Transcript 2. Minerals

6. Minerals and Rocks
6.1 Minerals are all around us
6.2 Rocks form in different ways
6.3 Natural processes break down rocks
6.4 Geologic maps show Earth’s surface
features
6.3 Natural processes break down
rocks
• Before, you learned:
– Minerals make up almost all rocks
– Different minerals have different properties
– Rocks are broken down to form sediments
• Now, you will learn:
– About the relationship between weathering and the
geological cycle
– How mechanical weathering breaks down rocks
– How chemical weathering changes rocks
– What factors affect the rate at which weathering
occurs
Warm-up Questions
• The set of natural processes that form, change,
and re-form rocks is called the rock cycle (t/f)
• True
• A rock is usually made up of only one type of
mineral
• A rock is usually made up of more than one type
of mineral
• Igneous rock develops from layers of sediment
• Sedimentary rock develops from layers of
sediment
Weathering helps change
Earth’s features over time
• Water, ice, and wind can weather mountains down into
small pieces of sediment
– Weathering: set of natural process that break down rocks
• Physical or Chemical
• The sediment can be changed back into large rocks
• New mountains can be pushed up (tectonic plate
movement)
• These are processes in the geologic cycle
– Includes all the processes by which Earth’s features
are worn down and built up
– Rock cycle?
– Weathering?
Geologic Cycle
Mechanical weathering produces
physical changes in rocks
• Does smashing a rock with a hammer
change into a new substance?
• Mechanical weathering – the breaking up
of rocks by physical forces
1.
2.
3.
4.
Ice Wedging
Pressure Release
Plant Growth
Abrasion
Mechanical Weathering: 1. Ice
Wedging
• When water freezes, it expands
– Can freeze in the cracks and pores of rocks
– The expansion can split the rocks apart
• Common in places where temperatures
rise above and fall below the freezing point
of water (0oC or 32oF)
link
Mechanical Weathering: 2.
Pressure Release
• Rocks deep within Earth are under great pressure
– Processes within Earth may push the rock up to the
surface, or the rock above may wear away
– The pressure inside the rock (the internal pressure) is
still high (it was balancing the external pressure from
the surrounding rock)
– When the external pressure decreases:
• External pressure < internal pressure
• Causing the rock to expand – it cracks, leading to
exfoliation
– Exfoliation: a process in which layers or sheets of rock
gradually break off (onion-skin weathering)
Mechanical Weathering: 3.
Plant Root Growth
• Trees, bushes,
and other plants
may take root in
cracks in rocks
• The roots grow
and wedge open
the cracks –
splitting rocks
Mechanical Weathering: 4.
Abrasion
• Water can wear down rocks on riverbeds
and along shorelines
– Worn down by friction (rubbing of one object
or surface against another)
– The water can wear down rocks or can cause
a rock to tumble, wearing down other rocks
Chemical weathering changes
the mineral composition of rocks
• Ex: nail rusting: the steel in the nail
contains iron; oxygen in air and water
react with the iron to form rust!
• Chemical weathering – the breakdown of
rocks by chemical reactions that change
the rocks’ composition
• Minerals in contact with air and water may
dissolve or change into different minerals
Chemical Weathering Dissolving
• Will water cause chemical weathering?
• It is the main cause of it!
– Water can become acidic from the carbon
dioxide in the atmosphere and also soils
• The acids can dissolve rocks, and also make them
more likely to break apart
– Air pollution can make water more acidic
Chemical Weathering - Rusting
• Oxygen in the air will rust minerals that
contain iron
– Iron “oxidizes” (combines with oxygen) and
produces “iron oxides”, or rust
Weathering occurs at different
rates
• Mostly over long periods of time
– Hundreds or thousands of years for a very
hard rock to wear down a few millimeters
(1000 mm = 1m !)
• Factors affect the rate of weathering:
– Surface area (amount of the surface exposed
to the weathering element)
– Rock composition
– climate
Weathering Rates
• Surface area – the more surface exposed to air/water, the
faster the rock will break down
– (think about crushed or cube ice – which melts faster?)
• Rock Composition – granite breaks down much more slowly
than limestone
• Climate – chemical weathering speeds up in hot, wet regions
vs cold, dry regions
– Mechanical weathering caused by freezing/thawing may
occur more in cold regions
How are rocks classified?
• By how they form
– Rocks change, typically over thousands to
millions of years – they break down and reform
• Rock types:
– Igneous Rock
– Sedimentary Rock
– Metamorphic Rock
Math in Science
• Each mineral makes up a certain proportion, or fraction,
of a granite sample
– You can compare mineral amounts by expressing
each mineral’s fraction as a percentage
• To change a fraction to a percentage, you must find an
equivalent fraction with 100 as the denominator
– 1/5 to percent?
• First, divide 100 by the denominator 5 = 20
• Then multiply the numerator and denominator by
20 = 20/100 = 20%
Rocks in the Crust
• Igneous, sedimentary and metamorphic rocks are all
found in Earth’s crust
• 95% is igneous and metamorphic rock
• 5% is sedimentary, a thin covering on Earth’s surface
– Surface of Crust: 75% sedimentary, 25% Ign & Met.
– Entire Crust: 5% sedimentary, 95% Ign & Met.
• Sedimentary is most common at surface because
formed by processes that occur at the surface
– Igneous and metamorphic are formed by process that
occur deeper within Earth
Chapter Investigation:
Weathering (p.204-205)
• Overview and Purpose: Mechanical and
chemical weathering break down rocks.
Different types of rocks react in various ways to
different weathering processes.
• In this investigation you will:
– Observe conditions that allow rusting, a form of
chemical weathering, to occur
– Design a procedure to model the effects of
mechanical and chemical weathering on different
types of rocks
Lab:
Weathering
• Materials: Parts I&II
• Procedure:
– Part I
• 1. Make a data table (p. 205)
• Follow steps 2-5.
– Part II
1.Design a procedure to model the effects of different weathering
processes on several rock samples. You might use some or all of
the materials available. Your procedure should enable you to
collect data, determine if the rocks changed, and describe how
they changed
2.Record your procedure in your science notebook
3.Create a data table to organize the data you will collect
4.Measure and record the mass or volume of each rock sample
5.Carry out your procedure…
Rock
before
Rock
during
Rock
after
Mass of
rock