix melting point, rather there is a range below which whole rock is solid and above which whole rock is ...
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159 views

Rock ID: Bone and potato-shaped, copper-colored [closed]

I found these two intriguing rocks on the UK's seaside. They have an unusual potato shape and copper/green color. Whrn knocked against each other, the sound is clear, metallic and the vibrations keep ...
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South American Rocks

I'm thinking of visiting South America at some stage, to look at some interesting geology in the Atacama, Patagonian Andes, etc. Does anyone know where the best geological museum is in South America? ...
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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 ...
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325 views

What is the maximum grain size of a sediment that can be point counted using a standard thin section?

Point counting is an extremely useful tool for determining the composition of a clastic sedimentary rock. It is also very useful in provenance studies to determine the sediment source of a sedimentary ...
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624 views

Is porphyritic texture always indicative of a 2 stage cooling process?

Is porphyritic texture always indicative of a 2 stage cooling process? Can't the phenocrysts and groundmass be formed at the same time depending upon the chemical constituents forming the rock? The ...
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Can we see lava in a divergent tectonic boundary?

Can we actually see an orange line of lava if we look down to the crack of the divergent boundary of tectonic plates?
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What are the dominant elements that oxygen is bonded to on earth?

Elements are found in different minerals and chemical compounds on earth. Oxygen is dominantly found in silicate minerals which, aside from silica, contain other elements. One way of looking at the ...
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What is the source of lava of volcano? From magma below crust? Or from molten crust by rubbing?

Ok, it looks like a very simple question but I still have some point want to clarify... Most images explain volcanos contain some orange stuffs as magma below volcanoes as magma chamber, but the ...
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Are trends in crossplots of compositional data from different analyses valid?

Some geochemistry papers that I have read contain crossplots of compositional components that resulted from different analyses of a rock sample. For example, one is the percent volume of TiO2 versus ...
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Does flow banding only occur in silicic rocks?

I made a video about what I learnt in my A Level Geology course so far, about igneous rocks. One thing I mentioned was that flow banding is mostly only found in silicic rocks (and so mostly rhyolite) ...