( ) φ =126 o. At 3 cm per year in E - W direction, time taken to move 3000 km is =10 8 a =100 Ma

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Henry Gregory McGee
5 years ago
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1 The Solid Earth Chapter 2 Answers to selected questions (1) (a) B v D = B v C + C v D ( ) B v D 2 = cos 25 B v D = sin( 25) sin( φ) = 5.22 φ =126 o (b) A v J = A v B + B v J At 3 cm per year in E - W direction, time taken to move 3000 km is =10 8 a =100 Ma (Diagrams are sketches and are not exactly to scale.)

2 (2) A v B = A v F + F v B (Diagrams are sketches and not exactly to scale) The B-F ridge approaches the trench eastwards at 0.8 cm per year. 2,200 km to the trench will take 275 Ma. Triple junction J approaches the northern trench at 4.2 cm per year km takes 36 Ma when the last remnant of the Cooler plate is subducted in this region. (3) Lat Long Location Plate pair Azimuth Rate (mm/yr) 54 N 169 E W. Aleutian Trench N Am - Pac N 169 W E. Aleutian Trench N Am - Pac N 122 W San Francisco - San N Am - Pac Andreas Fault 26 N 110 W Gulf of California N Am - Pac S 112 W East Pacific Rise Naz - Pac S 110 W East Pacific Rise Pac - Ant S 150 W Antarctic - Pacific Ridge Pac - Ant S 169 E S. New Zealand Pac - Aus S 159 E Macquarrie Island Pac - Aus S 140 E Southeast Indian Ridge Aus - Ant S 74 E Southeast Indian Ridge Aus - Ant N 60 E Carlsberg Ridge Ind - Afr N 38 E Red Sea Ar - Afr S 5 E Southwest Indian Ridge Afr - Ant S 5 E Mid-Atlantic Ridge Afr S Am

3 9 o N 40 o W Mid-Atlantic Ridge Afr S Am o N 35 o W Mid-Atlantic Ridge Afr N Am o N 18 o W Iceland Eur N Am o N 8 o W Gorringe Bank Afr - Eur o N 25 o E E. Mediterranean Afr - Eur o S 120 o E Java Trench Au Eur o N 72 o E Himalayas Ind - Eur o S 74 o W S. Chile Trench Naz S Am o S 82 o W N. Peru Trench Naz S Am o N 106 o W Middle America Trench Co N Am (5) λ p = 45, φ p = 0, ω = radians per year, R = 3400 km. (a) subduction, strike-slip and ridge. (b) Magnetic lineations between a and b, b and c. (c) Use method of Sect a: λ x = 0, φ x = 0 b: λ x = 0, φ x = 90 c: λ x = 0, φ x = 180 d: λ x = 0, φ x = -90 At a: [ ( )] [ ] a = cos 1 cos( 45)cos 0 = cos = 45 o cos( 45)sin( 0) C = sin 1 sin( 45) = 0 o β = 90 o

At b: At c: At d: (d) v = ωrsin( a) ( ) =10 10 3400 10 3 10 3 sin 45 = 0.

4 At b: At c: At d: (d) v = ωrsin( a) ( ) = sin 45 = 0.24 mm a -1, azimuth 090, strike slip [ ( )] a = cos 1 cos( 45)cos = 90 o cos( 45)sin( 90) C = sin 1 sin( 90) = 45 o β = 45 o v = ωrsin a ( ) ( ) = sin 90 = 0.34 mm a -1, azimuth 45, strike slip/extension [ ( )] a = cos 1 cos( 45)cos =135 o cos( 45)sin( 180) C = sin 1 sin( 135) = 0 o β = 90 o v = ωrsin a ( ) = 0.24 mm a -1, azimuth 090, strike slip [ ( )] a = cos 1 cos( 45)cos = 90 o cos( 45)sin( 90) C = sin 1 sin( 90) = 45 o β =135 o v = ωrsin( a) = 0.34 mm a -1, azimuth 135, oblique subduction (sketches only)

5 Stable with hemispheric plates for all pole positions. Plates stay the same size but are subducted along half of their margin and created along the other half. (6) Aω B 3x10-9 radians a -1, 30 N 0 E Bω C -6x10-9 radians a -1, 90 N 0 E R=6000 km. (a) Use method of Sect Aω C = A ω B + B ω C x AC = cos 90 ( )cos 0 ( ) cos 30 ( )cos 0 ( ) = y AC = cos( 90)sin 0 = 0 Aω C = pole position : z AC = sin 90 = ( ) cos 30 ( ) sin( 30) = λ AC = sin = 60 = 60 o S ( )sin 0 0 φ AC = tan 1 = (b) At b: λ X = 0, φ X = 90. B relative to A: β = 030, v = 18 mm a -1 ( ) C relative to B: β = 270, v = 36 mm a -1 C relative to A: β = 300, v = 31 mm a -1 (not to scale) bc would go through C if plate B is subducting beneath plate C. At d: λ X = 0, φ X = 90. B relative to A: β = 150, v = 18 mm a -1 C relative to B: β = 270, v = 36 mm a -1 C relative to A: β = 240, v = 31 mm a -1

6 (not to scale) Stable if plates B and C are subducting beneath A, or if plate A is subducting between plates B and C. Extension is taking place between plates A and B and between plates A and C at b. The magnetic stripes on each plate are 78 km wide and strike E-W. Extension is also taking place between plates B and C at d. The magnetic stripes on each plate are 180 km wide and strike N-S. Ridges: d to North Pole; between a and b, b and c. Strike-slip: at a and c. Subduction: North Pole to b; between c and d, d and a. (8) (Sketch) Ridge half-spreading rates are 2.0 cm per year and 3.45 cm per year. Triple junction moves at 3.9 cm per year at 275 relative to plate C.

7 (10) (Sketch only) (a) 11.6 cm per year (b) 6 cm per year parallel to B v N (e) (Sketches only, not to scale) At A At B (f) BJN: 6 cm per year for 2 Ma, 120 km spacing, 1.6 cm per year for 2 Ma, 32 km spacing. (Sketch, not to scale)

8 (11) Similar to Q6. λ AB = 0, φ AB = 90, ω AB = deg per year (a) λ BC = 90, φ BC = 0, ω BC = deg per year ( ( ) cos( 90)cos( 0) ) 10 7 x AC = 3.82cos( 0)cos 90 = 0 ( ( ) cos( 90)sin( 0) ) 10 7 y AC = 3.82cos( 0)sin 90 = ( ( ) sin( 90) ) 10 7 z AC = 3.82sin 0 = Aω C = ( ) 10 7 = λ AC = sin 1 = 36.8 o φ AC = sin 1 = 90 o 0 (b) Magnetic lineations formed at ridge along equator between plates A and C from 0 to 90 W. Maximum spreading rate is 5 cm per year at 0 N, 0 W (the triple junction) - spreading is oblique. On far side of planet the boundary between plates A and C is a subduction zone with oblique subduction. (c) The triple junction on the visible side of the planet is FFR, on the far side the triple junction is FFT. See Fig for stability of triple junctions. The FFR junction is not stable. The FFT junction will be stable if plate A is subducting beneath plate C (since then ac and bc will be coincident).

Crustal Boundaries. As they move across the asthenosphere and form plate boundaries they interact in various ways. Convergent Transform Divergent

Crustal Boundaries. As they move across the asthenosphere and form plate boundaries they interact in various ways. Convergent Transform Divergent Name: Date: Period: Plate Tectonics The Physical Setting: Earth Science CLASS NOTES Tectonic plates are constantly moving and interacting As they move across the asthenosphere and form plate boundaries