Transcript Stress - 地質科學系

Chapter 9: How Rock Bends, Buckles,
and Breaks
How Is Rock Deformed?
 Tectonics forces continuously squeeze, stretch, bend,
and break rock in the lithosphere.
 The source of energy is the Earth’s heat, which is
transformed into to mechanical energy.
Stress 應力
 Definition: the force acted on per unit of surface
area, SI unit = Newton/m2=Pascal.
 Uniform stress is a condition in which the stress is
equal in all directions.
 It is confining stress or confining pressure (圍壓)
applied to rocks because any body of rock in the
lithosphere is confined by the rock around it.
 Differential stress is stress that is not equal in all
directions.
Differential (Deviatoric) Stress
 The three kinds of differential
stress are:
 Tensional stress (張應力),
which stretches rocks.
 Compressional stress(壓
應力), which squeezes
them.
 Shear stress (剪應力),
which causes slippage (滑
移)and translation (位移).
Stages of Deformation (變形)
 Strain (應變) describes the deformation of a rock.
 When a rock is subjected to increasing stress, it passes
through three stages of deformation in succession:
 Elastic deformation (彈性變形) is a reversible change in the
volume or shape of a stressed rock..
 Ductile deformation (塑性變形) is an irreversible change in
shape and/or volume of a rock that has been stressed beyond the
elastic limit (彈性限度).
 Fracture (破裂) occurs in a solid when the limits of both elastic
and ductile deformation are exceeded.
Strain (應變)
 Normal Strain (正應變)
(1) Tensional strain
 Shear Strain (剪應變)
(2) Compressional strain
Figure 9.2
Figure 9.3
Ductile Deformation Versus Fracture
 A brittle (脆性) substance tends to deform by
fracture.
 A ductile substance deforms by a change of shape.
 The higher the temperature, the more ductile and
less brittle a solid becomes.
 Rocks are brittle at the Earth’s surface, but at depth,
where temperatures are high because of the
geothermal gradient, rocks become ductile.
A and B have the same
elastic deformation, but B
experiences larger ductile
deformation because the
temperature of A is lower
than that of B or the
confining pressure applied to
B is higher than that to B.
Figure 9.4
Confining Stress
 Confining stress is a uniform squeezing of rock
owing to the weight of all of the overlying strata.
 High confining stress hinders the formation of
fractures and so reduces brittle properties.
 Reduction of brittleness by high confining stress is a
second reason why solid rock can be bent and
folded by ductile deformation.
Fracture
 All the constituent atoms of a solid transmit stress
applied to a solid.
 If the stress exceeds the strength of the bond
between atoms:
 Either the atoms move to another place in the crystal
lattice in order to relieve the stress, or;
 The bonds must break, and fracture occurs.
Strain Rate (應變速率)
 The term used for time-dependent deformation of a
rock is strain rate.
 Strain rate is the rate at which a rock is forced to change
its shape or volume.
 Strain rates in the Earth are about 10-14 to 10-15/s.
 The lower the strain rate, the greater the tendency
for ductile deformation to occur.
A has a high elastic
limit and a low ductile
deformation, so the
temperature is low and
the strain rate is also
low.
B has a lower elastic
limit, and high
temperature, so the
ductile deformation is
still high even B has a
high strain rate.
C has the lowest elastic
limit and largest ductile
deformation because its
temperature is high and
strain rate is low.
Figure 9.6
Enhancing Ductility
 High temperatures, high confining stress, and low
strain rates (characteristic of the deeper crust and
mantle):
 Reduce brittle properties.
 Enhance the ductile properties of rock.
Composition Affects Ductility (1)
 The composition of a rock has pronounced effects on its
properties.
 Quartz (石英), garnet (柘榴石), and olivine (橄欖石) are very brittle.
 Mica (雲母), clay (黏土), calcite (方解石), and gypsum (石膏) are
ductile.
 The presence of water in a rock reduces brittleness and
enhances ductile properties.
 Water affects properties by weakening the chemical bonds in
minerals and by forming films around minerals grains.
Composition Affects Ductility (2)
• Rocks that readily deform by ductile deformation
are limestone (石灰岩), marble (大理石), shale (頁
岩), phyllite (千枚岩) and schist (片岩 ).
 Rocks that tend to be brittle rather than ductile are
sandstone (砂岩) and quartzite (石英岩), granite
(花崗岩), granodiorite (花崗閃長岩) , and gneiss
(片麻岩).
Rock Strength (1)
 Rock strength (岩石強度) in the Earth does not change
uniformly with depth.
 There are two peaks in the plot of rock strength with
depth.
 Strength is determined by composition, temperature, and pressure.
 Rocks in the crust are quartz-rich, so the strength
properties of quartz play an important role in the
strength properties of the crust.
