SCI1192 Lecture 4

1. The Rock Cycle
The Blue Planet: Chapter 7

2. From Rock to Regolith

3. Outline
• From Rock to Regolith
• From Regolith to Rock
• New Rock from Old
• From Rock to Magma and Back Again
• The Rock Cycle, Tectonic Cycle and
Earth’s Landscapes

4. From Rock to Regolith
• As soon as the fresh rock of the geosphere
is exposed, it is attacked by the
hydrosphere, atmosphere and biosphere
• Rocks of all kinds are physically broken
apart and chemically altered as deep as air
and water penetrate
• These processes are collectively known as
weathering, which has the end result of the
formation of the regolith

5. From Rock to Regolith
• Minerals in igneous and metamorphic rocks form
at much higher temperatures and pressures
than exist at Earth’s surface
• Chemical weathering occurs as these minerals
are exposed and chemically changed into new,
more stable minerals
• The principal agent of chemical weathering is
weak carbonic acid which is the result of
rainwater dissolving small amounts of carbon
dioxide from the atmosphere

6. From Rock to Regolith

7. From Rock to Regolith
• Some regolith consists of rock fragments
identical to the underlying bedrock
• These fragments have experienced
physical or mechanical weathering
• That is the disintegration of the rock as a
result of physical break up, which results
from pressure reduction, frost wedging,
salt wedging, fire, and plant roots

8. From Rock to Regolith

9. From Rock to Regolith
• Though chemical and physical
weathering are distinct, the two
processes generally work together
• As physical weathering breaks rocks
apart, the surface area increases, which
in turn increases the effectiveness of
chemical weathering

10. From Rock to Regolith

11. From Rock to Regolith

12. From Rock to Regolith
• Weathering breaks rocks into smaller
particles, and nature continuously sweeps
these particles away
• Nature’s “broom” can be
– Flowing water, gravity, wind, or ice
– This transportation is called erosion
– Transported regolith is called sediment
– After transport, the sediment accumulates in a
new location, this is deposition

13. From Rock to Regolith
• There are three principal families of
sediment
– Clastic: bits of broken rock and minerals
that are moved as solid particles
• Gravel, sand, silt and clay
– Chemical: dissolved substances are
transported in solution and precipitated
• Salt
– Biogenic: biochemical reactions in water
• Calcium carbonate shells, peat in bogs

14. From Rock to Regolith

15. From Rock to Regolith

16. From Rock to Regolith

17. From Rock to Regolith
• Both the flow of fluids and the downslope
movement of rock are controlled by gravity
• When the central motivating force is gravity,
this is called mass wasting (landslide)
• Landslides may appear to happen for no
reason, but are generally triggered by
– Earthquakes, volcanic eruptions, heavy rains,
stream erosion, storm surf on sea cliffs, and
human activities that modify slopes

18. From Rock to Regolith

19. From Rock to Regolith

20. From Rock to Regolith
• Locations where clastic sediment is
deposited, low-lying areas, are largely
controlled by plate tectonics
– Troughs
– Rift valleys
– Trenches and accretionary wedges
– Basins

21. From Rock to
Regolith

22. Outline
• From Rock to Regolith
• From Regolith to Rock
• New Rock from Old
• From Rock to Magma and Back Again
• The Rock Cycle, Tectonic Cycle and
Earth’s Landscapes

23. From Regolith to Rock
• The process by which sediment or
regolith becomes rock is lithification
• This produces sedimentary rock
• Bedding is a banded appearance in
many sedimentary rocks that results
from sediment deposition in layers

24. From Regolith to Rock

25. From Regolith to Rock
• The process of lithification has multiple
steps
– As sediment accumulates, the pile grows
thicker and causes compaction
– For the particles to adhere to each other,
either cementation or recrystallization must
take place
– These low-temperature and low-pressure
changes that happen to sediment after
deposition are collectively called diagenesis

26. From Regolith to Rock

27. From Regolith to Rock
• The end result of lithification is
sedimentary rock, which can reveal
information about the source region,
weathering and deposition processes
that formed it
• Besides bedding, the presence of
fossils is an important indicator of
sedimentary origin

