# L30 Synthesis Activity — Group Cards & Worksheet

*ESS 314 · Lecture 30 · Print one card per table and one worksheet per student.*

> **The task.** Your group receives one Earth target. Reconstruct how a geophysicist would
> figure it out. You must name **at least two methods** and **one ambiguity that their
> combination resolves**. You have 10 minutes, then 60 seconds to report.

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## The Synthesis Card (worksheet — one per student)

**Target / question:** ________________________________________________

**1 · Methods we would deploy** (2–4 from the course)
> ____________________________________________________________________

**2 · Observable → property** (what we measure → what Earth property it senses)

| Method | We measure… | It senses… |
|---|---|---|
| | | |
| | | |
| | | |

**3 · The null space** — what does *one* method, alone, fail to tell us?
> ____________________________________________________________________

**4 · The joint move** — method ___ + method ___ removes which ambiguity? (one sentence)
> ____________________________________________________________________

**5 · One number** — an order-of-magnitude estimate we can produce
> ____________________________________________________________________

**6 · The one-method trap** — a headline reports this from a single method. What hidden
assumption would you cross-examine?
> ____________________________________________________________________

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## Group cards (cut apart; one per table)

### Group 1 · The Whole Planet — *planetary interior, ~10⁴ km*
**How do we know Earth's outer core is liquid iron — and how would you find the core of Mars with a single seismometer?**
*On the table:* teleseismic body waves and the S-wave shadow · normal modes and PKIKP · mean density and moment of inertia (gravity) · the geodynamo.

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### Group 2 · Ocean Geophysics: The Spreading Seafloor — *ridge to abyssal plain, ~10³ km*
**For one patch of seafloor — how old is it, how deep should it be, and how much heat should it give off?**
*On the table:* marine magnetic reversal stripes (age) · bathymetry + gravity (depth, isostasy) · heat-flow probe + half-space cooling, q ∝ t^(−1/2). *(q = 510·t^(−1/2) mW m⁻², d = 2500 + 350·t^(1/2) m, with t in Ma.)*

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### Group 3 · Cascadia Earthquake & Tsunami Hazard — *subduction margin, regional*
**What will the next Cascadia megathrust do to Seattle and the coast?**
*On the table:* seismic imaging of slab geometry (Wadati–Benioff, tomography) · moment-tensor + paleoseismic estimate of M₀ · geodesy (GPS/InSAR) for locking · GMPE for shaking · shallow-water wave for tsunami. *(M₀ = μ·D̄·L·W; μ = 30 GPa.)*

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### Group 4 · Mountains, Basins & the Continents — *lithosphere, 10–500 km*
**Why is the Tibetan Plateau (or the Cascades) high — and what holds it up?**
*On the table:* free-air vs. Bouguer gravity · Airy / Pratt isostasy and flexure · crustal refraction / wide-angle reflection for Moho depth · the Nafe–Drake velocity–density bridge. *(Airy root r ≈ ρ_c·h / (ρ_m − ρ_c).)*

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### Group 5 · The Cryosphere & Climate–Solid Earth Coupling — *ice sheet to mantle*
**How do we weigh an ice sheet and watch it melt — and why does the solid Earth bounce back?**
*On the table:* time-lapse / satellite gravity (mass change) · glacial isostatic adjustment (rebound, mantle viscosity) · cryoseismology (icequakes, calving, basal slip) · DAS on fibre laid on ice.

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### Group 6 · The Reconstructed Past: Plate Kinematics — *whole planet, deep time*
**How do we rewind the plates 100 million years — and why isn't the hotspot frame fixed?**
*On the table:* paleomagnetism (paleolatitude, apparent polar wander) · seafloor magnetic isochrons · age-progressive hotspot tracks (Hawai'i–Emperor) · plate reconstructions.

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*When you report: state your target, your two best methods, and the one ambiguity your
combination resolves. Watch for a method another group used that senses a property yours missed —
every group is filling in one corner of the same diagram.*