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The processes forming obsidian are not well understood because an active obsidian-forming eruption has never been recorded by humans. However, we can make many inferences from the composition of the rock and settings in which it is found.
Obsidian is more than 70% weight percent SiO2 (i.e. rhyolitic), but has less than 0.5 weight percent H2O, and almost 0% ...
25
I'm a volcanologist and I have worked on erupting volcanoes. First of all, volcanologists almost never actually wear those suits. Heat is almost never the hazard that matters in the situations in which we work. The hazards are usually the chance of being hit by ballistics, or getting gassed. The reason you see those suits so often is that they look really ...
21
As always: It Depends.
Assuming enough water and sunshine, crop growth rate boils down to the concept of limiting nutrients. These may be: nitrogen (via ammonia or nitrates), phosphorous (via phosphates), potassium, and sometimes others. In typical continental settings (i.e. subduction related vulcanism), lavas may be enriched in potassium and phosphorous ...
20
The best argument I've heard supporting strong skepticism of plumes, if not total dismissal, is that the theory is too flexible. To put it more bluntly, this amounts to saying that it is unfalsifiable and therefore not helpful (in Popper's words, "A theory that explains everything, explains nothing.").
Erik Lundin, a Norwegian geologist, is a fairly ...
19
The Cascades (the volcanic range that Mt. St. Helens and Mt. Ranier are a part of) are "arc volcanoes" (a.k.a. "a volcanic arc", etc).
Volcanic arcs form at a regular distance (and fairly regular spacing) behind subduction zones. Subduction zones are areas where dense oceanic crust dives beneath more buoyant crust (either younger oceanic crust or ...
18
21.7%, by my calculations (338 / 1556 holocene-active volcanoes). I calculated perennial snow by combining 6 weeks of MOD10A2 data from winter and summer weeks in 2014 to figure out which pixels had snow both in the summer and the winter. My source code for that is available as a gist (an older version accidentally classified cloud as snow; that's been fixed)...
17
Obsidian is formed when a rhyolitic (or felsic) lava flows cool rapidly. This must mean that it's mostly available on the surface (and I think if you go near volcanos you can find pieces of Obsidian on the ground) because molten rock cools much faster above ground than it does below, allowing the melt to cool with small crystals (as opposed to intrusive ...
16
This is a complex question, and I cannot give you a complete answer I'd like to point out that there is a very important article published in February of this year on the subject: volcanoes are in an eruptable state very very short periods of time in the geologic timescale; the mobilization of magma is very rapid but its storage is kept near, but under the ...
15
Zeolites are very similar to clays, with one key difference. The molecular structure of clays is rather compact. In contrast, the molecular structure of zeolites has tiny molecular-sized holes, and these holes are wont to connect. The result is a porous, tunnel-filled structure at the molecular level.
The resulting tunnels make zeolites very good at ...
15
This is probably an observational effect that is quite common in the Earth Sciences. In scientifically progressing societies there is a higher proportion of observations due to a number of effects:
awareness of science (not interpreting it as a wonder)
ability to record events
ability to observe (think 12 hour day in the factory vs. free weekend)
technology ...
15
Surface temperatures on Earth are a consequence of solar heating and radiative cooling - in other words, the heat energy which drives climate comes from sunlight falling on the Earth "now".
On the other hand, the heat energy driving volcanism is left over from the formation of the Earth (4.5 billion years ago), stored deep inside the planet, with some ...
14
Most volcanism occurs at three major tectonic features: Subduction Zones, Rifting Centers (Like East Pacific Rise) and under hotspots.
I will start with the most contentious, hotspots. The clearest example of hotspot volcanism is the island chain Hawaii. Underneath the island, there is a chaotic discontinuity caused by the boundary conditions at the core-...
14
In keeping with your Lord of the Rings inspiration, the first reference is from New Zealand, where the Lord of the Rings movies were made.
There are three main types of magma that volcanoes currently produce: basalt, andesite and rhyolite.
Basalt magma has a high temperature, around 1200ºC, it is poor in silica, has a low viscosity and a low gas content. ...