28. From Regolith to Rock

29. From Regolith to Rock
• When clastic sediment is lithified, the
result is clastic sedimentary rock
– Conglomerate: rounded clasts > 2 mm
– Breccia: angular clasts > 2 mm
– Sandstone: clasts 0.5 - 2 mm
– Siltstone: silt and clay-szied particles
– Shale: mostly clay-sized particles in a rock
that easily splits into sheets
– Mudstone: shale that does not split

30. From Regolith to Rock
• Chemical sedimentary rock results from
lithification of chemical sediment formed by
precipitation of minerals from water
– Evaporite: formed by evaporation
– Banded iron formation: formed during an
atmospheric change from O2-poor to O2-rich
– Limestone: lithified shells and other skeletal
material from marine organisms
– Chert: tiny particles of quartz from siliceous
skeletons of microscopic sea creatures

31. From Regolith to Rock

32. From Regolith to Rock
• The principles of stratigraphy
– The principle of original horizontality: states
that sediment is deposited in a layer that is
horizontal and parallel to Earth’s surface
– The principle of stratigraphic superposition:
states that in any sequence of strata, the
order of deposition is from bottom to top
– The principle of lateral continuity: states that
a layer of sediment will extend horizontally as
far as it was carried, thinning laterally

33. From Regolith to Rock

34. From Regolith to Rock
• Stratigraphic correlation is the
determination of equivalence in age of
the succession of strata found in two or
more different areas
• This is accomplished by comparing
fossils and other characteristics of
sedimentary strata

35. From Regolith to Rock

36. From Regolith to Rock
• A sequence of strata deposited without
interruption is said to be conformable
• There are often breaks in a pile of strata that
represent times of nondeposition or erosion,
to which the term unconformity is applied
• There are 3 important types of unconformities
– Nonconformity
– Angular unconformity
– Disconformity

37. From Regolith to Rock

38. From Regolith to Rock
• The principle of cross-cutting
relationships: states that any geologic
feature must be older than any feature
that cuts it
• Similarly, a foreign rock that is encased
within another rock unit must predate the
rock that encloses it

39. From Regolith to Rock

40. Outline
• From Rock to Regolith
• From Regolith to Rock
• New Rock from Old
• From Rock to Magma and Back Again
• The Rock Cycle, Tectonic Cycle and
Earth’s Landscapes

41. New Rock from Old
• Metamorphic rocks undergo changes in
texture, mineralogy, or both while in the
solid state
– Low-grade: 150˚C–550˚C and low pressure
– High-grade: above 550˚C and high pressure
• Other factors also play an important role
in metamorphism: fluids, time, and stress

42. New Rock from Old

43. New Rock from Old
• Fluids trapped in the pores between
rock grains heat up during
metamorphism and can speed up
chemical reactions
• When there are abundant pore fluids
involved in metamorphism, it is called
metasomatism

44. New Rock from Old
• Rock can be heated by burial, exposure
to igneous intrusions, or collision
• Each of these can be associated with
different pressures so metamorphism
can rarely be due only to temperature
• The term stress implies direction, and is
a more useful term than pressure,
especially since metamorphic rocks
record differential stress in their textures

45. New Rock from Old
• Differential stress is stress that is not
equal in all directions
• Commonly this produces the parallel
alignment of certain minerals that gives
the rock a stripey pattern (gneiss) or a
planar fabric (foliation)
• Metamorphism also produces new
mineral assemblages that are stable at
the new pressure and temperature

46. New Rock from Old

47. New Rock from Old
• The processes that result from
changing temperature and pressure are
either mechanical deformation or
chemical recrystallization or both
• Different kinds of metamorphism reflect
the importance of the two processes
– Contact metamorphism
– Burial metamorphism
– Regional metamorphism

48. New Rock from Old
• Contact metamorphism
– Where magma intrudes rock, high temperatures
cause chemical reactions and recrystallization
• Burial metamorphism
– Buried sediment may attain temperatures
greater than 150˚C, causing recrystallization
• Regional metamorphism
– Differential stress, mechanical deformation and
recrystallization from mountain range formation