 Rock strength increases down to a depth about 15 km.
Above 15 km rocks are strong (they fracture and fail by
brittle deformation).
Rock Strength (2)
 Below 15 km, fractures become less common
because quartz weakens and rocks become
increasingly ductile.
 Rocks in the mantle are olivine-rich. Olivine is
stronger than quartz, and the brittle-ductile
transition of olivine-rich rock is reached only at a
depth about 40 km.
Figure 9.7
Rock Strength (3)
 By about 1300oC, rock strength is very low.
 Brittle deformation is no longer possible. The
disappearance of all brittle deformation properties
marks the lithosphere-asthenosphere boundary.
 In the crust large movements happens so slowly
(low strain rates) that they can be measured only
over a hundred or more years.
Abrupt Movement
 Abrupt movement results from the fracture of brittle
rocks and movement along the fractures.
 Stress builds up slowly until friction between the two
sides of the fault is overcome, when abrupt slippage
occurs.
–
–
The largest abrupt vertical displacement ever observed occurred
in 1899 at Yakutat Bay, Alaska, during an earthquake. A stretch
of the Alaskan shore lifted as much as 15 m above the sea level.
Abrupt movements in the lithosphere are commonly
accompanied by earthquakes.
Gradual Movement
 Gradual movement is the slow rising, sinking, or
horizontal displacement of land masses.
 Tectonic movement is gradual.
 Movement along faults is usually, but not always, abrupt.
台灣板塊構造圖
台灣的地震分布和地體
構造
資料:中研院地球所
從全球衛星測量系統
(GPS)看臺灣地殻的變動
台灣地區全球衛星測量(GPS)觀測 (水平分量)
Figure 9.9
Evidence Of Former Deformation
 Structural geology is the study of rock deformation.
 The law of original horizontality tells us that
sedimentary strata and lava flows were initially
horizontal.
 If such rocks are tilted, we can conclude that
deformation has occurred.
Dip (面的傾角) and Strike (走向)
 The dip is the angle in degrees between a horizontal
plane and the inclined plane, measured down from
horizontal.
 The strike is the compass direction (North) of the
horizontal line formed by the intersection of a
horizontal plane and an inclined plane.
Figure 9.10
Figure 9.11
Deformation By Fracture
 Rock in the crust tends to be brittle and to be cut by
innumerable fractures called either joints (解理 )or
faults (斷層).
 Most faults are inclined.
 To describe the inclination, geologists have adopted two
old mining terms:
–
–
–
The hanging-wall (上磐) block is the block of rock above an
inclined fault.
The block of rock below an inclined fault is the footwall (下磐)
block.
These terms, of course, do not apply to vertical faults.
Figure 9.12
Classification of Faults (1)
 Faults are classified according to:
 The dip of the fault.
 The direction of relative movement.
 Normal faults (正斷層) are caused by tensional
stresses that tend to pull the crust apart, as well as
by stresses created by a push from below that tend
to stretch the crust. The hanging-wall block moves
down relative to the footwall block.
Figure 9.13
Figure 9.13B
Classification of Faults (2)
 A down-dropped block is a graben (地塹), or a rift (張裂),
if it is bounded by two normal faults.
 It is a half-graben (半地塹) if subsidence occurs along a
single fault.
 An upthrust block is a horst (地壘).
 The world’s most famous system of grabens and half-grabens is the
African Rift Valley of East Africa.
 The north-south valley of the Rio Grande in New Mexico is a graben.
 The valley in which the Rhine River flows through western Europe
follows a series of grabens.
 The Basin and Range Province in Utah, Nevada, and Idaho is a series of
nearly parallel north-south striking normal faults which has formed
horsts and half-grabens. The horsts are now mountain ranges and the
grabens and half-grabens are sedimentary basins.
Figure 9.14
Classification of Faults (3)
 Reverse faults (反斷層) arise from compressional
stresses. Movement on a reverse fault is such that a
hanging-wall block moves up relative to a footwall
block.
 Reverse fault movement shortens and thickens the crust.
Classification of Faults (4)
 Thrust faults (逆斷層) are low-angle reverse faults with
dip less than 15o.
 Such faults are common in great mountain chains.
 Strike-slip (走向斷層) faults are those in which the
principal movement is horizontal and therefore parallel
to the strike of the fault.
 Strike-slip faults arise from shear stresses.
 The San Andreas is a right-lateral strike-slip fault.
–
Apparently, movement (more than 600 km) has been occurring along it
for at least 65 million years.
Figure 9.17
Figure 9.18
Classification of Faults (5)
 Where one plate margin
terminates another commences,
their junction point is called a
transform.
 J. T. Wilson proposed that the
special class of strike-slip faults
that forms plate boundaries be
called transform-faults (轉型
斷層).
