What Processes Shape our Earth?

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22 Φεβ 2014 (πριν από 3 χρόνια και 5 μήνες)

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What Processes Shape our Earth?


Geology
:
the scientific study of the origin,
history, structure, and composition of the
Earth


Importance:


Understanding of the forces that shape our
Earth, so we can better forecast potential
disasters


Characterized by a gradual increase in
temperature, pressure, and density with depth




Crust is the outer most layer of Earth,
consists of two types:


Continental: makes up continents, older than
oceanic crust, thick


Oceanic: makes up the ocean floor youngest
rocks, thin, mostly made of basalt (igneous
rock)


Age of The Earth & Geologic Time

Principle of
Uniformitarianism
:


The order in which layers are deposited is
how they relate to time.


The same processes that are at work today
were at work in the past



*The present is the key to the past!


Age of The Earth & Geologic Time


Relative Dating:

Finding the age of something,
compared to something else. We use a number of
principles and laws to do this:

1.
Law of Original Horizontality



soil is deposited
horizontally, then forms rock layers

2.
Law of Superposition

-

the layer below is older
than the layer above.


Scientist use law of superposition by using
relative age
(something compared to the age of
something else)


Problem:
uncomformity



buried surface that
represents a break in the rock record.


Age of The Earth & Geologic Time



Relative Dating


3. Lateral Continuity
-

layers of sediment extend
in all directions when they form.

4. Law of Cross
-
cutting Relationships

-

A rock is
older

than any rock across which it cuts.
Folds &
faults are younger than the layers they cut across.



Age of The Earth & Geologic Time

5.
Inclusions:
the inclusions (rock pieces) are older
than the surrounding rock

6.
Faunal Succession:
fossils can be used to identify
relative layers of rock


Index fossil:

1.
Lived in a certain time span in many places

2.
Lived in great numbers

3.
Distinct features to identify


Age of The Earth & Geologic Time


Correlation:
matching rocks by index fossil in
different places


1
st

person to use correlation was William Smith


Smith & others developed the Geologic Column


a diagram of the sequence of rock layers in an
specific area,

in order of age

Early Estimates of Earth’s Age



Methods

1.
Sedimentation


estimated the total thickness of
the earth’s sedimentary rocks (rate of
sedimentation)


Range from 3 million to 1.5 billion years


Problems

1.
Rate can vary at different times & places

2.
No accurate way to measure thickness

3.
Started with different measurements

Early Estimates of Earth’s Age


2. “Salt” Method


estimated the salt content of the
oceans, then compared it with the rate at which salt
is being added to the oceans.



Range from 9 million to 2.5 billion years


Problems


1. assumed at first the oceans contained fresh
water


2. did not account for all way salt is added to or
removed from the oceans


3. Each scientist estimated amounts of salt

Early Estimates of Earth’s Age


3. Kelvin Method


assumed Earth was hot
molten rock, he measured rate of Earth’s cooling
to present. Also, took into account heat coming
from within the Earth and from the Sun.


Range from 20 million years to 100 million years


Problems

1.
No one knew about radioactivity

Absolute Age Dating

Absolute Dating:

Enables scientists to determine the exact
numerical age of rocks and other objects

Radioactive Decay
:


Radioactive isotopes break down into other elements by
emiting

radiation


An element is defined by the number of protons it
contains


As the number of protons changes with each emission,
the original radioactive isotope (parent) gradually
converted to a different element (daughter)


Ex
: Uranium


238, parent
,

changes into Lead
-
206, daughter


Dating an object using radioactive
isotopes


As the number of parent atoms decrease
during radioactive decay the number of
daughter atoms increase


Rate of decay never changes, but is
based on elements half life


Half life: time it takes for half of the isotope to
break down into another element


Uranium 238 use to determine age of non
-
living things


Carbon used to determine age of once living
things




Dendrochronology: use tree rings to
determine absolute age


Uses tree rings called annual rings (early & late
season)


The width of the rings depends on certain
conditions in the environment


Trees from the same geographic region tend to
have the same patterns of ring width for a give
span of time


By matching the rings in these trees scientists
have established tree ring chronologies up to
10,000 years


Ice Cores: they contain a record of past
environmental conditions in annual layers
of snow deposition


Summer ice tends to have more bubbles
and larger crystals than winter ice


Ice
-
core
chronologies study
glacial cycles
and climate



Varves
: bands of alternating light and
dark colored sediments of sand, clay
and silt


Occur with seasonal deposition of sediments
around lakes usually


Sand
-
sized particles in summer (more) and
thinner, fine
-
grained particles in winter (few)


Scientists can date cycles of glacial sedimentation
over periods as long as 120,000 years




Half Life: The amount of time required for
half of a substance to decay


After one half life there is 1/2 of original
sample left.



After two half
-
lives, there will be

1/2 of the 1/2 = 1/4 the original sample.

You have 100 g of radioactive C
-
14. The
half
-
life of C
-
14 is 5730 years.



How many grams are left after one half
-
life?



Answer:50 g



How many grams


are left after two


half
-
lives?


Answer:25
g



The half
-
life of iodine
-
131 is 8 days.


If you start with 36 grams of I
-
131, how
much will be left after 24 days?


24 days/8days = 3 half lives



36 g


18 g


9
g


4.5 g

1 half
life

2

half
lives

3

half
lives