| #86079 - metamorphic, igneous, and all that... |   |
hoov
Member since:
2009-03-10 - 16:06:19
Quote:
Guess my High School Geology failed me, remember only 3 types of rock..igneous, metamorphic, sedimentary....remember ig came from melted mantle and lava, SOOOO fig hydro fit there...And since I saw that Flake started from organics deposited a then the ol heat and pressure trick, it would be classed as sedimentary. But since I was obviously WRONG, and Zens stuff really wasn't a Lava, maybe Metamorphic?????
No, you're not obviously wrong, although you did make a mistake, above. Flake graphite is found in a metamorphic host, the organics (and the rock containing them) having been altered by heat and pressure, making them a metamorphic product. When sediments have been metamorphosed, they are sometimes called metasediments. Similarly, volcanics can be metamorphosed into metavolcanics. Metamorphic rocks can be metamorphosed again. And, as you might expect, the nice clean boundaries of our definitions are really quite blurry in the real world. So, they discuss variables that describe how extensive the metamorphic changes were, i.e. its metamorphic character.
There are two categories of metamorphic events, called contact metamorphism and regional metamorphism. Contact metamorphism occurs when a magma flow (magma becomes lava only when it emerges at the surface) intrudes into a volume of pre-existing rock. If the forward momentum stalls, it will gradually erode out a pocket, forming a magma chamber. You may have read of intrusives: intrusive plugs, plutons, sills, dykes, and so on. They describe different shapes of solidified magma intrusives.
Because magma is hot, and because its forcing its way through solid rock, you have conditions of heat and pressure. The surrounding rock undergoes a progressive transformation called metasomatism. Closer to the heat, the effect is more profound. Farther away, the effects are partial. So you'll have a halo, with bands of alteration surrounding the intrusive. The bigger the heat source, the larger the halo of metasomatism. The Albany deposit arises from contact metamorphism, but there's more to the story.
Regional metamorphism occurs when large volumes of existing rock are somehow pushed deeper into the earth, and subjected to the heat and pressure below. Tectonic collisions (continents crashing together, for example) are common explanations for this sort of event. The heat and pressure metamorphose the rock on a bulk scale. If those rocks are later uplifted once more, broad regions of the Earth's surface may be composed entirely of metamorphic rock. Those regions are often referred to as provinces, or if more linear in shape, as belts. And they're very common.
Sedimentary rocks form almost invariably under water. Fine sediments and debris from living organisms settle to the bottom, and build up in layers, which if understurbed, later solidify into limestome. Coarser particles may form a sandstone. If the sediment is mostly clay, it may form shale. And so on. If those rocks are later metamorphosed, they transform into marble, quartzite and slate, respectively. Flake graphite is formed from sedimentary rocks with some organic matter (bodies of living organisms, algae especially), which were regionally metamorphosed. And that happened many times, in different places, all over the world.
When I was talking about the Albany deposit, above, I left off saying there was more to the story. During contact metamorphism, when hot magma interacts with cold host rocks, they begin to have chemical reactions with one another. Heat alone can drive water out of a sedimentary rock, as it metamorphoses. Or, exchange reactions between two types of rock can have water as a by-product. In either case, the water has nowhere to go. The volume of space is already occupied by rock. And water cannot be compressed (it's the root word for hydraulic), so pressure builds exponentially. The water forces its escape from the heat and pressure that contains it by punching right through the solid rock. That now becomes a hydrothermal event. And the rupture of the containing rock is called hydraulic fracturing, which you now hear of all the time in the energy industry. Mother Nature did it first.
The Albany deposit is an "eruption" of this hydrothermal water. Under the conditions of temperature and pressure during its formation, it could dissolve immense amounts of other substances. And somehow, those substances were almost entirely limited to silica, feldspar, and carbon. It would have dissolved other stuff if it could have, so the contact zone must have been rather pure. Freak of nature, absolutely. And immense.
Somewhere, beneath the Albany deposit, there must be a chamber which contained the hydrothermal fluids between eruptions. If you've ever seen a pressure cooker releasing steam, imagine it came out all in one burst, and then immediately began reloading for the next eruption. There were pulses, waves of hydrothermal eruptions. These pipes we're seeing almost without doubt are only a fraction of the size of this hydrothermal reservoir. And in turn, that reservoir would only be a fraction of the size of the magma chamber that fed it.
I kind of got off subject there, but I hope I helped with some of the concepts.
Lar |
