GEOL LAB 2

Introduction: To understand plate tectonics, one must be familiar with the lithosphere, Earth’s solid outermost layer consisting of the crust and the uppermost part of the upper mantle. The lithosphere is divided into plates that move relative to each other and interactions along their boundary zones produce earthquakes, volcanoes, mountain ranges, mid-ocean ridges, and trenches. In divergent plate boundaries, plates are moving away from each other resulting in the partial melting of the asthenosphere, seafloor spreading, or new crust to be added to both sides of the boundary. Furthermore, there are convergent plate boundaries, where two plates move towards one another; this can potentially cause the subduction of oceanic lithosphere and earthquakes along this subduction zone. Lastly, with transform plate boundaries, plates move in the same direction along fracture zones; however, they move in opposite directions between ridge segments and transform faults. Four major factors influence the behavior of Earth’s materials: strength, gravity and buoyancy, and heat flow. Strength refers to the amount of stress a material can withstand, which is directly influenced by its mineral composition, grain size, temperature, and pressure. Additionally, when a mass of greater density is surrounded by a fluid of lesser density, the mass with the greater density will move downward; however, if a mass with lesser density is surrounded by a fluid of greater density, such as magma, the less dense mass will be pushed upwards. Finally, heat flows within the solid Earth by conduction in the lithosphere and convection in the mantle. Activity 2.2: Plate Motion and the San Andreas Fault A) Displacement Along the San Andreas 1. The distance between Neenach and Pinnacles along the San Andreas Fault is approximately three inches or approximately 75 millimeters. Given that each millimeter on the map represents 4km on Earth’s surface, the approximate distance from Neenach to Pinnacles is 300 km. 2. Estimated average rate of displacement since 19 Myr: 15.789 km/Myr Displacement = ?𝑖?????? ?𝑖?? Displacement = 300 ?? 19 ?𝑦? 3. Estimated age of faulting: 14.151 Myr Age = ?𝑖?????? ???? Age = 300 ?? 21.2 ?? Age = 14.151 Myr B)Motion of the Crust in a Plate Boundary Zone 1. 2 cm = 50 mm/yr, 1 cm = 25 mm/yr Atlantic Plate: a) 1.1 cm = 27.5 mm/yr b) 1.2 cm = 30 mm/yr c) 1.3 mm = 32.5 mm/yr d) 1.4 cm = 35 mm/yr Average = 31.25 mm/yr North American Plate: a) 0.3 cm = 7.5 mm/yr b) 0.4 cm = 10 mm/yr

c) 0.5 cm = 12.5 mm/yr d) 0.6 cm = 15 mm/yr Average = 11.25 mm/yr Based on the average of four vectors for each plate boundary, the Pacific Plate is moving faster by an estimated 20 mm/yr. 2. Add half-arrows along the San Andreas Fault to show the sense of motion across the fault. Figure A2.2.2. C) The crust along the Big Bend might be affected by this difference in motion compared to the northwest and southwest because the northwest and southwest sectors move in a parallel motion to each other and the transform fault plane; therefore, in this location, the plates are more likely to slide past each other. However, at the Big Bend, the GPS velocity vectors show a change in motion direction, hence, the slower velocity of the northern portion of the plane against the faster velocity of the southern portion of the plane makes it subject to a collision. This location experiences more compression and stress given they move at different rates and in slightly different directions now. Such characteristics can produce thrust faulting, or rocks being pushed against one another, and as a result, there is a potential for increased seismic activity. Activity 2.4: Hotspots and Plate Motions

1.) If the Emperor and Hawaiian Islands Chains developed as a result of the same mantle hotspot, the change in direction of the hotspot trail at ~42 Myr can be a result of the change in direction or velocity of the Pacific Plate. Specifically, when the Pacific Plate was moving over a hotspot, magma would rise to the surface and create volcanic islands; however, the trajectory of this plate could have been affected by mantle convection patterns, collision of tectonic plates, or if a new ocean crust is formed. 2.) The rate of Pacific Plate motion relative to the Hawaiian hotspot as it was developing the 2,300 km-long Emperor Seamount Chain from 65 Myr to 42 Myr was 100 mm per year . By studying the age and location of volcanic features, the Pacific Plate was moving in a north-northwest direction relative to the hotspot during this time interval. Work: Rate of motion = ?𝑖?????? ?𝑖?? Rate of motion = = 2,300 ?? 65 ?𝑦? − 42 ?𝑦? 2,300,000 ? 23,000,000 𝑦? Rate of motion = 0.1 ?????? 1 𝑦??? Rate of motion = 100 ?? 1 𝑦??? 3.) The rate of Pacific Plate motion relative to the Hawaiian hotspot from 5.1 to 0.8 Myr was 93 mm/yr. Work: Rate of motion = ?𝑖?????? ?𝑖??

Rate of motion = = 400 ?? 5.1 ?𝑦? − 0.8 ?𝑦? 400,000 ? 4,300,000 𝑦? Rate of motion = 0.093 ?????? 1 𝑦??? Rate of motion = 93 ?? 1 𝑦??? 4.) The rate of Pacific Plate motion relative to the Hawaiian hotspot, Lo’ihi Seamount, from 0.8 Myr to today is 287.5 mm/year. Work: Rate of motion = ?𝑖?????? ?𝑖?? Rate of motion = = 230 ?? 0.8 ?𝑦? − 0 230,000 ? 800,000 𝑦? Rate of motion = 0.2875 ?????? 1 𝑦??? Rate of motion = 287.5 ?? 1 𝑦??? 5a.) The current motion of HNLC on Oahu compared to the direction of Pacific Plate motion relative to the Hawaiian hotspot over the past 42 million years is northwest. 5b.) Rate of movement of North = 34.607 mm/yr ± 0. 038 Rate of movement of West = 62.814 mm/yr ± 0. 041 Pythagorean Theorem = = ? 2 + ? 2 ? 2 Pythagorean Theorem = 34. 607 2 + 62. 814 2 = ? 2 Pythagorean Theorem = 1197.644 + 3945.598 = 5143.243 = ? 2 5143. 243 c = 71.716 mm/year The current speed of the Pacific Plate at Oahu relative to the NNR reference frame without considering the 0.038 mm/yr and 0.041 mm/yr error is 71.716 mm/year. 6.) Reflect and Discuss: Based on the above work, the directions, and rate of the Pacific Plate over the past ~70 million years have been mostly moving in the north, and eventually, the northwestern direction at an estimated average of 10 centimeters or 100 millimeters per year. This is subject to change based on its location or interaction with other plate boundaries.