1. Last time
Viscosity determines the ability of a melt to releases gasses coming out of solution
Viscosity depends on silica content and
TEMPERATURE
Mafic (Basaltic) melts are extremely hot and have low silica, do not explode much
Felsic (Granitic) melts are only moderately hot and have very high silica. Explode violently

2. Limestone dissolved by naturally acidic rainwater
Weathering and Soils
Lecture 5

3. Earth’s surface processes
Weathering – Physical breakdown and chemical alteration of rock at Earth’s surface
Erosion – The transport of rock fragments

4. Preview of Mechanical Weathering
Source: Tom Bean/DRK Photo

5. Mechanical weathering – breaking of rocks into smaller pieces
Four types of mechanical weathering
Frost wedging – freezing and thawing of water in cracks disintegrates rocks
Salt wedging is similar

6. Evidence of Frost Wedging in Wheeler Park, Nevada
Source: Tom Bean/DRK Photo

7. Rockfall caused by frost wedging ends up in a stream
Colluvium

8. Salt Weathering is similar, growing salt crystals in cracks
split the rock apart. At the beach and desert playas.

9. Unloading = Mechanical Exfoliation
Granite Batholith above subduction zone
Igneous rocks at Earth’s surface peeling like layers off an onion due to reduction in pressure

10. Unloading - Exfoliation of a Pluton

11. Mechanical Exfoliation in Yosemite National Park
Source: Phil Degginger/Earth Scenes

12. 3. Thermal expansion and contraction
Alternate expansion and contraction due
to heating and cooling
- Important in deserts

13. Tree Roots Growing in Rock Fractures Animal Burrows
Biological
Activity
Tree Roots Growing in Rock Fractures Animal Burrows
Source: Runk/Schoenberger/Grant Heilman

14. Mechanical weathering causes increase in surface area
Same volume of rock, but many pieces have much more surface area than one piece

15. Joint-controlled weathering in igneous rocks
Definition: Joints vs Faults
Increases surface area
for chemical weathering

16. Important for metals concentration
Chemical Weathering
Breaks down rock and minerals
Important agent in chemical weathering is water (transports ions and molecules involved in chemical reactions)
The ions form the cements in Sedimentary Rocks
These ions make the ocean salty
Important for metals concentration

17. Three types of Chemical Weathering

18. Dissolution of Calcite
By carbonic acid, CO2 in water CO2 + H2O => H2CO3
H2CO3 => H+ + HCO3-
Soluble ions contained in underground water
CaCO3 +2H+ => H2CO3 + Ca++
H2CO3 => H2O + CO2

19. Dissolution Weathered and Unweathered Limestone Boulders
Source: Ramesh Venkatakrishnan

20. Oxidation Chemical reaction where compound loses electrons
Usually while bonding with oxygen
Important in breaking down mafic minerals (contain Fe+2 or +3)
Rust- colored mineral (Fe2O3 ) from weathering of Basalt
[which contains Olivine (Fe, Mg) SiO4

21. Oxidation of Basaltic Lava Flows
Rust-colored Iron Oxide forms

22. Hydrolysis Water makes H+ and OH- ions
Hydrolysis is the reaction of any substance with water
Water’s ions replace different ions
in a mineral
Feldspars, most abundant crust minerals, become fine clay particles.
Clays are light weight, flat plates, easily transported by streams

23. Hydrolysis – Feldspar to Clay
Feldspars become
Mechanical fracture due to chemical weathering

24. Hydrolysis Cleopatra’s Needle, (Egypt)
Granite in a
Dry Climate
Source: New York Public Library, Locan
History and Genealogy Division

25. Hydrolysis Cleopatra’s Needle, (Central Park, NYC)
Granite in a
Wet Climate
Source: Runk/Schoenberger/Grant Heilman

26. Vegetation and Soil Development
Plants use Hydrolysis to get nutrient metals out of minerals

28. Bowens Reaction Series and Weathering

29. Soils
Also very important in recognizing past climates

30. Soil - combination of mineral and organic matter, water, and air
It is that portion of the regolith (weathered rock and minerals) that supports the growth of plants