Figure 9.19
Evidence of Movement Along Faults
 Movement of one mass of rock past another can
cause the fault’s surfaces to be smoothed,
striated (條紋細溝狀) and grooved (溝槽狀).
 Striated or highly polished surfaces on hard rocks,
abraded (磨損侵蝕) by movement along a fault, are
called slickensides (擦痕面).
 In many instances, fault movement crushes (壓碎) rock
adjacent to the fault into a mass of irregular pieces,
forming fault breccia (角礫岩).
Deformation by Bending
 The bending of rock is referred to as folding (褶皺).
 Monocline (單斜): the simplest fold (褶皺). The
layers of rock are tilted in one direction.
 Anticline (背斜): an upfold in the form of an arch.
 Syncline (向斜): a downfold with a trough-like
form.
 Anticlines and synclines are usually paired.
The Structure of Folds (1)
 The sides of a fold are the limbs (褶皺的翼).
 The median line between the limbs is the axis of the fold.
 A fold with an inclined axis is said to be a plunging fold.
 The angle between a fold axis and the horizontal is the
plunge (線的傾角) of a fold.
 An imaginary plane that divides a fold as symmetrically
as possible is the axial plane.
Figure 9.21
對稱
等斜
Figure 9.22 A, B
不對稱
翻轉
橫臥
Figure 9.22 C,D,E
The Structure of Folds (2)
 An open fold is one in which the two limbs dip
gently and equally away from the axis.
 When stress is very intense, the fold closes up and
the limbs become parallel to each other.
 Such a fold is said to be isoclinal.
The Structure of Folds (3)
 Eventually, an overturned fold may become
recumbent, meaning the two limbs are horizontal.
 Common in mountainous regions,such as the Alps and
the Himalaya, that were produced by continental
collisions.
 Anticlines do not necessarily make ridges, nor
synclines valleys.
Faulting causes a monocline to form.
Figure 9.23
The evolution of a recumbent
fold into a thrust fault
Figure 9.24
Topography resulting from different resistance to
erosion of rocks reveal the presence of the plunging
folds.
Figure 9.25
Central
Appalachians in
Pennsylvanvia
Strata are folded into
anticlines and
synclines as a result of
collision during the
assembly of Pangaea.
Figure 9.26
Examples of Faults (1)
 In the Valley and Ridge province of Pennsylvania, a
series of plunging anticlines and synclines were created
during the Paleozoic Era by a continental collision of
North America, Africa, and Europe.
 Now the folded rocks determine the pattern of the topography
because soft, easily eroded strata (shales) underlie the valleys, while
resistant strata (sandstones) form the ridges.
 The San Andreas Fault in California is a strike-slip fault.
Examples of Faults (2)
 The Alpine Fault is part of the boundary between the
Pacific plate and the Australian-Indian plate, and slices
through the south island of New Zealand.
 The North Anatolian Fault, also with right-lateral motion,
slices through Turkey in an east-west direction, and is the
cause of many dangerous earthquakes.
 The Great Glen Fault of Scotland was active during the
Paleozoic Era.
 Loch Ness lies in the valley that marks its trace.
Tectonism And its Effect On Climate
 Temperature decreases with altitude.
 The Sierra Nevada influences the local climate.
 It imposes a topographic barrier to flow that forces the
moisture-laden winds from the sea flowing upward,
causing rain, and snow on the western slopes. When the
winds flow down the eastern side of the mountains, most
of the moisture has been dried out, causing the dry lands
and deserts on Nevada and Utah.
地質科學導論作業(1)
繳交日期 03/01/2004
Assignments are due at 5 pm on the designated date. The deadlines are firm. If late
assignments are handed in within two weeks of the due date, the grade will be penalized by
10 pts. After two weeks of the due date, the grade will be penalized by 20 pts. Copies of
homeworks are not allowed and will not be graded.
1. (30pt) 同一岩石的強度會受到圍應力 (Confining Stress) 和應變速率 (Strain Rate) 所影響，圖
一為岩石標本受到單一方向壓縮的正應力 (Normal Stress) 對與應力同方向的正應變
(Normal Strain) 作圖。
(a) 在彈性變形的範圍之內，應力 σ 和應變 ε成線性關係，亦即σ=Eε，E 為該岩石的彈性
常數，請由圖一求出該係數E。
(b) 假設四組曲線代表同一岩石標本在相同的實驗條件下改變不同的應變速率，請問那一
條曲線代表最慢應變速率的結果，並解釋原因。
(c) 對C組實驗而言，當施予的正應力達到3.3 MPa (圖中x點) 時所產生的永久變形為何？
圖一
2. (20pts) 圖示說明岩石圈 (Lithosphere) 的岩石強度 (Rock Strength) 隨深度的變化，並解釋岩
石強度改變的主要因素。