Metamorphic Processes I

Crustal Deformation Changes in earth’s surface due to the folding, breaking, or flow of rocks Due to pressures that rocks experience within the earth Pressures can be due to changes in temperature or moisture, shifts in the Earth’s plates, sediment buildup or even gravity

Types of Pressure / “Stress” on the Crust Compressional Stress Forces that push inward or press upon rocks/layers Tensional Stress Forces that pull in opposite directions Shear Two forces acting parallel to each other but in opposite directions

Kinds of Crustal Deformation Elastic temporary reversed when the source of stress is removed Ductile a permanent change to the shape or size of the rock that persists even when the stress stops Brittle results in the breakage of the rock

Factors Affecting Rock Strength High Low 1. Temperature Molecules/bonds can stretch/move Materials will be more ductile Materials are more brittle 2. Confining Pressure (Equal stress on the material coming from all directions) Materials are less likely to fracture or break Materials will be more brittle and tend to break sooner 3. Time 4. Rock Type Some minerals, like quartz, olivine, and feldspars are very brittle Others, like clay minerals, micas, and calcite are more ductile  Dry rocks: more brittle Wet rocks: more ductile

Ductile Deformation Cause FOLDS Kinds of Folds: Monocline, Anticline, Syncline, Plunging Monocline Anticline Syncline Plunging Slight bend Upward fold Downward fold Occurs if the fold axis is not horizontal  The 2 sides of the bend are still horizontal The 2 sides of the fold dip away from the center of the fold The 2 sides of the fold dip inward toward the center of the fold Folds are caused by compressional stress

Ductile Deformation Cause FOLDS More Folds: Symmetrical, Assymetrical, Overturned, Recumbent Symmetrical Assymetrical Overturned Recumbent The 2 sides of the fold dip away from the center at the same angle The 2 sides of the fold dip away from the center at different angles The layers on one side of the fold have been folded beyond 90⁰, making it go nearly upside-down More extreme than an Overturned fold where the central axis of the fold is almost horizontal

Ductile Deformation Form Structures Ductile Structures: Domes, Basins, Hogbacks Dome Basin Hogback Looks like an upside-down bowl Looks like an upright bowl A long narrow ridge Due to compression and uplift Due to compression and down-warping Formed when inclined (monoclined) sedimentary rockbeds are eroded, while the harder rock layers on top resist erosion. Hogback

Brittle Deformation Joint Fault When rock layers are placed under a high enough stress, they can then fracture or break. Such fractures create either: Joint Fracture in rock that show no slippage or offset along the fracture Fault Fracture shows slippage or offset along the fracture Joints and faults can be caused by various stresses: compression, tension, shearing

Brittle Deformation Types of Faults Dip Slip Fault has an inclined fault plane the displacement or offset is along the dip direction Can either be: Normal Fault Tension acts in opposite directions. One block is displaced up and the other down  Reverse Fault Compression causes one block to be pushed up and over the other block Thrust Fault A special type of Reverse Fault with a dip below 45⁰, which can cause much more displacement of a much larger area Strike Slip Fault  has a vertical fault plane Shear stresses are exerted parallel to each other

Brittle Deformation Structures Formed Graben produced when tensional stresses on adjacent normal faults result in a block of rock dropping down Half-Graben Bound by only one normal fault with a curved fault plane which can cause the down-dropped block to rotate Horst produced when compressional stresses on adjacent reverse faults result in a block of rock being pushed up Also called a Fault-Block Mountain Fault Scarp The exposed cliff-like face of the fault plane

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