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# Chapter 9: How Rock Bends, Buckles, and Breaks - PowerPoint PPT Presentation

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.

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### Chapter 9: How Rock Bends, Buckles, and Breaks

• 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 stressor confining pressure (圍壓) applied to rocksbecause 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.

• 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).

• 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)

• Shear Strain (剪應變)

(1) Tensional strain

(2) Compressional strain

• 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 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.

• 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 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.

• 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 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.(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 the temperature is low and the strain rate is also low.

• 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 temperature is low and the strain rate is also low.

• 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 waterin 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) the temperature is low and the strain rate is also low.

• 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) the temperature is low and the strain rate is also low.

• 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) the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

Rock Strength (3) the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

• Gradual movement is the slow rising, sinking, or horizontal displacement of land masses.

• Movement along faults is usually, but not always, abrupt.

Figure 9.9 the temperature is low and the strain rate is also low.

Evidence Of Former Deformation the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.(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 the temperature is low and the strain rate is also low.

Figure 9.11 the temperature is low and the strain rate is also low.

Deformation By Fracture the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

Classification of Faults (1) the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

Figure 9.13B the temperature is low and the strain rate is also low.

Classification of Faults (2) the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

Classification of Faults (3) the temperature is low and the strain rate is also low.

• 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) the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

Figure 9.18 the temperature is low and the strain rate is also low.

Classification of Faults (5) the temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

Evidence of Movement Along Faults the temperature is low and the strain rate is also low.

• 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 temperature is low and the strain rate is also low.

• 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 temperature is low and the strain rate is also low.

• 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 the temperature is low and the strain rate is also low.

Figure 9.22 A, B

Figure 9.22 C,D,E

The Structure of Folds (2) the temperature is low and the strain rate is also low.

• 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) the temperature is low and the strain rate is also low.

• 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. the temperature is low and the strain rate is also low.

Figure 9.23

The evolution of a recumbent the temperature is low and the strain rate is also low.

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 rocks reveal the presence of the plunging folds.

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) rocks reveal the presence of the plunging folds.

• 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) rocks reveal the presence of the plunging folds.

• 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 rocks reveal the presence of the plunging folds.

• 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 rocks reveal the presence of the plunging folds.(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。