Transcript 25WxForms

Weathering Forms
Weathering
1. Weathering Products
2. Weathering Landscapes
1. Weathering Products
Quartz Sand: quartz is one of the last
minerals to decay – it survives weathering
& erosion to be deposited in
Rock
Coatings
Rock
Coatings
Nutrients - released from
mineral weathering
Calcium
Sodium
Magnesium
Potassium
Clay Minerals Formed
Clays represent Earth’s ultimate
decay of rock
If have too
much clay,
it shrinks &
swells
2. Weathering landscapes
Transport-limited
landscapes: where the
rate of transport
(detachment and
erosion) is smaller than
the rate of weathering.
Weathering > Transport
Weathering landscapes:
where
Transport > Weathering
The balance between weathering
and erosion defines the landscape
In deserts – transport is faster
You see bedrock, because
weathering particles eroded away
Humans can upset
the balance and
accelerate
erosion. So
when transport
(detachment and
erosion)
becomes faster
than weathering,
landscapes are
not sustainable.
Granitic weathering landscapes
• Consider a common rock – granitic rocks
(granite, granodiorite, tonalite, diorite …)
made up of interlocking minerals
Decay of
weak
minerals
(biotite,
feldspar)
separates
grains and
makes
granite sand
called GRUS
Grus
produced
most
rapidly
where
joints
intersect
Grus erosion from joints creates
rounded forms at Mt Rushmore
Core stones made
when corners of
granite blocks
weathered into
grus
Core stones in subsurface
are “emerge” onto the
surface as the grus
washes away with rain
and flowing water,
because they are too big
to be carried by water
Grus washes away easily with rain,
leaving piles of core stones - tors
Tors (piled up core stones) very
common in the Sonoran Desert
Tors often take on
significance to people
Used core
stones in
Portugal
Granite weathering took a long time in the
subsurface (from groundwater) – spheroidal
forms were then exposed by erosion of grus
Dome forms produced the same way:
subsurface weathering in joints
Rio de Janeiro - Sugar Loaf
Granite that is not heavily joined becomes
domes after grus washed away
Half dome was
made in the
subsurface in
tropical times
and exposed by
erosion of grus
Karst Topography: entire landscape
made by dissolution weathering
Other rocks can also dissolve to
form karst (gypsum, rock salt)
If exposed see grooves (karren)
If exposed see grooves (karren)
Solution doline – dissolve
fastest in joints
“Sinkhole” (doline)
Can also create doline by collapse
Florida – lots of
groundwater
pumping & roof of
cave collapses
Before Development
solution doline
After
Sinkholes merge to form Uvale valley
“Blind” rivers flow down sinkholes
into cavern systems
Limestone Cave
Caves Formation
Caves
Features
Speleothems:
Cave
formations
Stalagtite
Stalagmite
Limestone Caves
Step 1: Groundwater dissolves limestone, most
aggressively at the water table. Also,
groundwater follows lines of weakness in the
limestone enlarging caves.
Step 2. When the water table drops, stalactites
and stalagmites can form on the roof and floor,
respectively.
The water table usually drops
when the stream has “cut down” to
a lower level
Stalagmite –
requires lots of
time with water
table much lower
The southeast China
karst region has “tower
karst” forms