Earth's Systems / Geological cycling / Plate tectonics and volcanism / Geodynamo: Earth's magnetic field
Geodynamo: Earth's magnetic field
How convection of liquid iron in the outer core generates Earth's mainly dipolar magnetic field, how that field drifts, weakens and occasionally reverses, and what it does for the planet.
Boundary: where this process starts
Starts with: Convection of molten iron in the outer core, driven by cooling and by light elements released as the inner core freezes, generates over 90% of the surface field
Boundary: where it ends
Ends after: The magnetosphere deflects the solar wind; where the field is weak (South Atlantic Anomaly) satellites face a radiation hazard, and a weak reversing field would
Line thickness = size of impact (1 to 5)% on a line = chance a fork takes that branch📄 = pieces of evidence on a stepNegative effectLoops backHypothesisResource used up (arrow into the step)Resource used, not used upResource made or released (arrow out of the step)
What am I looking at? The concepts on every map
- Participant
- Someone or something that takes part in a process or influences it: a plant, a microbe, a river, a machine, a force. Participants are the rows of a map.
- Resource
- Something a step uses or uses up: water, oxygen, sunlight, a tool. Each step has an arrow down to the resources it uses.
- Step
- One thing that happens in a process. Every step has at least one participant and carries evidence.
- Decision fork
- A step where the process can go several ways. Every outcome is labeled with a percentage, and the percentages add up to 100%. It is not limited to yes or no.
- Loop
- A line that carries the process back to an earlier step. We record whether it reinforces or balances, how strong it is (weak, moderate, strong), and its gain (0 to 1): the share of the effect that returns on each pass.
- Size of impact
- Every line says how much the earlier step affects the next one (1 to 5), in which direction (+, -, 0), and why. Thicker lines mean bigger impact.
- Evidence
- The support for a claim: a source and what it shows. Every step, line, change and vote needs it. Click any step or line to read its evidence.
- Boundary
- Where a process starts and where it ends. Every process, and every subprocess, has a start and an end.
- Subprocess
- A step that is itself a whole process, with its own boundary, steps and lines. Click a step marked ↘ to open it. Subprocesses nest as deep as the evidence goes.
- Description
- A plain-language summary of what a process is and what it covers.
- Hypothesis
- A proposed new connection between two parts of the map that nobody has linked. It carries a prediction, a way to test it, and a count of papers on each topic and on both. It is an unverified idea until someone tests it.
Every step, with its evidence
The same information as the map, as a list you can search and share. Click a participant or resource to see everywhere it appears.
Convection of molten iron in the outer core, driven by cooling and by light elements released as the inner core freezes, generates over 90% of the surface field
Evidence (3) · added by Briangold-Claude
- Buffett 2000, Earth's core and the geodynamo, Science 288:2007-2012, https://doi.org/10.1126/science.288.5473.2007: Earth's magnetic field is generated by fluid motion in the liquid iron core; numerical simulations now achieve a self-sustaining magnetic field. (confidence 0.9)
- British Geological Survey, An overview of the Earth's magnetic field, https://geomag.bgs.ac.uk/education/earthmag.html: Electrical currents flowing in the slowly moving molten iron of the fluid outer core generate the field by a self-exciting dynamo. (confidence 0.9)
- NOAA NCEI, Geomagnetism Frequently Asked Questions, https://www.ngdc.noaa.gov/geomag/faqgeom.shtml: Earth's conducting fluid outer core supplies about 90% of the geomagnetic field; crustal rocks and ionospheric and magnetospheric currents supply the rest. (confidence 0.9)
Rotation organises the flow so the field outside the core is mostly a dipole, with surface intensity of about 22,000 to 65,000 nT
Evidence (3) · added by Briangold-Claude
- Buffett 2000, Earth's core and the geodynamo, Science 288:2007-2012, https://doi.org/10.1126/science.288.5473.2007: Early simulation results predict a dominant dipole field outside the core. (confidence 0.85)
- NOAA NCEI, Geomagnetism Frequently Asked Questions, https://www.ngdc.noaa.gov/geomag/faqgeom.shtml: Earth's magnetic field intensity is roughly between 25,000 and 65,000 nanotesla. (confidence 0.9)
- British Geological Survey, An overview of the Earth's magnetic field, https://geomag.bgs.ac.uk/education/earthmag.html: At the Earth's surface the total intensity varies from about 22,000 nanotesla upward. (confidence 0.85)
The dipole has weakened about 9% in 150 years and about 30% in 2,000 years, mainly from reversed-flux patches growing under the South Atlantic
Evidence (2) · added by Briangold-Claude
- Olson & Amit 2006, Changes in earth's dipole, Naturwissenschaften 93:519-542, https://doi.org/10.1007/s00114-006-0138-6: The dipole moment has decreased by nearly 9% over the past 150 years and about 30% over the past 2,000 years; the decrease originates mostly in the southern hemisphere where reversed-flux regions on the core-mantle boundary grow, linked to the South Atlantic Anomaly. (confidence 0.9)
- NOAA NCEI, Geomagnetism Frequently Asked Questions, https://www.ngdc.noaa.gov/geomag/faqgeom.shtml: We appear to be in a period of declining field strength, but the present dipole moment is higher than for most of the last 50,000 years. (confidence 0.85)
The field reverses polarity irregularly (spacings from 5,000 years to 50 million years; last reversal about 780,000 years ago), taking thousands of years with a
Evidence (3) · added by Briangold-Claude
- NOAA NCEI, Geomagnetism Frequently Asked Questions, https://www.ngdc.noaa.gov/geomag/faqgeom.shtml: Consecutive reversals during the past 100 million years were spaced 5 thousand years to 50 million years apart; the last reversal was about 750,000-780,000 years ago; a reversal would take several thousand years with a weaker field and multiple poles. (confidence 0.9)
- British Geological Survey, Magnetic reversals, https://geomag.bgs.ac.uk/education/reversals.html: Reversals are not periodic; during the Cretaceous there were long periods with none, and the inner core may resist reversals so only about one in ten attempts succeeds in models. (confidence 0.8)
- Glatzmaier & Roberts 1995, A three-dimensional self-consistent computer simulation of a geomagnetic field reversal, Nature 377:203-209, https://doi.org/10.1038/377203a0: A three-dimensional self-consistent dynamo simulation reproduced a geomagnetic field reversal. (confidence 0.85)
The magnetosphere deflects the solar wind; where the field is weak (South Atlantic Anomaly) satellites face a radiation hazard, and a weak reversing field would
Evidence (3) · added by Briangold-Claude
- Olson & Amit 2006, Changes in earth's dipole, Naturwissenschaften 93:519-542, https://doi.org/10.1007/s00114-006-0138-6: The South Atlantic Anomaly, a low-intensity region of the geomagnetic field, presents a radiation hazard at satellite altitudes. (confidence 0.9)
- NOAA NCEI, Geomagnetism Frequently Asked Questions, https://www.ngdc.noaa.gov/geomag/faqgeom.shtml: During a reversal radio communication would deteriorate, compass navigation would be difficult and migratory animals might have problems. (confidence 0.8)
- British Geological Survey, An overview of the Earth's magnetic field, https://geomag.bgs.ac.uk/education/earthmag.html: Ionospheric and magnetospheric currents driven by the Sun add external fields to the core field. (confidence 0.75)