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Types of Metamorphism Regional metamorphism – Large scale – large volume of rock is affected – Associated with convergent plate margins and mountain building – Folding and faulting increase thickness of the crust – Occurs over a range of temperatures and pressures – Fluids are also present – Low grade to high grade metamorphism – Will have zones of contact metamorphism Regional Metamorphism Metamorphic Grade The following diagram illustrates the rock cycle. Match the lettered responses to the blank ovals on the diagram. (Note: some letters are used more than once.) Magma 1 Metamorphic rock • • • • • a) Cementation & Compaction (lithification) b) Heat and Pressure 4 c) weathering, transportation, deposition d) Cooling and solidification e) Melting Sedimentary Rock 2 3 Igneous rock 6 5 7 8 Sediment Rock Cycle Geologic Structure Rock Deformation • Stress – Force applied to an area – Pressure – Differential stress – magnitude of stress is greater in one direction • Rock Strength – Rock’s resistance to deformation (stress) Rock Deformation Cont. • Strain – A rock’s response to stress – Deformation resulting in change of shape or position • Elastic deformation – Deformation is reversible, after stress is removed, materials return to original shape, i.e. A rubber band • Plastic deformation – Deformation is permanent – Rock flow in response to stress – Need high pressures and temperatures, generally deep in earth’s interior, i.e. Taffy Rock Deformation Cont. • Brittle deformation – Deformation is permanent – Rock break (fracture) or lose cohesion – Generally low temperature and low pressure, common in shallow crust, i.e. Peanut brittle Types of Stress • Tensional (Extension) Stress • Compressional Stress • Shear Stress Tension or Extensional Stress • Stress move away from each other (pull apart) • Rock lengthens • Common in divergent boundaries, but not sole locale • Ductile deformation – stretching and thinning of rock • Brittle deformation – fracturing and faulting Compressional Stress • Stress presses (squeezes) rock together • Rock shortens • Common at convergent boundaries • Ductile deformation – shortening and thickening of rock – rock folds • Brittle deformation – fracturing and faulting Shear Stress • Stress creates a lateral shift in the rock • Rock slides past other rock • Common at transform boundaries • Results in faults Folds • Ductile deformation • Compressional stress • Horizontal rocks are deformed Fold Terminology • Limb – the sides of the fold • Axial plane (hinge plane) – imaginary plane that divides the fold into two equal parts, marks location of maximum curvature • Fold axis (hinge line) – the line formed by the intersection of the hinge plane and a bedding plane Syncline • A fold with the youngest beds in the middle • Generally trough shaped, with limbs dipping toward the center Anticline • A fold with the oldest beds in the middle • Generally arched shaped, with limbs dipping away the center Monocline • Folds with only one limb • Often Associated with faults Fold Symmetry • Symmetrical fold – limbs of the fold dip at the same angle • Asymmetrical fold – limbs of the fold dip at different angles Overturned Fold • Fold where one limb has been tilted beyond vertical Applied to a 1 2 causes Daily Exercise-Complete the concept map 3 5 4 The material The material 7 8 Defined by youngest rock in middle 9 no displacement 10 a) b) c) d) e) f) g) Joints Folds Anticline Tension Faults Break Plastic deformation h) Syncline i) Brittle deformation j) Force/stress k) l) m) n) o) Deformation Normal Reverse Rock Compression 6 displacement 11 Each letter can be used only once, and it is possible that it will not be used. Faults • A results of brittle deformation • Fractures in the Earth's crust where displacement has occurred Fault Terminology • Hanging wall – block that lies below the fault plane • Footwall – block that lies above the fault plane Classes of Faults • Dip-Slip – Movement is primarily vertical • Strike Slip – Movement is along strike • Oblique-Slip – Movement is both vertical and along strike Dip-Slip Fault – Normal Fault • Extensional Stress • Hanging wall moves down with respect to footwall FOOTWALL HANGINGWALL Normal Fault • Horst – raised block of material bounded by two normal faults • Graben – trough (valley) bounded by two normal faults Dip-Slip Fault – Reverse Fault • Compressional Stress • Hanging wall moves up with respect to footwall Dip-Slip Fault – Thrust Fault • Compressional Stress • Low-angle reverse fault (fault plane < 45o) • Displacement can be 100’s km (~60 miles) • Typically associated with folds Strike-Slip Fault Left-lateral strike-slip faults: movement of opposite plate is to the left • Shear Stress • No vertical movement • Movement parallel to fault plane Right-lateral strike-slip faults: movement of opposite plate is to the right Joints • Brittle deformation at low pressure • Tension fractures with NO DISPLACEMENT