27

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% ...


19

The basic differences are: Geology study of rocks and minerals: the study of the structure of the Earth or another planet, especially its rocks, soil, and minerals, and its history and origins structure of area: the rocks, minerals, and physical structure of a specific area Petrology study of rocks: the study of sedimentary, igneous, and metamorphic ...


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 ...


17

It's an interesting question, but in practice I think it's impossible to answer. It's very difficult to measure the rates of many of those processes, and the divisions between rock types can be quite ill-defined (for example, in migmatites). There's no scientific instrument we can point at a chunk of the earth which will tell us "in this region, 29.4 ...


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 ...


14

I'll take the form of the question given by another person here and attempt to provide a different answer. So what you are asking is: "How did gold become so concentrated in certain parts of the world?" So yes, gold is all around but the concentration is too low to make extraction of it worthwhile. You need some process to take small amounts of gold from a ...


12

One can only speculate based upon a photograph - however they look very much like mineralized fractures. At some time in the past this rock mass may have fractured in response to thermal or tectonic stresses. Fluid may have then penetrated relatively long distances along the fractures into the area and infiltrated shorter distances into the wall rock along ...


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.


11

Why you should not do it The QAPF and related diagrams are intended for classification of rocks in the field, or preliminary classification with modal proportions as seen in the optical microscope. They are not designed with the chemical composition of the rocks in the mind. Furthermore, these diagrams are merely descriptive and not genetic. They do not ...


11

The best way to learn about rock types is to handle rock specimens guided by an experienced geologist. By handling rock specimens you get to feel the weight of the rock, its roughness or smoothness or if it feels slippery, soapy, glassy, firm or crumbly. Is it weak or strong? You will also be able to better see the colours in the rock, the sizes of the ...


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 ...


10

You are correct about halite and sylvite. I might add that carnallite also has an extremely bitter taste. These three minerals are chlorides and dissolve very easily, so that may be a part of the issue here. Differentiating mudstone (clay) from siltstone is actually not about taste, but rather about texture. Note that this can be misleading. If you have a ...


10

As far as I can judge from the picture, it is most probably slagstone, Swedish blue stone. Slag is a byproduct from ore smelting. In old foundries, the slag was molded and used as a constructing material for houses and roads. It is actually similar to obsidian, but is usually rather brittle. I believe that the hardness could be around 5-5.5, depending on ...


9

First, a short introduction to incompatible elements The Earth's mantle is mostly composed of the minerals olivine, pyroxene, anorthite, spinel and garnet. These minerals are made from the elements Si, Al, Fe, Mg and Ca. In the figure below I've put them in the MRFE field (Mantle Rock Forming Elements). The trace elements, the elements that occur in very ...


8

First of all, your statement implies that volcanism didn't occur in Troodos. That is not true. Troodos was even referred to as "Troodos Volcano" once in Miyashiro's 1973 article about Troodos. A geological map of Cyprus clearly shows that a large portion of the ophiolite is composed of lavas (volcanic) and dykes (sub-volcanic): (source) Note the red, ...


8

After some brief research, serpentinite diapirism occurs due to deep penetration of seawater at temperature around 100-200$^o$C [1]. To the south of the rift that was forming the ophiolite sequence there was an oceanic plate subducting as African plate moved north. This subducting oceanic plate provided the seawater that serpentinized the harzburgite in the ...


8

The blue spots are natural. This rock is indeed granite from K2. The blue spots are azurite. Malachite is also reported from this same location. From Geology.com: "K2 granite is named after a mountain in the Karakoram Range near the border between Pakistan and China. K2, also known as "Mount Godwin Austen," is the second-highest mountain in the world. The ...


7

According to the image that you provided, what you are seeing are not spherulites, but rather epoxy bubbles. More on that later. I will first answer what are spherulites: Spherical features observed in thin sections can form by several ways. Devitrification - this is the recrystallisation of glass. I think it is more common in rhyolitic rocks than in ...


7

The answer depends a lot on what you mean by "excluding shale oil". Tight oil production, commonly referred to as "shale oil" is about 4 million barrels per day currently (2014), compare to almost none in 2005. Canadian oil sands production is at 2 millon barrels per day, compare to about 700,000 per day in 2005. So compare to 2005, tight oil (shale oil)...


7

Geology/Petrology has proposed some models for the origin of basalts which are based upon chemical evidence from rocks. Modern origin-models have been constrained by isotopic chemistry for some time. I do not claim to be current in this subject, but I believe the conclusions in this classic paper are still accepted as the most likely origin models. Models ...


7

Your rock looks like a piece of light grey colored basalt with chalcedony vein running through it. I've collected pieces very similar last summer along the Bay of Fundy near Scot's Bay, NS


7

What you are looking for is information about the "Oslofeltet" (Oslo-field) in Norway (dated to be from the Ordovicium period, 443 - 488 million years old). This is a concentrated field of of fossils. There is a Sement museum ('cement') in a small place called Slemmestad where it is possible to view fossils from what was formerly an ocean bed. Geologicaly ...


7

Clastic sedimentary rocks are classified by size of the sediment particles making up the rock. Particle size descriptions like sand, silt, and clay have specific meaning in geology and engineering. (see chart below). Shales, mudstones and claystones are rock types that are very similar to each other. Siltstone - greater than half of the composition is ...


6

I am frequently identifying as an engineering geologist grain size distribution of fine-grained cohesive soil by biting it (for almost 3 decades by now). With some experience one comes close to the actual grain size distribution determined in the lab. This is very useful when immediate decision is neccessary at a construction site.


6

Volcanoes erupt due to increase in pressure, within the magma chamber (often 3-10 km deep), due to exolusion of volatiles. The magma chamber is usually heated from below. Geothermal plants in volcanic regions exploit heat from hot water in the "shallow subsurface", few hundreds of meters deep (and rarely exceeding 1-2 km). So as far as your second last ...


6

According the website http://www.tulane.edu/~sanelson/eens212/igrockclassif.htm from Tulane University silica content in igneous rock is based on mass. Below is copy of the relevant part of the web page where it states wt %, weight percent. General Chemical Classifications $\ce{SiO2}$ (Silica) Content > 66 wt. % - Acid 52-66 wt% - Intermediate 45-52 ...


6

Go to a good geological museum and spend time there. Alas, many such museums have dumbed down, and are not as informative as they used to be (Natural History Museum in Kensington, London - I'm writing about you!). Go walkabout with local rockhound / geology clubs. There are always geologists trying to impress with their knowledge. Get a geological map of a ...


6

The diopside–wollastonite–silica system ($\ce{CaMgSi2O6-CaSiO3-SiO2}$) was never studied for crystallisation paths in igneous systems. This is not a system encountered in terrestrial magmas, so no one bothered. However, it was studied as part of a more general lime–magnesia–silica ($\ce{CaO-MgO-SiO2}$) system by material scientists and they came up with ...


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