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This chapter explores how rocks are deformed through tectonic forces such as squeezing, stretching, bending, and breaking. It explains different types of stress, stages of deformation, and the effects of temperature, pressure, and composition on rock strength and ductility.
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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 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.
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) • Shear Strain (剪應變) (1) Tensional strain (2) Compressional strain
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 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) • 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.
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.
台灣的地震分布和地體構造 資料:中研院地球所
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.
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.
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.
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.
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.
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).
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.
