Transcript How old are rocks?

BELLRINGER #34
1. What is erosion?
 2. How does wind cause erosion?
 3. How does weather cause erosion?
 4. How does ice cause erosion?

How old are rocks?
The position of rocks
can tell its relative age.
Principle of superposition
says if there has been no
disturbance in the position
of the rock layers, the
oldest will be on the
bottom and the youngest
will be on the top.
Sedimentary rock layers in
Zion National Park, Utah.
Radioactive dating gives the absolute age of rocks.
The radioactive elements that make up minerals in rocks
decay at different but constant rates. Geologists use this
data to determine the age of rocks.
Scientists compare the amount of undecayed radioactive
material left in the rock with the decayed material in the
rock. Using the known decay rate for that particular element,
scientists are able to determine the absolute age of the rock.
Using this method is how they have determined some rocks
on Earth to be as old 4.6 billion years old.
S4. Weathering and Erosion
Key Ideas:
How does physical weathering affect rocks?
How are rocks affected by chemical weathering?
What is erosion, and what causes it to happen?
Two types of weather processes: physical and chemical
Physical weathering breaks rocks into smaller pieces but
does not change their chemical composition. Also called
mechanical weathering.
• abrasion is caused by particles suspended in wind
or water
• Frost wedging- water seeps into cracks in rock, freezes,
and pushes the rock apart a small amount. When the ice
thaws, the water seeps further into the rock, then freezes
again until it eventually breaks the rock.
Plant roots can do the same thing!!!
Chemical weathering can cause rocks to weaken
decompose, or dissolve. It can also affect the chemical
composition of the rock.
Badlands National Park,
South Dakota has sedimentary
rock formations with layers rich
in iron. Through the process of oxidation, the iron combines
with oxygen in the air to form iron oxide which gives the
red layers you see.
Carbon dioxide from the air dissolves in rainwater and
forms carbonic acid. This acid reacts with some minerals
and weakens some rocks like limestone.
This slightly acidic water runs underground and reacts with
calcite in limestone to form calcium bicarbonate. This
calcium bicarbonate dissolves in and is carried away by
water leaving underground pockets or caves.
The change in the composition of minerals when they react
chemically with water is called hydrolysis.
Leaching is when minerals dissolve in water from hydrolysis
and are carried to lower layers of rock.
When fossil fuels are burned, sulfur dioxide and nitrogen
oxides are released and react with water in clouds to form
acid precipitation.
Acid precipitation
can erode metal
and rock, especially
limestone and
marble.
The Clean Air Act is a federal law aimed at reducing the
amount of acid precipitation by requiring factories and
power plants to reduce harmful emissions.
Erosion is a process in which the
materials of Earth’s surface are
loosened, dissolved, or worn away
and transported from one place to
another by a natural agent, such as
wind, water, ice, or gravity.
Water erosion shapes Earth’s
surface.
Parunuweap Canyon
Zion National Park, Utah
Water is the most effective
physical weathering agent
and is the most effective
agent of erosion.
Water run-off, rivers,
waves on oceans and lakes
all shape the land we see.
Deposition is the process in which material is laid down.
Sediment moves by erosion, is dropped, and comes to rest.
Deposition happens
when rivers widen or
deepen to the point that
water slows and can’t
carry the sediment away
quick enough. Rivers
flowing into an ocean
do this and form deltas.
Deltas are large, fertile
deposits.
Mississippi River Delta
Glaciers erode mountains.
• The weight of the ice and the forward movement picks up
rocks and debris which grinds against other rocks and
breaks them down.
• Water from melting ice erodes lose material away.
• Glacial erosion is a long, powerful process.
Glacial Striations in Bedrock
Wind can also shape the landscape.
Wind that carries sediment creates
a sandblasting effect.
Arches National Park, Utah
Home to over 2000 “temporary”
natural sandstone arches.
Gravity moves weathered material downhill.
Mass movement- the movement of rock fragments down
a slope.
• Landslide- loose rock and soil suddenly fall down a slope.
• Mudflow- large amounts of mud rapidly move down a hill,
usually after a heavy rain.
• Creep- slow downhill movement of weathered rock
- moves more soil than any other type of mass
movement just not as noticeable.
Humans can cause erosion.
☝ Destroy plants whose strong root systems prevent soil
from eroding.
☝ Deforestation- clearing of trees from an area without
replacing them.
☝ Allowing livestock to overgraze an area.
60-70% of deforestation in the
Amazon is for cattle ranches.
HOMEWORK
Section 4 questions
 Page 761
 #1-9
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