8 votes
Accepted

What is the age of the earliest traces of life on Earth?

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 ...
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  • 5,238
8 votes

What is the age of the earliest traces of life on Earth?

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 ...
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  • 3,780
7 votes

Can concretions form in non-sedimentary rocks?

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 ...
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6 votes
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Exhumation of blueschist

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 ...
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6 votes

When metamorphic rocks undergo pressure, how are they changed?

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 ...
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  • 5,896
5 votes
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Which geological processes will destroy plastic?

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 ...
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  • 5,380
4 votes

Exhumation of blueschist

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 ...
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  • 3,780
4 votes

What is the age of the earliest traces of life on Earth?

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 ...
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  • 2,201
3 votes
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When metamorphic rocks undergo pressure, how are they changed?

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, ...
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  • 511
3 votes

How do the swirls and veins in marble form in detail?

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?...
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3 votes
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Identifying serpentinite reworked during high-T metamorphism

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 + ...
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2 votes

When metamorphic rocks undergo pressure, how are they changed?

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 ...
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2 votes

When metamorphic rocks undergo pressure, how are they changed?

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 ...
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2 votes

Where can I find papers on the geology of the Wastach Range in Utah

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 ...
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2 votes
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Is there any mineral that survives hard degree metamorphism?

Zircon is one mineral which survives metamorphism. Zircons can survive processes like erosion, transport and metamorphism, so they preserve a record of past geological processes. From Wikipedia, ...
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2 votes

Isotopic dating of metamorphic rocks

There are two ways (at least two I will discuss here) to date a metamorphic rock. The first is using an isochron. During a heating event, isotope ratios between different minerals (most often used are ...
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1 vote

Isotopic dating of metamorphic rocks

The main objection with regard to sedimentary rock is that you can date the individual mineral grains with isotopic techniques but not when they were laid down together. All metamorphic rocks show ...
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1 vote

When sediments are deposited, do they sink into the earth?

The podcast may have given the impression that sediment always gets buried or pushed down into the earth. This isnt the reality of the processes which are dynamic and dependent on geologic time as ...
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1 vote
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What is the metamorphic field gradient useful for?

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 ...
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1 vote

Does grade of metamorphism apply to igneous and sedimentary rocks?

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. ...
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1 vote

Does grade of metamorphism apply to igneous and sedimentary rocks?

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 ...
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  • 22.2k
1 vote
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are laccolith, lobolith, dikes and sills formed of porphyritic texture?

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 ...
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1 vote

How do the swirls and veins in marble form in detail?

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 ...
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