Geology Ch. 10 Mountain Building
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-a general term encompassing all changes in the shape or volume of rocks.
Most of the Earth's seismic activity, volcanism, and rock deformation, happen at convergent plate boundaries.
additions of new material to existing continents
folded and fractured rock layers resulting from deformation.
causes rocks to crumple into folds or to fracture, it is a force applied to a given area of rock.
if the stress is greater than the rocks internal strength, the rock undergoes strain.
deformation causes by stress.
What are the three varieties of stress?
compression, tension, and stength
- rocks or any other object are squeezed or compressed by forces directed toward one another along the same line.
- rock layers tend to be shortened in the direction of stress by either folding or fracturing.
- Rocks are much stronger in compression than they are in tension
- results from forces acting along the same line, but in opposite directions.
- Tends to lengthen rocks or pull them apart.
forces act parallel to one another, but in opposite directions, resulting in deformation by displacement along closely spaced planes.
What are the two types of strain?
Elastic strain and plastic strain
Elastic and Plastic Strain:
- when deformed rocks returnn to their original shape when the deforming forces are relaxed.
- As stress is applied, rocks first undero elastic strain, but when they are strained beyond their elastic limit, they undergo plastic strain, or they behave like brittle solids and fracture.
- In folding or fracturing, rocks do not recover their original shape.
What does the type of strain in a rock depend on?
- kind of stress applied
- pressure and temperature
- rock type
- length of time rocks are subjected to stress
exhibit a great deal of plastic strain before fracturing
little or no plastic strain before a fracture
Strike and Dip:
- Strike is the direction of a line formed by the intersection of a horizontal plane and an inclined plane.
- Dip is a measure of an inclined plane's deviation from horizontal, so it must be measured at right angles to strike direction.
- any feature resulting from deformation (crumpled or folded rocks, or fractured rocks).
- Present almost everywhere that rock is exposures can be observed.
- geologic structures in which planar features are crumpled and bent.
- Compression is responsible for most folding.
- Folding is permanent.
- Most folding takes place deep in the crust where rocks are more ductile.
What are thee basic types of folds?
monoclines, synclines and anticlines
- a simple bend or flexure in otherwise horizontal uniformly dipping rock layers.
- One half of an anticline or a syncline
an uparched or convex upward fold with the oldest rock layers in its core
a down-arched or concave downward fold in which the youngest rock layers are in its core.
Anticlines and Synclines:
- have an axial plane connectin the points of maximum curvature of each folded layer
- the axial plane divides folds into halves, each half being a limb.
a folds axial planes are vertical and both fold limbs dip at the same angle.
the axial plane is not vertical, the limbs dip at different angles, and the folds are characterised as inclined.
both limbs dip in the same direction
deformation is so intense that axial planes of folds become horizontal.
Which folds are particularly common in mountains?
overturned and recumbent folds
Plunging and Non Plunging Folds:
- Plunging Folds - fold axes are inclined so that they appear to plunge beneath adjacent rock. More common than non plunging folds.
- Non Plunging Folds - the fold axis is horizontal
a fold that is equidimensial and circular, all of the folded strata dip outward from a central point, and the oldest exposed rock is at the center of the fold.
has all strata dipping inward toward a central point and the youngest exposed rocks are at the fold's center.
fractures along which no movement has taken place parallel with the fracture surface.
a fracture along which blocks of rock on opposite sides of the fracture have moved parallel with the fracture surface.
the surface along which movement takes place
- -a bluff or cliff formed by vertical movement.
- Usually quickly eroded or obscured.
formed when movement on opposite sides of a fault plane are scratched and polished or crushed and shattered into angular blocks.
Hanging Wall Block:
the rock overlying the fault
the rock that lies beneath the fault plane
when you cannot tell which rock moved up or down, or if both rocks moved.
- all movement takes place parallel with the fault's dip. Movement is vertical, either up or down the fault plane.
- The hanging wall block moved down relative to the footwall block, giving rise to a normal fault. In contrast, in a reverse fault, the hanging wall block moves up relative to the footwall block.
- A thrust fault is when a hanging wall block also moved up relative to the footwall block, but the fault has a dip of less than 45 degree and is a special variety of reverse fault.
- -result from shear stresses and show horizontal movement with blocks on opposite sides of the fault sliding past one another.
- the San Andreas fault is considered a transform faultCharacterized as:
- 1. right-lateral - the block on the opposite side appears to move to the right
- 2. Left-lateral - the block on the opposite side appears to move to the left.
both types of movement take place, strike-slip and dip-slip
- any area of land that stands significantly higher than the surrounding country and has restricted summit area.
- Some are single isolated peaks.
- More commonly, they are parts of linear associations of peaks and ridges known as mountain ranges. A mountain system is a complex linear zone of deformation and crustal thickening and consists of several or many mountain ranges.
flat topped mountains
a way that mountains form, caused by deformation of the crust. Invonves movement on normal faults so that one or more blocks are elevated relative to adjacent blocks.
down dropped blocks
an episode of mountain building during which intense deformation takes place, generally accompanied by metamorphism, and emplacement of plutons, especially batholiths, and thickening of Earth's crust.
What characterizes orogenies at oceanic-oceanic boundaries?
Deformation, igneous activity, and the origin of a volcanic island arc
a subduction complex of intricately folded rocks cut by numerous thrust faults, resulting from compression.
What characterizes orogenies at oceanic-continental boundaries?
Volcanic activity, deformation of rocks, and seismic activity
What chatacterizes orogenies at continental-continental boundaries?
Mountain building, seismic activity, rock deformation.
small, accreted lithospheric blocks that clearly originated elsewhere. These are fragments of seamounts, island arcs, and small pieces of continents that were carried on oceanic plates that collided with continental plates, thus adding them to continental margins.
- departures from the expected force of gravity.
- May be positive, meaning that an excess of mass is present at some location, or negative, when a mass deficiency exists.
Principal of Isostasy:
the phenomenon of Earth's crust floating in the denser mantle
if loading by glacial ice or sediment depresses Earth's crust further into the mantle, it follows that when vast glaciers melt or where deep erosion takes place, the crust should rise back up to its equilibrium level.
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