Lithosphere Workshop — The Eleven-Attribute Comparison

Lithosphere Workshop — The Eleven-Attribute Comparison#

Format E — Predict-then-Reveal Active Learning Companion to L26 Relevance: Synthesis · Tectonics · Geodynamics

Three predict-then-reveal beats · The eleven-attribute matrix · “The lithosphere is not one thing”

This 40-minute workshop is paired with L26 — Lithosphere: Oceanic vs. Continental. Hand out (or have students open) a blank comparison-matrix worksheet with the eleven attributes pre-labeled and two empty columns for oceanic / continental. The whole comparison fits on one page so students can see the framework at a glance.

Before beginning, say explicitly: “You will predict each row before I show you the answer. The point is to surface what you already think you know.”

Setup (0 – 3 min)

Distribute the comparison-matrix worksheet. Eleven rows: composition, layering, density, thickness, seismic structure, heat flow, gravity, magnetics, age, geodynamic role, and the synthesis row.

State the rule: predict first, copy second. No transcribing the matrix before the reveals.

Beat A — Composition, density, subduction (3 – 13 min)

Predict (5 min). Read the four prompts aloud:

  1. What is the dominant rock type of oceanic crust? Of continental crust?

  2. Which lithosphere is denser?

  3. Which subducts?

  4. Sketch a density profile for each, surface to 250 km.

Walk around. Look at sketches.

Discussion (3 min). Before revealing, ask: “Who predicted that continental mantle is heavier than oceanic mantle?” — then “Who predicted the opposite?” This is the productive disagreement. Do not resolve it yet.

Reveal (2 min). Show F7 (oceanic \(V_p\)) and F9 (continental \(V_p\)). Walk through composition → density. Then drop the punchline:

Cratonic mantle is chemically depleted. It is colder than oceanic mantle lithosphere — and it is also LESS dense. That is why cratons are stable for billions of years.

This usually produces visible “huh!” reactions. Hold the moment.

Beat B — Thickness, seismic structure, and “what is the lithosphere?” (13 – 28 min)

This is the longest beat. It is the heart of the workshop.

Predict (5 min). Read the three prompts:

  1. Predict \(V_p(z)\), Moho depth, total lithospheric thickness for 100 Ma Pacific.

  2. Same for the Canadian Shield.

  3. Same for the East African Rift.

Walk around. Look at sketches.

Reveal (4 min). Show F11 (continental vs. oceanic \(V_p\)):

  • Oceanic Moho is sharp and shallow (~11 km from sea surface). Continental Moho is deeper and gradational.

  • Oceanic LVZ at ~60 km is clear. Continental LVZ is weak or absent under cratons.

The pivot (6 min) — this is where the workshop earns its tuition. Show F6 (the boundary-layers key figure). Walk through panel (a) — oceanic boundary layers thicken with \(\sqrt{\mathrm{age}}\), both definitions track each other. Then panel (b).

Pause. Point at the four horizontal lines in panel (b). Say:

Every one of these is a “lithosphere base.” They are all measurements of the same craton. They are 80, 150, 220, and 280 kilometers. They should agree. They don’t.

Wait. Then ask: “Why do they agree under oceans?” Take 2–3 answers. The right answer is some version of: oceanic lithosphere is young, thermally simple, and doesn’t have a chemical-buoyancy layer at the bottom to complicate things. Cratons have had billions of years to develop independent thermal, chemical, mechanical, and seismic structure, and the four properties have decoupled.

This is the workshop’s thesis. Make sure every student has it written down before you move on.

Beat C — Heat flow, gravity, magnetics, age (28 – 38 min)

This beat is faster because students are now in the comparison mindset.

Predict (3 min). Read all six prompts at once. Students predict on their worksheets.

