ESS 314 — L27 Ridges and Rifts

Ridges and Rifts

ESS 314 — Lecture 27

Module 7: Tectonics, Lithosphere, and the Cooling Earth

University of Washington · Earth & Space Sciences

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

By the end of this lecture, you will be able to…

  • Articulate the rifting continuum from stable craton through mature rift, magmatic break-up, and mid-ocean ridge — and identify the diagnostic geophysical signature of each stage.
  • Invert a spreading rate from a magnetic-anomaly profile using the Geomagnetic Polarity Time Scale and scipy.signal.find_peaks.
  • Decompose a continental-rift Bouguer anomaly into topography + Moho + LAB contributions.
  • Demonstrate gravity non-uniqueness at a mid-ocean ridge and identify the additional observables that disambiguate.
  • Evaluate failure modes of autonomous stripe picking as an AI-as-Tool case study.
Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

The geoscientific question

Mid-ocean ridges and continental rifts both extend the lithosphere. Why do they look so different in gravity, heat flow, and magnetics — and how do we know they are the same physical process?

  • Ridge: submarine mountain chain, basaltic crust, dense, recycling on Myr timescales.
  • Rift: continental depression, faulted, alkaline volcanism, sediment-filled.
  • Same physical process — extension — produces both.
  • The East African Rift preserves every stage in between.
Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Every active mid-ocean ridge on Earth

Global seafloor-age map: bright yellow zero-age bands trace the Mid-Atlantic Ridge, East Pacific Rise, Southwest Indian Ridge, and Juan de Fuca system; deep purple-black patches in the northwest Pacific mark the oldest oceanic crust at ~180 Ma

Müller / Seton 2020 grid. Narrow stripes = slow ridges (Atlantic). Wide stripes = fast ridges (Pacific). The Earth records 180 Myr of extension in one image.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

The East African Rift system

The East African Rift system map showing plates, rift axes, plate motion arrows, and volcanic centres, with rifting-continuum stage markers from Malawi (incipient) to Gulf of Aden (full spreading)

Spatially samples stages 2–6: Malawi (incipient) → Kenya (mature) → Afar (break-up) → Red Sea / Gulf of Aden (spreading).

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Governing physics: extension and the stretching factor

  • Stretching factor β=L0/Lthinned\beta = L_0 / L_{\text{thinned}}.
  • Stable craton: β=1\beta = 1.
  • Mature continental rift: β=2\beta = 2–33.
  • Magmatic break-up: β=4\beta = 4–66.
  • Mid-ocean ridge: β→∞\beta \to \infty (original lithosphere fully replaced).
  • Same physics. Different stage. Different geophysical signature.
Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

The ridge magmatic system

Schematic cross-section of a mid-ocean ridge showing layered crustal structure with axial magma lens, crystal mush zone, and mantle upwelling

Axial melt lens (AML) at 1.5–3 km depth — broader crystal mush zone below — sheeted dykes and pillow basalts above.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Magnetic stripes record spreading

Vine–Matthews–Morley:

  • New oceanic crust acquires thermal remanent magnetisation at the ridge axis as it cools through the Curie point.
  • The geomagnetic field reverses irregularly — the GPTS (Cande & Kent 1995) catalogues reversals back to ~180 Ma.
  • Symmetric polarity bands either side of the axis record the reversal history.

r=dn−dmtn−tmr = \frac{d_n - d_m}{t_n - t_m}

The simplest inverse problem in geophysics.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Ridge gravity and the mantle root

  • Free-air anomaly ≈0\approx 0: ridge is isostatically compensated.
  • Bouguer anomaly strongly negative (−250-250 mGal): low-density mantle root.
  • The Bouguer reduction strips out the topography but leaves subsurface density contrasts.
  • The mantle root — partially molten + thermally expanded — is the dominant signal.

The Bouguer signature is opposite to what naive intuition predicts.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

MAR gravity profile — three views

Three-panel cross-section showing bathymetry rising to a 2-km axial valley at the Mid-Atlantic Ridge, Bouguer anomaly dropping 250 mGal at the axis while free-air stays near zero, and a density model with low-density mantle root beneath the ridge axis

Topography + gravity + density model. The mantle root (orange) drives the Bouguer signature.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Round 1 — Predict

A clean Bouguer profile across a mid-ocean ridge.

  • Amplitude: ≈−250\approx -250 mGal at axis.
  • Half-width: ≈200\approx 200 km.

Sketch a 2D density-anomaly cross-section that fits.

You choose depth, width, and density contrast. 4 minutes.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Round 1 — Reveal

Two density models — shallow narrow body with high density contrast, and deep broad body with low density — fitting the same Bouguer anomaly profile

Both fit within data uncertainty. Amplitude is constrained; depth, width, density are not.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Round 1 — Discuss

  • What is the geological interpretation of each model?
  • Which additional observable would disambiguate?
    • Seismic VpV_p tomography? Heat flow? Mantle Bouguer anomaly?
    • All three together?
  • The pattern: single-method inversions are non-unique. Multi-method synthesis is the resolution.

Connects forward to L29 (focal mechanisms + GPS) and L30 (joint inversion).

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

McKenzie 1978 stretching → subsidence

  • Syn-rift subsidence: Si=aρmαTm(1−1/β)ρm−ρsS_i = a \frac{\rho_m \alpha T_m (1 - 1/\beta)}{\rho_m - \rho_s} — instantaneous.
  • Post-rift thermal subsidence: lithosphere re-thickens by conduction; t\sqrt{t} behaviour. Thermal time-scale τ≈62\tau \approx 62 Myr.
  • Steer's-head basin: narrow syn-rift + broad post-rift = every passive margin on Earth.

Same heat equation as half-space cooling (L26).

