Plate Tectonics - Week 4 Study Notes

Week 4 Study Schedule and Exam Plan

Use your weekly study guides
Compare yours to the guide posted on Canvas to ensure you’re covering everything
Textbook: look for vocabulary and images/diagrams
Lecture slides: focus on diagrams
Office Hours: Grote 218E (through 218 conference room & down the hall on the right)
- Tuesdays 3-5pm
- Wednesdays 10:30am – 12:30pm

Core questions: What forces cause tectonic plates to move? How do we know the speed and direction of moving plates?
Topics to cover:
- Forces
- The Wilson Cycle
- Hotspots
- Calculating plate motion

Driving forces include:
- Heat
- Gravity
- Density
- Slab Pull
- Ridge Push
Additional context:
- Rising magma at ridges contributes to buoyancy of the asthenosphere
- Slab pull and ridge push act as primary mechanisms moving plates

Concept: Hot asthenosphere and magma are buoyant and rise up, making ridges higher than the surrounding areas.
Effect: Plates tend to slide off the ridge due to gravity, pushing both plates away from the ridge.

Concept: Supercontinents form, then rift apart in a repeating cycle.
Timescale: The entire cycle takes approximately ext{(a few)} imes 10^8 ext{ years} (hundreds of millions of years).
Each cycle is unique, depending on the motion and arrangement of tectonic plates.

Evidence supports multiple supercontinents over time.
As you go further back in time, corroborating evidence becomes harder to obtain.

Relative Motion:
- Compare the motion of one plate to another
- Choose one plate as the reference point for others
Absolute Motion:
- Compare the motion of plates to a stationary reference point
- References include hotspots, Earth’s rotation axis, and magnetic north pole

Hotspot: location on Earth’s surface with unusually high heat flux and volcanism
Mantle Plume: rising masses of hot mantle in the asthenosphere, potentially sourced from the core/mantle boundary region

Plumes are large and slow-moving (especially compared to tectonic plate motion)
Essentially stationary (relative to Earth’s rotational axis) over tens of millions of years

Active volcanos form at hotspots
When the tectonic plate moves, the volcano moves with it
When the volcano moves off the hotspot, it loses magma supply and stops erupting
A new volcano forms over the hotspot

Hawaii serves as a classic hotspot track illustrating plate movement and hotspot behavior
The track records a sequence of volcanoes formed as the Pacific plate moved over the stationary hotspot

Oldest volcano (not active) and currently erupting volcano are part of the track
Direction of Pacific plate movement can be inferred from the arrangement of ancient vs. current volcanism

The oldest part of the track is toward the north; newer parts extend toward the northwest as the plate moves
This pattern reflects the northwestward motion of the Pacific plate relative to the stationary Hawaii hotspot

Question: How fast has the Pacific Plate been moving relative to the hotspot since the Kauai volcano stopped erupting?
Principle: Velocity is distance over time
Formula: v = \frac{d}{t}
Given data: distance = 480 km, time = 3.8 Myr
Calculations:
- v = \frac{d}{t} = \frac{480\ \text{km}}{3.8\ \text{Myr}} \approx 126\ \text{km/Myr}
- Equivalent in other units: \approx 12.6\ \text{cm/yr}
Therefore, the Pacific plate speed relative to the hotspot since Kauai stopped erupting is about 126\ \text{km/Myr} \; (\approx\ 12.6\ \text{cm/yr})

Montana
- Yellowstone Caldera (2-0.6 Ma)
- Heise Caldera (6.6-4.4 Ma)
Idaho
- Picabo Caldera (10.2 Ma)
- Twin Falls Caldera (10-8.6 Ma)
Snake River Corridor
- Bruneau-Jarbidge Caldera (12.5-11.3 Ma)
Wyoming
- Owyhee-Humboldt Caldera (13.8-12 Ma)
- McDermitt Caldera (16-15.1 Ma)
Scale: The list shows multiple calderas with activity ages spanning tens of millions of years across the Yellowstone region

Plate tectonics framework connects mantle convection, slab dynamics, and surface geology (earthquakes, volcanoes, mountain building)
Hotspot tracks provide a way to measure plate motion independently from plate boundaries
Absolute vs. relative motion highlights how geologists reconstruct past plate movements using fixed reference frames
Wilson Cycle explains long-term cycles of supercontinent formation and breakup

Understanding plate tectonics informs natural hazard assessment (earthquakes, volcanic eruptions) and resource distribution (minerals, geothermal potential)
Reconstructing past plate movements informs models of climate change, biological evolution, and continental configurations through deep time

Driving forces: ext{Heat}, ext{ Gravity}, ext{Density}, ext{Slab Pull}, ext{Ridge Push}
Ridge Push mechanism and its role in plate motion
Mantle convection and its link to surface tectonics
The Wilson Cycle and its timescale
Relative vs Absolute plate motion definitions
Hotspots and mantle plumes, their characteristics and significance
Hotspot tracks as evidence for plate motion
Hawaii hotspot track as a case study for tracking plate movement
Velocity calculations and unit conversions for plate motion
Calderas in the Yellowstone region and their eruption histories