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16 votes

Are fossil fuels really formed from fossils?

Let's look at this. A very large number of points for one question. First, the solar system. We do not see any hydrocarbons in the inner solar system (Mercury to Mars). This is because in this region ...
Andrew Jon Dodds's user avatar
8 votes

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

The nomenclature is confusing and recent studies have shown that among mid-ocean ridge basalts (generally called MORBs) that normal mid-ocean ridge basalts (NMORB) should reflect the statistically ...
dergeophysiker's user avatar
7 votes
Accepted

Curious natural patterns on the surface of basalt blocks that make up the sidewalk

These are most likely manganese dendrites. It is not a fossil, and not organic. These usually form in cracks in rocks, and most likely this slab was broken along an existing crack. You can read more ...
Gimelist's user avatar
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6 votes
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Is it possible to estimate the size of a meteorite from its remains?

Meteors enter the atmosphere at speeds ranging from 11 km/sec, to 72 km/sec. Those speeds are so large that if a meteorite were to hit the surface at that speed, the energy released would be more then ...
Camilo Rada's user avatar
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6 votes
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How did the terms "acidic" and "basic" come to be associated with $SiO_2$ in igneous rocks?

The idea came from the theory that silicic acid was the chief form of silicon occurring in rocks. Early attempts to classify minerals, placed some mineral specimens in groups based acid-base ...
Earth Science Expatriate's user avatar
6 votes

Where are the oldest igneous rocks found?

Zircons from Australia at 4.4Ga, and perhaps basalts from a Canadian island at 4.5Ga. It would interest you to know the quest of Boston University geochemist Matthew Jackson, who is searching for the ...
bandybabboon's user avatar
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6 votes
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What is lava called if it has 56% to 64% silica?

What is lava called if it has 56% to 64% silica? It's called intermediate. Or is it impossible for any lava to cool into rocks with that range of silica? Not impossible, but not common. The two ...
Gimelist's user avatar
  • 23.2k
5 votes

Molten salt seas on the future Earth

So will a tangerine sun loom large over red-hot waves softly breaking on abyssal beaches? Probably not. Interesting question, and one might think that yes - you could melt the evaporite deposits that ...
Gimelist's user avatar
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5 votes

Are fossil fuels really formed from fossils?

The definition of a fossil is "evidence of past life preserved by geologic processes". By this definition a coal bed is itself a fossil since it is the preserved organic matter from an ancient ...
verisimilidude's user avatar
5 votes

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

Is porphyritic texture always indicative of a 2 stage cooling process? Not necessarily. While it is a nice simplification for undergrad textbooks and it nicely explains phenocrysts in some simple ...
Gimelist's user avatar
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5 votes
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What is meant by intraplate volcanism?

The majority of Earth's volcanoes occur in plate boundaries. These can be at spreading ridges (green dots on the map) where they are mostly underwater, but sometimes are above the water (such as in ...
Gimelist's user avatar
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5 votes

What is meant by intraplate volcanism?

Terms: Intraplate volcanism - as the name suggests it is volcanism within the plates rather than at plate boundaries. These are also known as hotspots. Ocean island basalt (OIB) - is the basaltic ...
shul's user avatar
  • 133
5 votes

Why aren't there ultra acid igneous rocks?

Warning: not an igneous petrologist here, I'm more interested in what happens to magmas once they reach the surface than all the processes they might have encountered along the way... That being said, ...
Jean-Marie Prival's user avatar
5 votes

Why aren't there ultra acid igneous rocks?

As mentioned in a comment to another answer, the terms "basic" and "acid" in relation to igneous rocks are outdated and stem from an incomplete understanding of rock chemistry in ...
Gimelist's user avatar
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4 votes
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How can I tell the difference between granite and syenite?

It is confusing - both rocks are commonly pink overall. But, granite has abundant quartz, syenite has no quartz to very little. Quartz will be the grey translucent mineral that looks just like a blob ...
Gimelist's user avatar
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4 votes

What does "rapid" mean in terms of igneous rock formation?

Obsidian is volcanic glass of felsic composition (i.e. SiO2-rich) whereas basalt is a volcanic rock which may or may not be (partially) glassy of mafic composition (i.e. SiO2-poor). So when you're ...
Gimelist's user avatar
  • 23.2k
4 votes

What determines the scale of columnar jointing?

There is a paper about this in the Bulletin of Volcanology (Hetényi et al. 2012). Cooling rate seems to be the main parameter controlling column size (faster cooling yielding to thinner columns). ...
Jean-Marie Prival's user avatar
3 votes
Accepted

Why don't pyroxenites and peridotites appear in Streckeisen's QAPF diagram?

Why don't pyroxenites and peridotites appear on this diagram? Because they do not have any quartz, feldspars, or felspathoids. The same Wikipedia page also says this: QAPF diagrams are also not ...
Gimelist's user avatar
  • 23.2k
3 votes

Coarse grains in a sample of Rhyolite suggest that it formed by

(This homework question is probably long overdue, but here is an answer for the sake of site statistics and future readers.) Magmatic rocks are further classified into extrusive rocks (emplaced at the ...
Jean-Marie Prival's user avatar
3 votes

Coarse grains in a sample of Rhyolite suggest that it formed by

Rhyolite by definition doesn’t have coarse grains. A rock with the composition of rhyolite that is completely coarse grained is going to be a granite or microgranite of some sort. If what you are ...
Avana Vana's user avatar
2 votes

What is meant by intraplate volcanism?

Intra = within/on the inside, so intraplate volcanism would be volcanism happening far from plate boundaries, usually hotspots. If you have trouble remembering intra- vs inter- remember international ...
John's user avatar
  • 7,016
2 votes
Accepted

Effect of reducing conditions on melting point

As you probably know, the melting temperature in the mantle is a function of H2O contents. The more H2O you have, the lower the melting point of the rocks (i.e. dry vs wet solidus in the sketch). A ...
Gimelist's user avatar
  • 23.2k
2 votes

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

In fact, columnar jointing like this is common in nature and also appears in other rock types all around the world, especially in other lavas, subvolcanic intrusive (i.e. dikes, sills, laccoliths) ...
Avana Vana's user avatar
2 votes

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

This answer is based on Winter's presentation for his textbook. Look up chapter 7 on his website, and start at slide 39. First let's go through some of your statements: Felsic magmas crystallise ...
Gimelist's user avatar
  • 23.2k
2 votes
Accepted

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

I am not sure those are pegmatites. They may simply be granitic dikes. But in either case, you would need to assess the regional geology to look for potential sources of this material, which would be ...
Daniel's user avatar
  • 176
2 votes

Fissure eruption

A fissure eruption typically occurs in basaltic terrain, where the magma is relatively fluid. Hawaii is a great example. Fissure eruptions occur along dilated fault zones. You show a picture of an ...
Daniel's user avatar
  • 176
2 votes

Are the cores of every mountain range igneous?

examples of mountain ranges with sedimentary or metamorphic cores Mount Everest. At an elevation of 7000 metres and higher, it is all sedimentary rock. Below 7000 metres, it is metamorphic of ...
Gimelist's user avatar
  • 23.2k
2 votes
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Large Igneous Provinces are like Lunar Mare?

I disagree with Michael about the "not as large as the lunar mare" part. Head and Wilson (1992) mention that the "total area of exposed mare deposits is about 6.3 millions km2" (17 % of the Moon's ...
Jean-Marie Prival's user avatar
2 votes

What is the difference between mobile & molten rock?

Molten rock must be a liquid, mobile rock is any rock capable of moving, sand and mud can be mobile.
John's user avatar
  • 7,016

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