31. Components in topsoil that support plant growth

32. Climate also very important, see below
Factors related to bedrock composition (parent material)
Weathering-resistant sandstone (mostly quartz) yields little soil
Soil
Soil
Soil
Chemical
weathering
by hydrolysis
Iron-rich
basalt
Chemical
weathering
by oxidation
Feldspar-rich
granite
Chemical weathering
by dissolution
Limestone
Climate also very important, see below

33. Variations in soil development due to topography: slope and time

34. Equator to Poles Factors
Annual precipitation
Temperature
Equatorial
and tropical
rain forests
Low-latitude
deserts and
semi-deserts
Temperate
regions
and mixed
boreal forests
Arctic
and
tundra
regions
Grasslands
(steppes)
Savannahs
30 degrees Latitude
40ºC
Precipitation
1800 mm
Temperature
30ºC
Evaporation
20ºC
600 mm
10ºC
Rainforest
US & Europe
Bedrock at
or very near
surface
Desert
Shallow nutrients
Deep Regolith, shallow Soil
Soil
Deeply weathered
bedrock
(~ meters deep)
Bedrock
Increasing depth
of weathering
Factors related to Climate

35. Soil Profile The soil profile
Soil forming processes operate from the surface downward as water descends
Vertical differences are called horizons – zones or layers of soil

36. Soil The soil profile O horizon – organic matter
A horizon – organic and mineral matter
High Biological Activity (animals live here)
Together the O and A horizons make up topsoil
E horizon – little organic matter
Zone of leaching – soluble minerals removed
B horizon – zone of re-precipitation
C horizon – partly altered parent material

37. An idealized soil profile
ONLY
ACTIVE
EDUCATORS
BECOME
CHAMPIONS
An idealized soil profile

38. Remember the different horizonsA E B C
ONLY
ACTIVE
EDUCATORS
BECOME
CHAMPIONS
Organic
Animal Activity
Soluble minerals Exited
Soluble Minerals Back
Crushed Rock

39. Animal Activities in “A” horizon
Worms ingest mineral grains because they are covered with living organisms –their food. Their burrows, not their feeding, increase chemical weathering by exposing the minerals to water and air
Source: Runk/Schoenberger/Grant Heilman

40. E and B: Eluviation & Illuviation
exited
back

41. Illuviation and Eluviation Leaching and Precipitation of IronB
Source: Jens/Gutzmer/Rand Afrikaans University/Geology

42. Pedalfer Pedocal Laterite Soil Types Soil types
The characteristics of each soil type primarily depend on the prevailing climatic conditions
Three very generic soil types
Pedalfer
Pedocal
Laterite

43. pedalfer
pedocal
laterite
Evergreen forests
Shortgrass
tropics

44. Pedalfer
Accumulation of iron oxides and Al-rich clays in the B horizon. Brown B horizon
Best developed under temperate forest landscapes

45. Pedocal White calcium carbonate (caliche) in B horizon
Associated with dry grasslands and brush vegetation

46. Pedocal with Caliche in the B horizon
Organic
Activity
Leached
Accumulation
Crushed Rock

47. Laterite Hot and wet tropical climates Monsoonal Climate
Intense chemical weathering
Red Iron oxide - Topsoil not distinct from B horizon –bacterium responsible for dissolving soil iron is not present
Deep soil but usable nutrients shallow

48. Laterite in Sarawak, Borneo
The percolating rain water causes dissolution of primary rock minerals and decrease of easily soluble elements as sodium, potassium, calcium, magnesium and silicon. This gives rise to a residual concentration of more insoluble elements predominantly iron and aluminum.
Source: Fletcher & Baylis/Photo Researchers, Inc.

49. Earth’s surface processes
Erosion – the physical removal of material by mobile agents like water, wind, ice, or gravity

50. Natural rates of soil erosion depend on:
Soil characteristics
Climate
Slope
Type of vegetation
Headed for the Sea
Source: Ramesh Venkatakrishnan

51. Soil
Soil erosion
In many regions the rate of soil erosion is significantly greater than the rate of soil formation
Farmers level fields to slow loss of topsoil
Farmers have been building terraces for thousands of years