  1. Heat flow at 100 Ma Pacific seafloor.

  2. Heat flow on the Canadian Shield.

  3. Bouguer gravity over a 5-km ocean basin.

  4. Bouguer gravity over the Himalayas.

  5. Dominant magnetic pattern over the Juan de Fuca Plate.

  6. Dominant magnetic pattern over the North American craton.

Reveal (5 min). Speed-walk through the answers:

  • 100 Ma Pacific: \(\sim 50\) mW/m² (HSC prediction).

  • Canadian Shield: \(30\)–\(45\) mW/m². Why is this lower? — pause for the answer (low radiogenic, depleted mantle).

  • Bouguer over ocean basin: strongly positive. Over the Himalayas: strongly negative.

  • Juan de Fuca: Vine–Matthews stripes. NA craton: terrane-mapped basement structure.

The wrap (2 min). Display the full matrix. Read aloud:

Oceanic lithosphere is globally homogeneous and young. Continental lithosphere is heterogeneous and preserves Earth history.

Then say: “The geodynamic role of each — row 11 — follows mechanically from rows 1 through 10. We will see this play out in L27, L28, L29, and L30.” This is the bridge to the rest of Module 7.

Pacing checks

If you are behind schedule mid-workshop:

  • Skip Beat A’s discussion phase and go straight to the reveal. The chemical-buoyancy punchline still lands.

  • Do not skip Beat B’s pivot. That is the workshop. If you must cut, cut Beat C’s wrap.

  • Beat C can be compressed to 6 minutes — students will recover the content from the matrix in their notes.

If you are ahead of schedule:

  • Add a Beat-A side discussion: “What does the cratonic chemical depletion tell us about Earth’s early differentiation?”

  • Add a Beat-B side discussion: “If the cratonic LAB is a gradient, can we still talk about plates as rigid?” — a Module 7 capstone preview.

  • The Siletzia §9 anchor naturally absorbs any remaining time.

Common student misconceptions to watch for

  1. “Continental crust is more rigid than oceanic crust because it’s thicker.” Both crusts are mostly elastic on seismic timescales. The rigidity-relevant property is the temperature and rheology of the mantle lithosphere below, not the crust above. Address this in Beat B.

  2. “The seismic LAB is the real LAB.” This is the most common error. Address it directly when showing F6: there is no single “real” LAB; there are different measurements of different aspects of the lithosphere–asthenosphere transition.

  3. “Subduction happens because oceanic crust is heavier than continental crust.” Crust density alone is not the right comparison; the mantle lithosphere below matters, and oceanic mantle lithosphere becomes denser than asthenosphere only after it has cooled for ~10 Ma. Address in Beat A.

  4. “All continental crust is Precambrian.” Only the cratonic cores are. The continental margins (Cordillera, Andes, Cascadia) are Phanerozoic and very young. Address in Beat C row 10.

Relevance

Synthesis: The eleven-attribute matrix is the framework students will use for every Module 7 lecture that follows — divergent margins, convergent margins, transforms, and the geodynamic capstone all reduce to choices about which lithospheric attributes dominate at which kind of plate boundary.

Methods reuse: Every method from Modules 2–6 reappears here. Refraction sets the oceanic \(V_p\) stack. Tomography sets the seismic LAB. Gravity sets \(T_e\). Magnetics sets the Vine–Matthews record. Heat flow sets the thermal LAB. The workshop teaches students to read a combined observation, not a single method in isolation.

PNW anchor: The Siletzia section in L26 §9 is the local payoff — students can predict the gravity, magnetic, and heat-flow signature of the Cascadia forearc basement from the matrix alone, then compare against Anderson et al. 2024.

Go Deeper

  • Richards et al. 2018 — modern global re-fit of the oceanic-thermal models.

  • Holdt et al. 2025 — most recent plate-cooling parameters.

  • Levin et al. 2023 — the cratonic LAB as a gradient, not a sharp boundary.

  • Anderson et al. 2024 — Siletzia from gravity and magnetics, open access via USGS.

One name: Dr. Maureen Long, Yale — receiver-function imaging of the LAB and the mechanics of continental margins.