14
You're asking
Why do human populations concentrate near fault lines, volcanoes, etc.?
But the real question is
Do human populations concentrate near fault lines, volcanoes,
etc.?
And the answer is not necessarily positive. Let's look at where earthquakes are:
and active volcanism:
Both maps are from Volcano Discovery. Here's a population density ...
14
As it was explained to me at university there are two factors; buoyancy and erosion.
Rock buoyancy is a major factor, fresh Basalt is hot and dry and has a much lower density than older oceanic crust. This is why the mid-Atlantic ridge rises above the surrounding seabed.
The effect is even more pronounced with seamounts because they're composed of even ...
13
Whether volcanic activity fluctuates depends rather on the timescales you are looking at. Crisp (1984) compiles available data on igneous activity lasting for longer than 300 years and concludes that over the past 180 million years the annual average magmatic output each year is around 30km3, of which ~75% is produced at mid-ocean ridges.
The most obvious ...
13
The answer is Volcanos. There might be other inorganic processes capable to produce $\text{CO}_2$, but on Earth, the main inorganic source of $\text{CO}_2$ are volcanoes.
In some period of Earth's history, there is evidence of large glaciations events, some of them are thought to have been triggered by the lack of $\text{CO}_2$ (like the Snowball Earth and ...
12
Yellowstone is thought to be above a mantle plume, of with there are tens to hundreds on our planet, although there is plenty of debate on this matter. The exact nature of mantle plumes is a huge area of hotly contested research, but generally the are thought to be large melts originally sourced from the D'' layer (core / mantle boundary). Below the park, ...
12
First things first about Laki: it's in Iceland, which means its source is a combination of an ocean ridge and a mantle plume. The plume contribution means that the source composition is relatively undegassed (unlike the source for most mid-ocean-ridge basalts (MORBs)!) and very basic, so contains relatively high concentrations of sulphur and other volatiles.
...
12
I have seen it on the surface in some the lava fields of Iceland. This is consistent with @Neo answer.
Obsidian is not that often present, but if present, there is usually plenty around. It occurs in rather large pieces. This photo is from Landmannalaugar.
12
As you said, the Andean belt is divided into four segments, usually called the northern, central, southern, and austral volcanic zones (NVZ, CVZ, SVZ and AVZ, respectively; your map lacks the AVZ). This has been interpreted as a difference in the angle of subduction. Under the active zones, the subducting plate (called "slab") sinks into the mantle, ...
11
Complementing the answer by @seismo_steve, two other geophysical ways of
detecting magma chambers are through their density and conductivity anomalies.
Seismic tomography is able to detect the chambers because they cause an anomaly
in the propagation velocity of seismic waves.
Likewise, the chambers cause an anomaly in the gravity field because
their ...
11
I am a seismologist, not a volcanologist, but we can use a method called seismic tomography to understand the size of magma chambers.
This method is similar to a medical CT scan - the magma mush will have a slower seismic wavespeed than the surrounding 'normal' rock. Therefore, we can use this to determine the size of magma chambers from a tomographic ...
11
the mantle is in a highly reduced state
This is not entirely correct. The core is in a highly reduced state, but the mantle is not necessarily reduced, and is quite oxidised in some places. The mantle is heterogeneous.
2.
however I justify it's emission because carbon dioxide is continually being fed into the mantle by the subduction of carbonate ...
11
I think the answer to "Why don't scientists use fire entry suits to study volcanoes?" is that this is a question of professional ethics rather than of technology. For some institution to support, condone, or fund such a proposal, it would first have to conduct some sort of a safety review. At a significant cost, this approach would encourage risky behavior, ...
11
No, there is no consensus.
In fact, there's quite a bit of debate about it — around the 2 causes you mentioned. To sum up the proposed triggers:
The eruption happened about 2 days after the 2006 Yogyakarta earthquake, which killed over 5000 people. Lupi et al (2013), Sawolo et al. (2011), and others including the Indonesian government, favour this as the ...
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