How does rock deform

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Can jelqing deform your penis? Why do NFL kickers deform the ball on the ground before they kick? Elastic Deformation -- wherein the strain is reversible. Ductile Deformation -- wherein the strain is irreversible. Fracture - irreversible strain wherein the material breaks. We can divide materials into two classes that depend on their relative behavior under stress. Brittle materials have a small or large region of elastic behavior but only a small region of ductile behavior before they fracture.

Ductile materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.

We all know that rocks near the surface of the Earth behave in a brittle manner. Crustal rocks are composed of minerals like quartz and feldspar which have high strength, particularly at low pressure and temperature. As we go deeper in the Earth the strength of these rocks initially increases. At a depth of about 15 km we reach a point called the brittle-ductile transition zone.

Below this point rock strength decreases because fractures become closed and the temperature is higher, making the rocks behave in a ductile manner. At the base of the crust the rock type changes to peridotite which is rich in olivine.

Olivine is stronger than the minerals that make up most crustal rocks, so the upper part of the mantle is again strong. But, just as in the crust, increasing temperature eventually predominates and at a depth of about 40 km the brittle-ductile transition zone in the mantle occurs. Below this point rocks behave in an increasingly ductile manner. Deformation in Progress.

Only in a few cases does deformation of rocks occur at a rate that is observable on human time scales. Abrupt deformation along faults, usually associated with earthquakes occurs on a time scale of minutes or seconds.

Gradual deformation along faults or in areas of uplift or subsidence can be measured over periods of months to years with sensitive measuring instruments. Evidence of deformation that has occurred in the past is very evident in crustal rocks.

For example, sedimentary strata and lava flows generally follow the law of original horizontality. Thus, when we see such strata inclined instead of horizontal, evidence of an episode of deformation. Since many geologic features are planar in nature, we a way to uniquely define the orientation of a planar feature we first need to define two terms - strike and dip. For an inclined plane the strike is the compass direction of any horizontal line on the plane.

The dip is the angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike. In recording strike and dip measurements on a geologic map, a symbol is used that has a long line oriented parallel to the compass direction of the strike.

A short tick mark is placed in the center of the line on the side to which the inclined plane dips, and the angle of dip is recorded next to the strike and dip symbol as shown above. For beds with a 90 0 dip vertical the short line crosses the strike line, and for beds with no dip horizontal a circle with a cross inside is used as shown below. For linear structures, a similar method is used, the strike or bearing is the compass direction and angle the line makes with a horizontal surface is called the plunge angle.

As we have discussed previously, brittle rocks tend to fracture when placed under a high enough stress. Such fracturing, while it does produce irregular cracks in the rock, sometimes produces planar features that provide evidence of the stresses acting at the time of formation of the cracks. Two major types of more or less planar fractures can occur: joints and faults. As we learned in our discussion of physical weathering, joints are fractures in rock that show no slippage or offset along the fracture.

Joints are usually planar features, so their orientation can be described as a strike and dip. They form from as a result of extensional stress acting on brittle rock. Such stresses can be induced by cooling of rock volume decreases as temperature decreases or by relief of pressure as rock is eroded above thus removing weight.

Joints provide pathways for water and thus pathways for chemical weathering attack on rocks. If new minerals are precipitated from water flowing in the joints, this will form a vein. Many veins observed in rock are mostly either quartz or calcite, but can contain rare minerals like gold and silver. These aspects will be discussed in more detail when we talk about valuable minerals from the earth in a couple of weeks.

From an engineering point of view, joints are important structures to understand. Since they are zones of weakness, their presence is critical when building anything from dams to highways. For dams, the water could leak out through the joints leading to dam failure.

For highways the joints may separate and cause rock falls and landslides. Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is small, the displacement can be easily measured, but sometimes the displacement is so large that it is difficult to measure.

As we found out in our discussion of earthquakes, faults can be divided into several different types depending on the direction of relative displacement. Since faults are planar features, the concept of strike and dip also applies, and thus the strike and dip of a fault plane can be measured.

One division of faults is between dip-slip faults, where the displacement is measured along the dip direction of the fault, and strike-slip faults where the displacement is horizontal, parallel to the strike of the fault. Recall the following types of faults: Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane and along which the relative displacement or offset has occurred along the dip direction. Note that in looking at the displacement on any fault we don't know which side actually moved or if both sides moved, all we can determine is the relative sense of motion.

Normal Faults - are faults that result from horizontal tensional stresses in brittle rocks and where the hanging-wall block has moved down relative to the footwall block. In such a case the down-dropped blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has recently affected the crust, the grabens may form rift valleys and the uplifted horst blocks may form linear mountain ranges.

The East African Rift Valley is an example of an area where continental extension has created such a rift. The basin and range province of the western U. Nevada, Utah, and Idaho is also an area that has recently undergone crustal extension. In the basin and range, the basins are elongated grabens that now form valleys, and the ranges are uplifted horst blocks.

Half-Grabens - A normal fault that has a curved fault plane with the dip decreasing with depth can cause the down-dropped block to rotate. In such a case a half-graben is produced, called such because it is bounded by only one fault instead of the two that form a normal graben.

A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 45 o. Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can result in older strata overlying younger strata. Since movement on a fault involves rocks sliding past each other there may be left evidence of movement in the area of the fault plane.

When rocks deform in a ductile manner, instead of fracturing to form faults or joints, they may bend or fold, and the resulting structures are called folds. The dip is indicated in terms of angle and direction e. Layered rocks folded into arches are called anticlines whereas troughs are referred to as synclines. Figures The two limbs come together to form an imaginary line called the fold axis.

The direction in which the fold axis points indicates the strike of the fold. Rock bands appearing on one side of the fold axis are duplicated on the other side. For basins and domes, strata exposed at the surface form concentric circles around a central point Figure Rock exposures become progressively younger towards the axis of synclines.

Rock layers dip away from the fold axis in anticlines, but dip toward the fold axis in synclines. The axial plane divides a fold as symmetrically as possible. The line formed by the intersection of the axial plane with the beds define the fold axis. If the axis is not horizontal, the structure is said to be a plunging fold. The plunge of a fold can be described as the angle a fold axis makes with a horizontal surface.

However, once you've made this change, you can't 'unstretch' it, and the same is true for rocks in this stage of deformation. Finally, if rocks are stressed enough, they fracture , which is when the change is irreversible and the rock breaks.

If you fall down hard enough, you may fracture a bone in your body, and rocks experience the same thing when the stress is great enough. Some people handle stress better than others, and rocks are the same way. So, just like there are various types of rock stress, there are also different responses to the stress for different types of rock material. The ability of a rock to handle stress depends on its elasticity , or the flexibility of the rock.

You might be surprised to learn that rocks are flexible. Under the right conditions, they can 'flow' very slowly. This movement is similar to silly putty. Push on silly putty very hard and it feels like a solid, but pull it apart and it moves like a liquid. Rocks are not quite as fluid as silly putty, but sometimes they can act similarly.