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Continental-rift Bouguer decomposition

Continental rift gravity decomposition showing three contributions — topography (narrow positive), Moho relief (intermediate positive), lithosphere-asthenosphere thinning (broad negative) — summing to the observed Bouguer anomaly, with cross-section below showing rift structural elements

LAB thinning dominates; Moho relief partially cancels; topography modulates near axis.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

EAR Kenya cross-section

East African Rift Kenya cross-section showing Bouguer gravity low with central magmatic high, broad rift uplift with axial graben, and density/Vp model with thinned crust and low-velocity Kenya dome

Mature rift signature: broad uplift + axial graben + thinned crust (Moho 22 km) + mantle dome.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Three rifts, three stages

Three-panel comparison of Basin and Range Province with broad gravity low and thinned crust, East African Rift Kenya with deep gravity low and mantle dome, and Keweenawan failed rift with central gravity high from gabbroic intrusion flanked by sediment-filled basins

Active broad low (B&R) → mature rift with magmatic high (EAR) → failed rift with central gabbroic high (Keweenawan).

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

CONUS Bouguer gravity

Conterminous US Bouguer gravity anomaly map showing strong negative anomalies over the Basin and Range and Rocky Mountains, neutral values over the craton, localised feature over the Midcontinent Rift, moderate negative over the Appalachians, and positive offshore

Western lows (active extension) — Midcontinent Rift high (failed) — Appalachian low (orogenic root). After USGS FS 78-95 (public domain).

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

§4 — Code Block D: spreading rate from magnetic stripes

import xarray as xr
import numpy as np
from scipy.signal import find_peaks

url = "https://www.ngdc.noaa.gov/geomag/EMAG2/EMAG2_V3_20170530.nc"
emag = xr.open_dataset(url)
profile = emag["z"].sel(lat=47.0, method="nearest").sel(lon=slice(-132, -126))
distances_km = (profile.lon - (-129.0)) * 111 * np.cos(np.radians(47))

peaks, _ = find_peaks(profile.values, prominence=80, distance=20)
# Match to GPTS (Cande & Kent 1995); fit slope through origin.

The fourth canonical data-access block of the course.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

JdF Ridge — stripes and inversion

Two-panel figure showing predicted polarity stripes from the GPTS above, and the magnetic anomaly profile across the Juan de Fuca Ridge below with peak detections and an inverted half-spreading rate of 2.17 cm per year against a reported value of 2.85 cm per year

Find peaks → match to GPTS → linear fit → spreading rate. One line, in principle.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Round 2 — Predict, then reveal the rifting continuum

Six stages: stable craton → incipient → mature → break-up → slow MOR → fast MOR.
For each, predict sign + magnitude of: Bouguer · heat flow · topography · β\beta · magma · seismicity. 5 minutes.

Six-panel rifting continuum cartoon showing cross-sections and attribute tables for stable craton through fast MOR

Same six observables, six qualitatively different signatures. EAR spatially preserves stages 2–4.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Slow vs. fast ridges

Two-panel figure with axial magma chamber depth versus spreading rate scatter showing slow ridges with deep AMCs, fast ridges with shallow AMCs, and Juan de Fuca highlighted as intermediate, alongside schematic Vp profiles for slow and fast end-members

Trend after Bell et al. 2022 (CC-BY 4.0): fast → shallow continuous AMC; slow → deep, episodic.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

PNW connection: Juan de Fuca + Axial Seamount

  • JdF Ridge sits 250 km off the Washington and Oregon coast.
  • The plate it creates is the same one that subducts under Cascadia.
  • Axial Seamount = ridge + Cobb hotspot interaction.
  • Continuously monitored since 2014 by the OOI Regional Cabled Array — the only fully cabled MOR observatory in the world.
  • Erupted in 2015 — the only MOR eruption ever recorded by a real-time cabled network.

Cascadia = ridge offshore + trench onshore + arc through the city. Drive distance from Seattle.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

AI as a Tool — autonomous stripe picking

  • scipy.signal.find_peaks succeeds in the data-rich middle: clean profiles, strong signal, smoothly varying noise.
  • It fails at the magnetic equator (weak induced field), at slow ridges with rough basement topography, near the poles (interfering geology), and under thick sediment cover (low-pass filtered signal).
  • A deep-learning picker (Galloway 2024) does better on noisy data but inherits the same physical failure modes.

You must know enough geophysics to recognise when the tool will lie.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Research horizon (open access)

  • Bell et al. 2022 (Frontiers, CC-BY) — modern review of MOR magma chamber and crystal mush imaging. Database of 277 imaging studies.
  • Biggs et al. 2021 (Nat Comms, CC-BY) — comprehensive synthesis of volcanic activity and hazard in the East African Rift.
  • La Rosa et al. 2025 (Solid Earth, CC-BY) — Afar rift strain partitioning between faulting and dyke intrusion.

Open question: Why do some continental rifts break to spreading and others fail?

Module 7 · Tectonics, Lithosphere, and the Cooling Earth
ESS 314 — L27 Ridges and Rifts

Concept checks

  1. Spreading rate: C5 chron (10.95 Ma) at 410 km from axis. What is the half-rate? Slow, intermediate, or fast?
  2. Disambiguation: Round 1 Bouguer profile + one extra measurement. Heat flow, VpV_p tomography, or magnetics? Justify.
  3. Stage: Rio Grande Rift — β=1.2\beta = 1.2, heat flow 80 mW/m², Bouguer −100 mGal. Which continuum stage?
  4. AI failure: 12 peaks per side, only 4 expected, slow Reykjanes Ridge. Diagnose two failure modes.

See you next time for L28: convergent margins.

Module 7 · Tectonics, Lithosphere, and the Cooling Earth