Earth Science Lecture 20 pt. 1 Weathering (Mechanical/Physical Weathering)

weathering and erosion are...

*NOT* the same thing!

Wetting and Drying

-Alternate wetting and drying of soft or poorly-cemented rock (shales) causes fairly rapid breakdown of the rock -Most disintegration occurs during the drying cycle

Pressure relief

-Deep in the crust, rocks experience high *CONFINING PRESSURE* equal to the weight of all the overlying rock (similar to the pressure you feel on your eardrums at the bottom of a deep pool, which is equal to the weight of the water above you)

weathering/erosion example

-On the steep rock faces at the top of the cliff, rock fragments have been broken off by ice wedging, and then removed (eroded) by gravity -Other important agents of erosion that also have the effect of removing the products of weathering include water in streams, ice in glaciers, and waves on the coasts

Freeze-Thaw Action(Frost Wedging/Ice Wedging)

-Water entering cracks in rock expands upon freezing, forcing the cracks to widen -Extremely effective in areas where temperatures regularly fluctuate around 0o C -High mountains in temperate climates -Polar regions

Pressure Relief (Process)

-When a mass of rock is exposed by weathering and removal of the overlying rock, there is a decrease in the confining pressure on the rock, and the rock expands -This unloading promotes cracking of the rock, known as *exfoliation* -Granitic rock tends to. exfoliate *parallel to the exposed surface* because the rock is typically homogenous, and it doesn't have predetermined planes along which it must fracture -Sedimentary and metamorphic rocks tend to exfoliate along predetermined planes

Salt Crystal Growth

-When salt water seeps into rocks and then evaporates on a hot sunny day, salt crystals grow within cracks and pores in the rock -The growth of these crystals exerts pressure on the rock and can push grains apart, causing the rock to weaken and break

Biological Activity (Weathering)

-animals and plants help to open cracks -outward growing force of a tree root has the same effect, though at a lower pressure, as the force of crystallization -tiny root hairs seek out small cracks and pits in rock -root hairs grow and expand -expansion causes great pressure and cracks the rock

Mechanical or Physical Weathering

-breaking apart rocks and minerals -result = smaller components of the same material -*no change in composition!* -produces mostly sand-sized and larger grains -most of the fracturing occurs along mineral boundaries

Salt Crystal Growth (Process)

-crystal exerts force in the direction it's growing -most crystals nucleate on one wall and grow their elongate dimension directly perpendicular to it -force directed against the opposite wall (or against the ends of other crystals growing from the opposite wall)

Impacts/Abrasion

-rock falling from a cliff or bouncing down a steep slope not only breaks itself as it lands, it damages any other rock it impacts en route -Sand grains blown by wind or carried by water abrade both themselves and anything they strike -Boulders carried by glacial ice scour, abrade, and fragment themselves and any rock they contact in the floor or walls of the glacial valley

Types of Mechanical/Physical Weathering

1. Impacts/Abrasion 2. Freeze-Thaw Action 3. Temperature Changes 4. Salt Crystal Growth 5. Pressure Relief 6. Wetting and Drying 7. Biological Activity (biologic weathering)

Weathering Processes

1. Mechanical or Physical Weathering 2. Chemical Weathering (work simultaneously and reinforce each other)

Freeze-Thaw Action(Frost Wedging/Ice Wedging): process of frost wedging on a steep slope

1. Water gets into fractures and then freezes, expanding the fracture a little 2. When the water thaws it seeps a little farther into the expanded crack 3. The process is repeated many times, and eventually a piece of rock will be wedged away

Temperature Changes

Heat=Expand Cool=Contract -expansion and contraction can cause *physical fracturing* -whole rock expands and contracts -*individual crystals do too!* -when many different minerals exist and are oriented in many different directions, effect of crystal expansion is really important

Temperature Changes: Enchanted Rock (near Fredericksburg, TX)

is so named from groaning, creaking, and popping noises it makes as morning sun warms it and evening air cools it

pressure is called "confining" because...

it is equally forceful in all directions -- turning your head sideways does not change the way your ears feel underwater

water increases in...

volume by 9% when it freezes (outward force exerted by this expansion is ~30,000 lb/in2)

Mechanical weathering is...

greatly facilitated by erosion, which is the removal of weathering products, allowing for the exposure of more rock for weathering