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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6
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2answers
74 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 ...
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2answers
84 views

Large Igneous Provinces are like Lunar Mare?

Would Large Igneous Provinces like the Deccan Traps have looked like ancient lunar Mare from space? Glowing lava oceans shinning in the night? Are these two things similar in nature? Would smoke from ...
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4answers
529 views

Are the cores of every mountain range igneous?

Every volcano is a mountain, but not every mountain is a volcano. Still, it strikes me that--at least for the mountain ranges I can think of in this moment--they all seem to have igneous cores. Is ...
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3answers
69 views

Fissure eruption

Can fissure eruptions dislocate/deform ( fault, rotate, bend, folding) the surrounding strata which fissure ascending into. Or, can a laccolith reach to the surface (and flow) after folding the ...
3
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1answer
59 views

Is it possible to know what was the flow direction in a pegmatite dike?

The following picture (Painted wall cliff in the Black Canyon of the Gunnison National Park, Colorado, USA) shows a pegmatite dike: is it possible to tell what was the flow direction? From thin (see ...
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1answer
54 views

Possible cavern?

In eastern Ontario, Canada, there's a place where, when walking and stomping, you can hear a hollowness in the ground. The area is primarily igneous rock with many large boulders, and the spot in ...
5
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2answers
106 views

Molten salt seas on the future Earth

As the sun gets brighter, the oceans are expected to evaporate by the next billion years or so (the vapor slowly gets stripped off into space). The salt, which is less dense than most rocks, will be ...
2
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1answer
34 views

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
26 views

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 ...
4
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1answer
91 views

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 ...
2
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1answer
66 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 ...
3
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1answer
264 views

Mineral identification; black & metallic [closed]

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 ...
4
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1answer
191 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
635 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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1answer
190 views

How can I tell the difference between granite and syenite?

(I'm not a geologist, just helping my kids collect rocks).
4
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1answer
8k 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.
2
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1answer
49 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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5answers
814 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
372 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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2answers
3k views

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
1k 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, ...
3
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1answer
90 views

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, ...
5
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1answer
301 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 ...
4
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1answer
68 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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0answers
51 views

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
761 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
918 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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228 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 ...
3
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1answer
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 ...
6
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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 ...
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1answer
124 views

Do discordants dykes ever travel concordantly (are transgressive dykes a thing)?

Transgressive sills "jump" between bedding planes, following joints: ...
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2answers
847 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
489 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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1answer
3k views

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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3answers
8k views

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 ...