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Questions tagged [petrology]

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

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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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105 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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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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32 views

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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179 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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86 views

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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58 views

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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159 views

I found this rock in my back yard. What is it?

I found this rock in my back yard. I suppose it has been there for ages. It weighs about 10 kg and when I tried to cut it with machinery, I couldn't. Some facts about the rock: I live in Cyprus. It ...
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1answer
41 views

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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490 views

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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Where can I get mineral testing done?

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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327 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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323 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?
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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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58 views

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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237 views

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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112 views

Rock ID: Bone and potato-shaped, copper-colored

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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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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391 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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2answers
353 views

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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359 views

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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698 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 ...
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1answer
56 views

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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1answer
326 views

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) ...
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2answers
93 views

Relative dating of granite pluton

In selecting a sample for age-dating of a granite pluton, which site is a better site to collect a sample from, the centre, top or side? Why was this site a better choice? Which dating method would ...
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Lithologic and hydrostratigraphic logs for the Deccan volcanic province, India

I need data on depth/thickness of of different lithologic units under Deccan Volcanic Province in MP, Maharashtra and Gujarat. For this if anyone can help in providing help in obtaining well log data ...
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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, ...
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453 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 ...
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2answers
732 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 ...
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60 views

Identifying serpentinite reworked during high-T metamorphism

What kind of mineralogy results when a serpentinite is reworked during high-T or UHT metamorphism? I am envisaging a possible scenario where an obducted ophiolite terrane is buried and metamorphised ...
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293 views

What is the natural replenishment rate of oil? [duplicate]

What is the natural replenishment rate of oil? Are extinction events connected with higher production?
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486 views

What processes produced the basalt columns of the Giant's Causeway?

The Giant's Causeway is, according to the Wikipedia page was formed during during the Paleogene Period, Antrim was subject to intense volcanic activity, when highly fluid molten basalt intruded ...
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2answers
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What is the geothermal potential of a volcano?

If there were lots of geothermal plants—even mobile ones—near a volcano, how much power could this provide? Could the sapping of some of the heat energy make the volcano less likely to erupt?
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Acidity of rock

I read that the acidic character, or, better, persilicic character of a rock is determined by a content of more than $65\%\ \ce{SiO_2}$. A rock is defined, by the texts available to me, as ...
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What's the partition coefficient (D) of Nb for apatite?

Was there any experimental study that determined $D$ for apatite and silicate liquid? The GERM Kd database only lists one study with carbonatite liquid, but that's not what I'm looking for.
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174 views

When did/will peak conventional oil happen?

Global oil reserves are, for practical purposes, limited. We're using them at a rate far exceeding the rate of deposition. At some point, we will have to spend more energy to extract remaining ...
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1answer
20k views

What are the high field strength and large ion lithophile (HFS or HFSE & LIL or LILE) elements?

There are two groups of elements that are frequently mentioned when discussing incompatible trace elements. They are the high field strength elements (HFS or HFSE) and the large ion lithophile ...
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187 views

Why would magma have high amounts of nickel and chromium?

I am doing a research project on Mount Lamington and through my research I have found that its magma has unusually high amounts of chronium and nickel. What would cause this to happen? Is there ...
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How can I find a large petrology dataset?

I need a large dataset of the oxide components of peridotitic rocks and their manual classification for a project of mine. The only one that I managed to find was from the EarthChem database and it is ...
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1answer
6k views

How do you use the streckeisen (QAPF) classification ternary diagram to identify igneous rocks based on chemical rock composition?

I have been given the following diagrams: and and a database that is structured like this: ...
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1answer
1k views

How are spherulitic structures formed in basalt rock?

After having studied the thin sections of basalt under the microscope, I have come across many spherulitic structures occurring in clusters. How can this happen? Is it due to devitrification or is ...
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Where can obsidian be found?

Where is obsidian found? Is it typically found on the surface or underground? If underground, how far under (meters or feet would be perfect)? Also, is it found everywhere on Earth, or just in areas ...