Vine, Matthews and Morley’s magnetic stripes (1963)

Fred Vine and Drummond Matthews, and independently Lawrence Morley, saw that ocean crust records the reversing geomagnetic field as it cools at the ridge. Distance over half-spreading rate gives the age of each stripe, so the pattern is a mirror image about the ridge. The polarity timescale below keeps only the main chrons and subchrons of the last 4 million years.

import matplotlib.pyplot as plt
import numpy as np

from earthsciencekit.plate_tectonics import spreading_age
from earthsciencekit.units import convert

# Boundaries of alternating polarity, normal first [million years].
boundaries = np.array([0.0, 0.78, 0.99, 1.07, 1.78, 1.95, 2.58, 3.58, 4.0])
half = convert(2.0, "cm/year", "m/s")
distance = np.linspace(-80e3, 80e3, 1601)
age = convert(spreading_age(np.abs(distance), half), "s", "year") / 1e6
polarity = np.where(np.searchsorted(boundaries, age, side="right") % 2 == 1, 1, -1)
np.testing.assert_array_equal(polarity, polarity[::-1])
brunhes = spreading_age(15.6e3, half)
assert np.isclose(convert(brunhes, "s", "year"), 0.78e6)

fig, (top, bottom) = plt.subplots(2, 1, sharex=True, figsize=(8, 4.5), height_ratios=[1, 2])
top.imshow(polarity[None, :], aspect="auto", cmap="gray_r", extent=[-80, 80, 0, 1])
top.set(yticks=[], title="Normal (black) and reversed (white) crust")
bottom.plot(distance / 1e3, age)
bottom.set(xlabel="distance from ridge [km]", ylabel="crustal age [million years]")
fig.tight_layout()

plt.show()