Hot answers tagged

17

I'd like to add to Brian's answer, and also point out some inaccuracies. First of all, it is not true that felsic minerals have lower melting temperatures than mafic minerals. Here are some melting temperatures of common minerals, sorted from high to low: Forsterite (mafic): 1890 °C Quartz (felsic): 1713 °C Anorthite (felsic): 1553 °C Diopside (mafic): ...


17

Good question! As you know, Bowen's reaction series describes the order of crystallization of silicate minerals in a cooling magma. The complex anion of silicates is a tetrahedron of four oxygen atoms surrounding one silicon atom, connected with strong covalent bonds. Each tetrahedron may be isolated from one another or they may be bonded together ...


11

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 of the solar system, dissociation by solar UV rapidly destroys primordial hydrocarbons. This effect is much weaker further out. Oil well 'replenishment' will ...


10

"Basalt" per definition is a fine grained rock (that is, you can't see the individual crystals with the naked eye, aka aphanitic) with a certain chemical composition. The coarse grained form of this rock is called a "gabbro". A diabase (or dolerite) is something in between, but let's ignore it for the meanwhile for the sake of simplicity. So quoting from ...


8

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 usual composition while enhanced MORB (EMORB) and depleted MORB (DMORB) should reflect end-members of the MORB population. Gale et. al 2013 proposes the use of ...


6

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 reactions and the hypothetical acid that mineral was derived. Sulfates, phosphates, nitrates , and tungstates etc. This thinking was the result of 19th century ...


6

Stable isotope signatures in igneous rock (primarily $\delta$18O) can be affected by climate, but the effect is complicated and unreliable enough that I doubt it can really be used to say much about palaeoclimate. You might be interested in my friend Lara Owens' dissertation in which she considers the influence of climate on $\delta$18O in lava from Mt ...


6

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 most common lava types are indeed mafic (mostly basalts) and felsic (mostly rhyolite). A well known intermediate lava type is andesite. Why? This question of ...


6

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 oldest basalt and mantle rock that exists. His work isn't entirely agreed upon, as they are using all kinds of difficult isotopes which are a subject of ...


5

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 Iceland). They can also be on convergent margins, for example the Pacific ring of fire. A less common type of volcano is the intraplate volcano, which are ...


5

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 rocks associated with intraplate volcanism. Relation between mid-ocean ridges and hotspots: Hotspot–ridge interaction produces a wide range of phenomena ...


5

These are basalts with columnar joints. Basalt is an igneous rock that forms when magma (liquid + crystals) erupts onto the surface as a lava flow or gets injected into cooler shallow crust and forms an intrusive dike or sill. Because basalt cools quickly, basalt commonly contains glass as well as crystals. The physical properties of magma changes rapidly ...


5

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 will be left after all the water is gone. The problem is that molten salts are extremely reactive materials. They will corrode any rock they get into contact ...


4

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 basalts, there is much more to it. Availability of chemical constituents Can't the phenocrysts and groundmass be formed at the same time depending upon the ...


4

Yes, they are. It's not the sills or dykes that "jump", it's the magma. Depending on the various parameters such as viscosity, stress, temperature, pressure, and local conditions and availability of joints, the magma will flow either as a sill or a dyke (or some other intrusive body). Notice that your two sketches are basically the same thing - it's just ...


4

One more trick - felsic magmas are basically fractionated derivates of other rocks. During the rock cycle, the most volatile components tend towards the felsic rocks. Water and fluxes generally reduce the melting point. And felsic rocks usually have alkalis compared to Fe/Mg in mafic rocks. Alkalis are more reactive and volatile. If you partially melt ...


4

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 enough to to vaporize the meteorite on impact, therefore leaving nothing of the original impacting body. However, during their fall, meteorites slow down due ...


4

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 with no distinct shape. You can use this website: https://www.virtualmicroscope.org to see examples of syenite and granite (use the search box).


3

The stone is Ignimbrite, the black crystals are either Hornblende or Pyroxene phenocrysts.


3

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 swamp. Oil and Natural Gas that are formed by the change of dead algae under the heat and pressure of being buried in the earth. These would also be fossils. ...


3

Potassium–argon (K-Ar) dating specifically and only works with igneous rocks. The idea being that when molten magma solidifies into, say, granite or basalt, any potassium ends up locked into crystaline forms. Some of that potassium will be the unstable K-40 isotope, decaying into A-40 with a half-life of 1.26 billion years. Since virtually no A-40 would have ...


3

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 saying obsidian as opposed to basalt ...you're comparing apples and oranges. So instead of discussing obsidian versus basalt, let's discuss phaneritic versus ...


2

Atmospheric pressure affects the bubbles that form in lava, and this effect can be used to estimate paleopressure at the time of emplacement of a lava flow. You might need some special conditions, but it is possible to make such a study. The technique is described in Sahagian et al. (2002). Sahagian, D. L., Proussevitch, A. A., & Carlson, W. D. (2002). ...


2

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 in the crust, unlike mafic magmas which tend to make it to the Earth's surface Not necessarily. It is true the upper continental crust is composed mostly of ...


2

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 trade = trade from one nation to others, domestic trade would be intra-national trade even though no one uses that term.


2

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 less known phenomenon is the decrease of melting point with the addition of CO2. The CO2 melting curve has a weird shape. At low pressures nothing much happens, ...


2

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 used if mafic minerals make up more than 90% of the rock composition (for example: peridotites and pyroxenites). Instead, we use a different diagram, that has ...


1

Probably both but I'd expect to see more in a peraluminous melt. The more mafic the melt the more iron based mineralisation you're going to see.


1

I figure you're looking for more exotic examples, and some geologists may well have some offhand, but I want to say "yes" because of the example of carbonatite rocks described in this geology page from San Diego State University: Carbonatites are perhaps the most unusual of all lavas. They are defined, when crystalline, by having more than 50% carbonate ...


1

First, read my highly related answers to your other questions: https://chemistry.stackexchange.com/q/67403/8083 https://chemistry.stackexchange.com/q/72483/8083 The forsterite might make decent clear glass Probably not, see above. fayalite can be processed with hydrogen to get iron, water, and silica, and the silica can be used to make glass Again, ...


Only top voted, non community-wiki answers of a minimum length are eligible