1. Weathering and Soil
Geology 101
Ted Brewster

12. Weathering The act of breaking down existing rock
Can be done by mechanical or chemical processes
Both methods serve to create sediment from existing rock types

13. Mechanical Weathering
Physical Disintegration of the rock
The rock breaks into smaller pieces
More surface area exposed to chemical weathering processes
The rock changes physically but not chemically
Examples
Frost wedging, roots, pressure release, and abrasion

15. Change in Surface Area

16. Frost Wedging Water fills into small cracks within the rock
When water freezes into ice it expands
This small expansion slowly makes the crack larger, so more water can fill in
Over time and successive melting and freezing cycles the cracks open up enough to split the rock into pieces

17. Frost Wedging

18. Frost Wedging

21. Roots – Biological Weathering
The act of plant roots breaking rocks apart
The roots of the plant find cracks and slowly grow downward into the crack
Over time the effect of the root growth breaks the rock into pieces

22. Tree in Limestone

23. Tree Roots

25. Pressure Release
Most rocks are formed at depth and then are moved to the surface
The pressure at depth is high, as the rocks move to the surface they expand
This is similar to compressing a sponge, then letting expand in water
The expansion causes the mineral grains to break away from each other

26. Pressure Release - Sheeting

27. Spheroidal Weathering

28. Abrasion Removing small pieces of rock over time by wind or water
Like using sandpaper on the rock
Slowly the rock is just worn away
Causes the rounding of angular pieces

29. Abrasion – Death Valley, CA

30. Differential Weathering
Layers of rock are not of the same resistance to weathering
Some layers are left intact while others are weathered away almost completely
Most prominent in layers that have been tilted onto their sides

31. Differential Weathering - Dike

32. Sorting and Rounding
As pieces of sediment move away from their parent rock they change
Sorting – the ratio of different sizes of material within the rock
More uniform size indicates farther from source
Rounding – the shape of the edges of the sediment pieces
More rounding indicates farther from source

33. Sorting

34. Rounding

35. Chemical Weathering
Rock decomposition that actually dissolves the rock minerals and forms new ones
Most minerals form at depth with little water
Surface has abundant water, so the reaction can form new minerals
Some minerals are soluble in water and dissolve
Limestone, halite, gypsum, etc

36. Weathering of Granite

37. Chemical Weathering

38. Slate vs Limestone

39. Acid Weathering Many carbonate minerals are susceptible to acids
If the water around them contains an acid, then the rock is more likely to dissolve quickly
We see this in cities with acid rain
Buildings, Statues, Tombstones, etc

40. Cleopatra’s Needle

41. Salt Weathering - Greece

42. Climate
Climate plays a big role in determining the type of weathering most common
Desert – mechanical abrasion
Tropical (warm and wet) – chemical
Arctic – mechanical frost wedging

43. Climate and Weathering

44. Soils Weathered, unconsolidated materials on top of bedrock
Capable of supporting plant growth
Other material is called regolith
Soils form over long periods of time
Exact time depends on the climate conditions
Soils have layers called horizons

45. Soil Horizons O horizon A horizon
Dark colored layer just below the vegetation
Contains decomposed plant material called humus that forms organic acids
A horizon
Zone of leaching formed by percolation of rain water through the O horizon
Wet climates have larger A horizons than deserts

46. Soil Horizons B horizon C horizon
Accumulation of leaching products from horizon A
Tends to be clay rich and stained red by hematite and limonite
C horizon
Incompletely weathered bedrock
Transitional layer between bedrock and evolving soil above

47. Soil Profile by Climate

48. Soil Horizons - Hawaii

49. Soil Horizons in AZ

50. Sedimentary Structures
Ancient structures that can determine the depositional environment of the rock

51. Graded Beds
Graded Beds – sorted layers of sediment

52. Ripple Marks
Ripple marks – shallow ripples that indicate a current direction

53. Cross Bedding
Cross bedding – angled sediment layers that indicate either wind or water flow direction

54. Fossil Roots
Fossil plant roots – filled in holes made by roots

55. Mud Cracks
Mud cracks – filled in cracks from muddy and dry environments

56. Dinosaur Footprint
Fossil footprints, tracks, or burrows – any impression in the sediment made by living creatures that was then filled in