2. What eons are included in the Precambrian, and what are their beginning and e

Question

2. What eons are included in the Precambrian, and what are their beginning and end dates?
3. Why don't we find any rocks that have formed on Earth in the first 560 million years of the planet's history?
4. Explain why we can use meteorites to provide a date for when the Earth formed?
5. What two things do the zircon crystals found in Australia tell us about the early Earth?
6. Why are the oldest rocks exposed on Earth found in the middle of continents? Think of the very old rocks found in Canada for example.
7. What is the significance of ULTRAMAFIC lavas in Archean rocks? Do these form today?
8. Describe plate tectonics in the Archean. How is it different today?
9. What does the absence of oxides such as hematite tell us about the conditions on the planet in the Archean?
10. Describe Archean life based on the fossil evidence we have. Did it need oxygen? Do we have anything like these organisms today?

Explanation / Answer

ques 2

Precambrian (end date - begining date)

proterozoic ( .540 to 2.5 by)

archean   (2.5 - 4.0 by)

haden ( 4.0 - 4.6 by)

ques 3

In the first 560million year the earth was evolving from the moltane stage . so they are getting cristallise after the 560 million years so we are unable to find the rock below that age.

ques 4

the meteorites are derived from the protostar sun when our earth was derived from the sun many other planet also derived and the planetory dust too. so they remained with the same chemical composition. but our earth changing its chemical composition thats why we can use the meteorite data to get the earth age.

ques 5

1 the trace element inside the zircon crystal tell us that they came from water-rich, granite-like rocks such as granodiorite or tonalite,That means Earth cooled quickly enough for surface water and continental-type rocks.

2 the early earth was same as it is today.

ques 6

the oldest rock found at the middle of the contenent because at the side area of the contenent it may get either eroded or it may get transformed into another rock.

ques 7

the ultramafic lava of archean age are restricted with contenental settings, they are mainly peridotitic in composition and they are rich in trace elements. no in present these are not forming because at that time our crust are forming and the mantle was depleting but now our mantle is depleted in trace elements and REE.

ques 8

Archean Earth was significantly hotter than it is today,so they make the less denser contenental crust and the oceanic lithosphere too. so at that time there must be no subduction or the less denser oceanic lithosphere subducte at very shallow level due to less densty.

ques 9

in the archean time there were either no oxigen or they have oxidising environment . so in oxidizing environment there were no precipitation of oxidised mineral im the last archean time . but in the early time there were no oxigen in the earth at all.

ques 10

in the archean life we have onli soft tissue like organisms Archea that was similar to present bacteria. no it need not oxigen since it is single celled and chemically precipitated organism. at present we have Bacteria similar to that.

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