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Earths History, most fossils are found in sedimentary rocks., A comparison…
Earths History
Fossils
Relative Dating
Law of superposition
; an undisturbed sequence of rock layers (strata), the oldest layer is at the bottom, and layers become progressively younger as you go upwards.
Law of original horizontality;
layers of sediment, which form sedimentary rocks, are deposited horizontally or nearly horizontally due to gravity, parallel to the Earth's surface. If there is a disturbance in the line that means a major event happened on the surface causing it to shift.
Law of Cross cutting
relationships:any feature that cuts across another feature must be younger than the feature it cuts
Law of Inclusions
: If there are pieces of rock in a strata, then those chunks are older than what they are in.
Absolute Dating
Uses terms mya (million years ago) and bya (billion years ago)
Used mainly in igneous rocks.
Analyses the chemical properties in rocks/fossils. Uses half lifes: the constant, predictable time it takes for half of the radioactive atoms (parent isotopes) in a sample to decay into stable atoms.
Carbon- 14 Dating: the half life is 5,730 years. C-14 dating only works for fossils that are 60,000 or younger.
Potassium - Argon Dating: The half life is 1.3 billion years old so this is used for older fossils.
Zircon/Uranium led dating: the half life equals 4.5 bil. years and is the most useful method for old rocks. This method works by finding chunks of led in the fossil to analyse.
The Fossil Record:
a timeline of rocks relative ages. We can use this to figure out the ages of newly found rocks or cycles.
Petrified fossi
ls: minerals that replace all or parts of an organsim.
Molds and casts are the most common type of fossil.
Molds
: the hollow area in the sediment that's in the shape of an organism.
Casts:
a copy of the shape of an organism in sediment.
Carbon film:
an extremely thin coating of carbon on rock.
Trace Fossil
s: they provide evidence of activity an organsim has made. Ex: footprints.
Preserved remains:
Fossils can be preserved in tar, amber, or by being frozen.
Index fossils
: Widespread, abundant, popular during a small time period(distinctive). Used to determine the relative age of rocks by comparing the organisms in different strata across different locations.
Geological Time Scale
Paleozoic: 544 Mya-245 Mya.
Periods: Cambrian, Ordovician,Silurian,Devonian,
Carboniferous, Permian.
Important events:
Cambrian explosion
,
End-Ordovician
*Extinction, Late- Devonian extinction, end-Permian extinction.
During this era plants colonized the land and Arthropods (scorpions, millipedes) and early amphibians came onto land, followed by reptiles. The Cambrian explosion was a very important part of this era because it marks the rapid appearance of nearly all major animal groups (phyla) and complex body plans in the fossil record
Cenozoic :
65.5 Mya-Now. Periods; Tertiary, Quaternary
Earth is really old!! Over 4.6 Billion years old.
Mesozoic: 245 mya -65.5 mya Periods:
Triassic, Jurrasic, Cretaceous.
Important events;
End-triassic Extiction, K/T extiction
Extinctions: The geologic time scale can help dating back when important extinctions and mass extinctions happened.
The Geologic Time Scale is broken up into Epochs, Periods, and Eras
Eras
: The largest chunk of time
Period
: Second largest, break up Eras
Epoch
: Smallest chunk of time, break up Periods
Eras from Oldest to youngest:
Paleozoic, Mesozoic, Cenozoic *
we are currently in the Cenozoic Era.
Periods from oldest to youngest:
Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceaus, Tertiary, Quaternary.
*we are currently in the Quaternary Period
Before the Paleozoic Era there were the
Precambrian Times
from 4.6 bya - 544 mya. These aren't separated into Eras because we haven't dug deep enough to gather enough information to.
Rock Cycle
Metamorphic
Metamorphic rocks
are formed from heat and pressure from existing igneous, sedimentary, or metamorphic rocks.
Sedimentary
Sediments:loose, solid particles of minerals, rocks, and organic matter (like dead plants/animals) that are weathered and eroded
Sedimentary rocks are formed from sediments. They go through deposition and burial, then they are compacted and cemented
Igneous
Igneous rocks are formed through cooling and solidification from magma
most fossils are found in sedimentary rocks.
A comparison of 2 or more rocks ages. No exact numbers.
Heat comes from the magma beneath and the pressure comes from the strata above.