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Questions tagged [igneous]

concerning rocks and processes relating to the cooling and solidification of magma, typically involving temperatures above approximately 800 degrees Celsius for at least some of the rock's history

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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
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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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1answer
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What is lava called if it has 56% to 64% silica?

We've learnt in science at school that "Mafic" lava forms rocks with 45% to 55% silica, meanwhile "Felsic" lava forms rocks with >=65% silica. So what is lava called in between? Or is it impossible ...
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57 views

The magnetite is in which type of magma

In which type of magma can I find magnetite? Peraluminous or peralkaline? I've been looking and the peraluminous magma have a lot of oxides so I thought it would be reduced magma so it would have ...
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1answer
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Mineral identification; black & metallic

During a recent trip to the Portuguese island of Madeira, I found a couple of interesting rocks on a beach. Both the rocks pictured in this question contain the same black mineral. I first guessed ...
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1answer
115 views

Is it possible to estimate the size of a meteorite from its remains?

I live near this ophiolite (Sasso di San Zanobi, Firenzuola) and they used to say it is a meteorite. There is no evidence of a crater but let's pretend it is the remain of a meteorite: how big would ...
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3answers
492 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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136 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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1answer
6k views

What is the difference between N-MORB, E-MORB and OIB?

What are the differences between N-MORB, E-MORB and OIB with regards to: Tectonic setting Geochemical features There are several types of basalts, but I can't find a clearly explained article.
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1answer
48 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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4answers
613 views

Where are the oldest igneous rocks found?

I'm trying to catch up on my homework for earth science, but I'm having a difficult time searching for an answer to my question.
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2answers
339 views

Water and explosive volcanoes in the Pacific ring of fire

Most volcanoes in the Pacific Ring of Fire have an explosive nature. This is due to high concentrations of water in their magma. Obviously, this magma comes from several kilometres below the earth's ...
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Are fossil fuels really formed from fossils?

Hydrocarbons have been found in great abundance elsewhere in the solar system where there is unlikely to be evidence for life past or present. No fossils involved. Petroleum and natural gas wells ...
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1answer
880 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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1answer
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Can the atmosphere affect the composition of igneous rocks?

Recently I was reading 'How to build a habitable planet' by Langmuir and Broecker and in chapter 15 they speak about how Life has led to a difference in oxidation potential in the atmosphere, ...
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1answer
237 views

Why does the loss of water from a felsic magma encourage crystallisation?

Felsic magmas crystallise in the crust, unlike mafic magmas which tend to make it to the Earth's surface, for the following reasons: They are more viscous than mafic magmas and therefore it is ...
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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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What processes cause basalt to form a columnar structure? [duplicate]

I have run across many pictures of rocks that look like almost perfectly carved columns. For example: the Twyfelfontein Organ Pipes, the Basaltic Prisms of Santa Maria Regla, and the waterfall ...
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1answer
621 views

are laccolith, lobolith, dikes and sills formed of porphyritic texture?

Laccolith, lobolith and batholith are shapes of igneous rocks which exist below the surface of the earth, dikes and sills too. And the way they formed by makes me think that they have a porphyritic ...
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1answer
724 views

What does “rapid” mean in terms of igneous rock formation?

Specifically, what time period would it take for obsidian as opposed to basalt, to form? (I need to know how much faster obsidian cools than basalt.)
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210 views

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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1answer
550 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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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 ...
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1answer
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Do discordants dykes ever travel concordantly (are transgressive dykes a thing)?

Transgressive sills "jump" between bedding planes, following joints: ...
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2answers
728 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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3answers
438 views

What, if any, paleoclimate data can be derived from igneous rocks?

Paleoclimate data often derives from sedimentary rocks. Metamorphic rocks can also contribute to paleoclimate information in a wide variety of ways. What about igneous rocks? I guess that this can ...
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Is there such a thing as a “basalt dike”?

I've always understood that basalt is an extrusive rock (formed by eruption), and dikes are intrusions (no eruption), so I would have thought that a dike could not be composed of basalt rock. It's ...
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Why do felsic materials have lower melting points than mafic?

It is clear from Bowen's reaction series that more felsic minerals have lower melting points than mafic minerals. As far as I know, the same is true of quenched glasses. Felsics have a higher degree ...