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To complement @MarkRovetta answer, McLoughlin & Grosch reported this year during EGU findings of carbonaceous fragments that they think are biogenic in the 3.4 Ga Buck Reef Chert. Chemical tests are still ongoing however. Schopf (2006) in his review of archean life reported a dozen of fossils in the 3 to 3.5Ga range. They are all "putative" fossil, ...


8

Fossils are our strongest, and to most people most accessible, evidence of the great age of life on earth. The fossils in the Burgess Shale are clearly the imprints of critters, but are a mere 505 million years old. The oldest cyanobacteria-like fossils known are nearly 3.5 billion years old, among the oldest fossils currently known. There is Evidence for ...


7

Not really. Concretions are features of sedimentary rocks almost by definition. Here's my go at a definition; it essentially presupposes a sedimentary rock: Concretions are spatially discrete zones of above-average (compared to the rest of the rock) cementation by authigenic minerals. Common cements include calcite, quartz, siderite, and pyrite. ...


6

Fast exhumation is not necessary to retain blueschist mineral assemblage. This is the classical difference between prograde and retrograde metamorphism, and it's not limited to blueschists. Why do we have eclogite on Earth's surface? What about granulites? Amphibolites? In fact, why do we see any metamorphic rocks on the surface instead of just various clays ...


6

Metamorphic rocks are formed when a rock (sedimentary, igneous or a previous metamorphic rock) comes under high pressure and/or temperature. Pressure and temperature forces the atoms to form new minerals and thereby a new kind of rock. It's not necessary fragmented, but the rock rather morph through recrystallization into a new state in response to the ...


4

Some simplifying assumptions This is quite a broad and complex question to answer, so I'm going to simplify it shamelessly to make it a little more answerable. Firstly, there are a huge number of different plastics with a huge variety of physical characteristics. You mention that you're interested in ‘the most common type of plastics’, so I'm going to ...


4

I don't know whether its currently the favoured explanation, but in California people made reference to something informally referred to as the watermelon seed theory . The general idea being that the compression in a subduction zone managed to eject these rocks back up and out of depth by a process like squeezing (and shooting) a wet watermelon seed between ...


4

The oldest (fairly) definitive fossils date from about 3.48 billion years ago (Ga) and consist of sedimentary structures associated with microbial mats living in coastal environments.[1] Beyond this there are is no known direct fossil evidence so instead we have to rely on geochemical evidence. As the OP mentioned, the oldest known sedimentary rock, the ...


3

Metamorphic rocks are changed by transformations deep underground. Being deep underground there is immense pressure and heat. The transformations can be just crystal size of the particular mineral, or different minerals can be in fact formed. For a particular mineral there are also may be different crystal structures which depend on the pressure and ...


3

What you see in marble are not veins (for example in the picture of yours). By the term vein in geology we mean a magmatic intrusion in the rock in the form of vein. Can a marble have magma real veins? Yes. The process is explained here: https://en.wikipedia.org/wiki/Intrusive_rock The 'veins' you are referring to are coming from the different beds of the ...


3

Of course, any ultramafic rock will ultimately melt at about 2000 deg C, but long before that there will be some interesting phase transitions, possibly involving serpentine > talc > olivine + orthopyroxene. Various subtle changes occur in the rock during its original hydration from peridotite to serpentinite. There is high- and low-temperature serpentinite....


2

Metamorphic can have been halfway to the state of lava for a short time, it may have been like quite tough dough of bread. You can imagine the viscosity of it to be the same as ordinary glass which is at 200 degrees, if you have a window at 200 degrees, you can bend it into a U shape in a time ranging from 20 minutes to 20 seconds depending on the ...


2

The existing answers are correct but I think they miss an important aspect of your question, which is the effect of pressure. Do they become fragments of rocks? No. Fragmentation of rocks is a relatively low temperature and pressure process. This is the kind of stuff you would see in near-surface environments. Once rocks become hot under pressure, they ...


2

This report on the stratigraphy is probably a good place to start. Additonally, searching Google Scholar for Wasatch Range geochronology brings up a number of papers on the geochronology of specific formations if you are interested in that.


1

You are entirely correct. Todd and Engi write: The mineral assemblages of rocks at the surface lie on a locus of P–T-conditions… It must be stressed…does not imply a P–T-distribution or geotherm realized at any one time during the orogenic evolution of this area. Even assuming the P–T-values reflect maximum metamorphic conditions, these were ...


1

Metamorphic "grade" ONLY applies to metamorphic rocks; the "grade" referring to how much pressure and temperature the rock, be it sedimentary, igneous, and even metamorphic, has been subjected to. This is deduced by noting the presence of specific minerals: at key temperatures and pressures a sedimentary rock will produce specific minerals, an igneous rock,...


1

I'm not sure I entirely understand your questions but I'll have a go. Does grade of metamorphism apply to igneous and sedimentary rocks? Yes. Both igneous and sedimentary rocks can be metamorphosed. For example, a high grade metamorphic rock of the amphibolite facies could have been originally either a sedimentary rock or an igneous rock. It's not always ...


1

Laccoliths, lopoliths, dikes and sills are all characteristic modes of occurrence of hypabyssal rocks, generally small in scale compared to most magma chambers. They are magmas which have cooled in a near-surface environment. None of these terms necessarily implies a particular petrologic texture. So, for example a dike or sill may be aphanitic, porphyritic, ...


1

particles that go into marble (to my knowledge) don't have any noticeable elongation Not necessarily. Marble is mostly made of either calcite (CaCO3) or dolomite (CaMg(CO3)2), both of which are carbonates that do not exhibit any foliation in metamorphic rocks. But marble is rarely pure and commonly has some clays or other minerals that can contribute to any ...


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