49. New Rock from Old

50. New Rock from Old
• Classification of metamorphic rocks is based
on rock texture and mineral assemblage, and
primarily names the metamorphic derivatives
of
– Shale -> slate -> phyllite -> schist -> gneiss
– Basalt -> greenschist -> amphibolite -> granulite
– Limestone -> marble
– Sandstone -> quartzite

51. New Rock from Old

52. New Rock from Old
• The concept of metamorphic facies states
that for a given range of temperature and
pressure and for a given rock composition,
the assemblage of minerals formed during
metamorphism is always the same
• Plate tectonics explains the regional
distribution of metamorphic facies and
regionally metamorphosed rock

53. Outline
• From Rock to Regolith
• From Regolith to Rock
• New Rock from Old
• From Rock to Magma and Back Again
• The Rock Cycle, Tectonic Cycle and
Earth’s Landscapes

54. From Rock to Magma and
Back Again
• When rock is heated to the point of
melting, even partial melting, it becomes
magma, which will become igneous rock
• Cooling and crystallization determine the
properties of the igneous rock
– Crystals grow in an interlocking texture
– Rate of cooling determines crystal size

55. From Rock to Magma and
Back Again
• Rocks that contain a lot of silica are
called felsic, rocks that contain little
silica are called mafic
• When magma or lava solidifies the
mineral assemblage is the same for
both intrusive and extrusive rock,
however the texture is different

56. From Rock to Magma and
Back Again

57. From Rock to Magma and
Back Again
• Rapid cooling: volcanic rock
– Volcanic rocks have characteristically fine
grained texture, lava cools so rapidly that
minerals do not have time to grow large
– Some lava cools so rapidly it forms glass
– Pyroclastic rock is transitional between
igneous and sedimentary, forming tephra
• Fused ash forms welded tuff
• Bomb-sized tephra is called agglomerate
• Lapilli or ash-sized tephra is called tuff

58. From Rock to Magma and
Back Again

59. From Rock to Magma and
Back Again
• Slow cooling: plutonic rock
– Intrusive igneous rock tends to be coarse
grained because magma within the crust
cools slowly and has time to grow crystals
– Extremely coarse-grained rock is called
pegmatite
– A mixture of large and small grains is
called porphyry

60. From Rock to Magma and
Back Again
• There is an enormous diversity of
igneous rocks that arise from the three
principal magma compositions
• Fractional crystallization contributes to
the diversification of igneous rocks
– Crystallization is halted, the crystals are
separated from the melt, or the melt is
injected with additional magma

61. From Rock to Magma and
Back Again

62. Outline
• From Rock to Regolith
• From Regolith to Rock
• New Rock from Old
• From Rock to Magma and Back Again
• The Rock Cycle, Tectonic Cycle and
Earth’s Landscapes

63. The Rock Cycle, Tectonic Cycle,
and Earth’s Landscapes
• The major components of the Earth
system meet at the land surface
• Constant changes of Earth’s surface
reflect the ongoing contest between
internal forces that raise the lithosphere
and external forces that wear it down
• Uplift, isostasy, and volcanism are
driven by Earth’s internal heat energy

64. The Rock Cycle, Tectonic Cycle,
and Earth’s Landscapes
• Gravity and the Sun’s energy drive
denudation: the destructive effects of
weathering, erosion, and mass wasting
• The net result is the progressive
sculpting of the land into varied relief
• Uplift rates are variable and change
through time, as are denudation rates

65. The Rock Cycle, Tectonic Cycle,
and Earth’s Landscapes

66. The Rock Cycle, Tectonic Cycle,
and Earth’s Landscapes
• Landform development in any given
location is controlled by
– Process
– Climate
– Lithology
– Relief
– Time

67. The Rock Cycle, Tectonic Cycle,
and Earth’s Landscapes

68. The Rock Cycle, Tectonic Cycle,
and Earth’s Landscapes
• Major landscape features of Earth have
developed over long intervals of time
• Change may be started by a tectonic
event, by substantial sea level change,
or by a shift in climate
• A landscape never achieves a state of
equilibrium, it is, and likely always has
been, a dynamic surface

69. The Rock Cycle, Tectonic Cycle,
and Earth’s Landscapes