Notes for Test #3 Chapter 7 Rock Deformation * Rock deformation * Looking at rocks in the field, we see that they can be broken (faulted ) or bend (folded ) by (deforming) forces FIG 7.1 o Faulting shows brittle behavior of rocks o Folding shows ductile behavior o The latter occurs only for rocks at considerable depth inside the earth * To understand “structures” formed by rocks in the field, we need to measure the orientation or rocks in the field; we need to measure the orientation of rocks in outcrops. o Measure, dip , and strike of beds. o Strike (compass) direction – e.g. N-S, NE-SW o Dip has a (compass) direction as well as an angle associated with it • (FIG 7.2/3) * DIP AND STRIKE o Strike – the direction of a rock layer as it intersects the horizontal surface o Dip is measured at right angles to this, and measures the amount of tilting. o NOTE: strike and dip directions are always at right angles . o EXAMPLES: o Horizontal bed. Dip = 0 O • Has no strike or dip direction o Bed strikes NW-SE (northwest-southeast) • Dip can either be to NE or SW, and angle can be 1 O to 89 O o Dip and strike measurements are used to work out structure (see fig 7.4) * Stress – deforming force on rocks * Strain – the resultant change in shape or size * Summary of deformation o Three types of stress: o Compression o (Ex)tension Shear o°
o If rocks behave in ductile manner, folding , stretching , and shearing , respectively. o In faulting, movement of two blocks of rocks occurs across the fault plane . o This fault plane (intersects the horizontal surface) has both dip and strike. o If rocks break, but there is no movement between the two blocks, it’s called the joint . * Brittle behavior – faults (FIG 7.7) o If rocks behave in brittle fashion: o Compression forces create reverse faulting o Extension forces create normal faulting o Shearing forces create strike-slip faulting o Faults are divided into two types o Dip-slip • Movement occurs in direction of dip (has vertical component) • UP OR DOWN o Strike-slip . • In latter, movement occurs in strike direction (only horizontal movement) • LEFT OR RIGHT * Dip-slip faults o Normal fault – caused by extension o Found at or near divergent plate boundaries . o Reverse fault – caused by compression o Found at or near convergent plate boundaries o In mountain building, there can be extreme shortening, blocks move tens of kilometers along low angles (< 15 O ) o Called thrust fault • FIG 7.7 * Strike-slip faults o Caused by shearing. o Found at or near transform fault boundaries * Can also have oblique slip fault, caused by some amount of shearing and some amount of tension or compression. * Two dimensional figures of faults. o Normal faults – extension
o Reverse faults – compression o Block faulting – two normal faults, central part falls down to make a “rift valley” 3/12/08 * Brittle VS. ductile behavior o Deep burial (higher temperature and confining pressure) favors ductile behavior – under these conditions the rocks tend to FOLD (folding behavior) * Folding o
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- Spring '07
- Plate Tectonics, rocks, Fig, convergent plate boundaries