Do Now

• What are the two types of metamorphism

• What type of rock is “born from fire” as its Latin name suggests?

• How can a metamorphic rock become an igneous rock?

Objective

• SWBAT: Describe how a fossil is formed

• Compare and contrast mechanical and chemical weathering

Fossils

• Fossilization - refers to a variety of often complex processes that enable the preservation of organic remains within the geological record.

• Conditions:• rapid and permanent burial• oxygen deprivation • continued sediment • absence of excessive heating or compression

Fossils

Usually formed beneath sediments deposited beneath water

Why do you think fossils are not as common on land as they are in the marine environment?

Fossil Formation

• Fish returns to its birth place to spawn

• After spawning the fish dies and sinks to the seafloor.

• After several weeks the soft body tissues have mostly decayed

• Nearby tectonic activity causes sediment to mobilize and buries the specimen

• Several months pass and all that remains of the buried fish is its skeleton

• As times passes more sediment accumulates above the fish and the skeleton is gradually compressed and permineralised.

Permineralization

• Process of fossilization • Mineral deposits form internal casts of

organisms

• Minerals fill the spaces within organic tissue

Permineralization

• Silicifaction- Silicate minerals from weathering

• Minerals permeate pores and cells of dead organisms and becomes a gel.

• Gel will dehydrate over time and form a crystal structure that is an internal cast of the organism

• Most common type

• Over time the rock is distorted and uplifted by geological forces associated with continental movement, raising it above sea level

• The uplifted rock is exposed to weathering and gradually erodes away, eventually exposing the tip of the fish's skull at the surface.

• A palaeontologist recognises the fish by the small area of skull exposed and begins to carefully extract the specimen.

• A Pomognathus fish from Houghton Quarry - the skull is clearly visible, and what parts of the skeleton remain are obscured within the chalk matrix.

Your Turn….

• You and a friend are walking along Herne Bay when your friend discovers something on the ground.

• Knowing you have strong science skills, your friend shows you the amazing discovery…..

• Wow! What a find! Your friend has stumbled across a fragment of a turtle shell.

• Write out an explanation to your friend about how this fossil was formed.

Project

• Journey of a sedimentary rock

• Due friday

Objective

• SWBAT: Compare and Contrast Mechanical and Chemical weathering

Weathering: the disintegration, or breakdown of rock material

Weathering

• Mechanical• no change in

chemical composition--just disintegration into smaller pieces

• Chemical• breakdown as a

result of chemical reactions

• CaCO3+CO2+H2O --> Ca2+ + 2HCO3-

Mechanical Weathering

Physical breakup• pressure release• water: freeze - thaw cycles• crystallization of salt in cracks• thermal expansion and contraction

All this increases the total surface area exposed to weathering processes.

Mechanical Weathering

Exfoliation:Rock breaks apart in layers that are parallel to the earth's surface; as rock is uncovered, it expands (due to the lower confining pressure) resulting in exfoliation.

WeatheringMechanical Weathering

Stone Mountain, GA

Half Dome, Yosemite, CA

Sheet Joints(Exfoliation)

Exfoliated Domes, Yosemite

Frost Wedging: rock breakdown caused by expansion of ice in cracks and joints

Stone Mountain, GA

Stone Mountain, Georgia, showing the product of exfoliation due to unloading

Shattered rocks are common in cold and alpine environments where repeated freeze-thaw cycles gradually pry rocks apart.

WeatheringThermal expansion due to the extreme range of temperatures can shatter rocks in desert environments.Repeated swelling and shrinking of minerals with different expansion rates will also shatter rocks.

Role of Physical Weathering

1) Reduces rock material to smaller fragments that are easier to transport

2) Increases the exposed surface area of rock, making it more vulnerable to further physical and chemical weathering

Surface Area and Weathering

Joints in a rock are a pathway for water – they can enhance mechanical weathering

Rates of weathering

Chemical Weathering

Definition: transformation/decomposition of one mineral into another

Mineral breakdown• carbonate dissolves• primary minerals --> secondary minerals (mostly clays)

Net loss of elements retained in the soil.

Chemical Weathering

Definition: transformation/decomposition of one mineral into another

Mineral breakdown• carbonate dissolves• primary minerals --> secondary minerals (mostly clays)

Net loss of elements retained in the soil.

• Water is the main operator:– Dissolution

• Many ionic and organic compounds dissolve in water

– Silica, K, Na, Mg, Ca, Cl, CO3, SO4

Chemical Weathering

Dissolution

H2O + CO2 + CaCO3 --> Ca+2 + 2HCO3-

water + carbon dioxide + calcite dissolve into calcium ion and bicarbonate ion

Biological activity in soilsgenerates substantial CO2

Bicarbonate is the dominant ion in surface runoff.

Solution: process by which rock is dissolved in water

• Is strongly influenced by pH and temperature• When water becomes saturated, chemicals

may precipitate out forming evaporite deposits.

• Calcium carbonate (calcite, limestone), sodium chloride (salt), and calcium sulfate (gypsum) are particularly vulnerable to solution weathering.

Chemical Weathering

Biological Weathering

Can be both chemical and mechanical in nature.

• roots split rocks apart

• roots produce acids that dissolve rocks.

• tree throw

• burrowing animals

Rates of weathering• Climate

– Temperature and moisture characteristics

– Chemical weathering• Most effective in areas of warm, moist climates –

decaying vegetation creates acids that enhance weathering

• Least effective in polar regions (water is locked up as ice) and arid regions (little water)

– Mechanical weathering• Enhanced where there are frequent freeze-thaw

cycles

Weathering

Mechanical and Chemical Weathering

– Fracturing, disintegration caused by mechanical weathering exposes more surface area.

– Greater surface area, means more places for chemical action to occur.

Venn diagram

• Create a venn diagram to compare and contrast mechanical and chemical weathering

Exit Ticket

• Name 3 conditions necessary for fossil formation

• Describe exfoliation. Include what type of weathering it is (mechanical or chemical).

• How does weathering affect surface area?