.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "api/gallery/geophysics/plot_15_cox_doell_dalrymple_polarity_timescale.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        :ref:`Go to the end <sphx_glr_download_api_gallery_geophysics_plot_15_cox_doell_dalrymple_polarity_timescale.py>`
        to download the full example code or to run this example in your browser via JupyterLite.

.. rst-class:: sphx-glr-example-title

.. _sphx_glr_api_gallery_geophysics_plot_15_cox_doell_dalrymple_polarity_timescale.py:


Cox, Doell and Dalrymple's polarity timescale (1964)
====================================================

Allan Cox, Richard Doell and Brent Dalrymple dated lava flows from around the
world by potassium-argon and measured their polarity. Flows of the same age had
the same polarity everywhere, so reversals are global and can be put on a
clock: the Brunhes normal epoch, the reversed Matuyama with short normal
events, and the normal Gauss. The timescale turned the magnetic stripes at
ridges into a speedometer. Spreading crust records it as blocks of alternating
magnetization, whose anomaly profile is symmetric about the ridge.

.. GENERATED FROM PYTHON SOURCE LINES 13-47



.. image-sg:: /api/gallery/geophysics/images/sphx_glr_plot_15_cox_doell_dalrymple_polarity_timescale_001.png
   :alt: Reversals recorded by a spreading ridge
   :srcset: /api/gallery/geophysics/images/sphx_glr_plot_15_cox_doell_dalrymple_polarity_timescale_001.png
   :class: sphx-glr-single-img





.. code-block:: Python


    import matplotlib.pyplot as plt
    import numpy as np

    from earthsciencekit.geophysics import block_magnetic_anomaly
    from earthsciencekit.units import convert

    normal_chrons = [(0.0, 0.773), (0.988, 1.072), (1.775, 1.934), (2.595, 3.596)]  # Ma
    boundaries = sorted({0.0, 3.596, *(t for chron in normal_chrons for t in chron)})
    rate = convert(2.0, "cm/year", "m/s")  # half spreading rate
    x = np.linspace(-1e5, 1e5, 801)
    anomaly = np.zeros_like(x)
    for start, end in zip(boundaries[:-1], boundaries[1:]):
        sign = 1 if any(lo <= start < hi for lo, hi in normal_chrons) else -1
        near, far = rate * convert(start, "Ma", "s"), rate * convert(end, "Ma", "s")
        for left, right in [(near, far), (-far, -near)]:
            anomaly += block_magnetic_anomaly(x, left, right, 3000, 3500, sign * 5)
    anomaly = convert(anomaly, "T", "nT")
    np.testing.assert_allclose(anomaly, anomaly[::-1], atol=1e-9)
    matuyama = np.abs(x - rate * convert(1.4, "Ma", "s")).argmin()
    assert anomaly[x.size // 2] > 0 > anomaly[matuyama]

    fig, (top, bottom) = plt.subplots(2, 1, sharex=True, figsize=(8, 5), gridspec_kw={"height_ratios": [3, 1]})
    top.plot(x / 1e3, anomaly)
    top.axhline(0, color="k", lw=0.5)
    top.set(ylabel="anomaly reduced to the pole [nT]", title="Reversals recorded by a spreading ridge")
    for start, end in normal_chrons:
        for side in (1, -1):
            span = sorted([side * rate * convert(start, "Ma", "s") / 1e3, side * rate * convert(end, "Ma", "s") / 1e3])
            bottom.axvspan(*span, color="k")
    bottom.set(xlabel="distance from ridge [km] (black: normal polarity)", yticks=[])
    fig.tight_layout()

    plt.show()


.. rst-class:: sphx-glr-timing

   **Total running time of the script:** (0 minutes 0.154 seconds)


.. _sphx_glr_download_api_gallery_geophysics_plot_15_cox_doell_dalrymple_polarity_timescale.py:

.. only:: html

  .. container:: sphx-glr-footer sphx-glr-footer-example

    .. container:: lite-badge

      .. image:: images/jupyterlite_badge_logo.svg
        :target: ../../../lite/lab/index.html?path=api/gallery/geophysics/plot_15_cox_doell_dalrymple_polarity_timescale.ipynb
        :alt: Launch JupyterLite
        :width: 150 px

    .. container:: sphx-glr-download sphx-glr-download-jupyter

      :download:`Download Jupyter notebook: plot_15_cox_doell_dalrymple_polarity_timescale.ipynb <plot_15_cox_doell_dalrymple_polarity_timescale.ipynb>`

    .. container:: sphx-glr-download sphx-glr-download-python

      :download:`Download Python source code: plot_15_cox_doell_dalrymple_polarity_timescale.py <plot_15_cox_doell_dalrymple_polarity_timescale.py>`

    .. container:: sphx-glr-download sphx-glr-download-zip

      :download:`Download zipped: plot_15_cox_doell_dalrymple_polarity_timescale.zip <plot_15_cox_doell_dalrymple_polarity_timescale.zip>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    `Gallery generated by Sphinx-Gallery <https://sphinx-gallery.github.io>`_