Please consider that the strength of the magnetic field determines atmospheric density and pressure. Planets with strong magnetic fields have thick oceans and atmosphere, while planets with very weak magnetic fields have no oceans and very thin atmosphere. That's because hydrogen atoms from solar winds are attracted and condensed around magnetic fields and the strength of the MF dictates the atmospheric density, and atmospheric pressure dictates the ocean levels.
Mars for example shows signs of erosion as it had oceans and atmosphere but when the dynamo inside went through a hard double flip (most probably) and slowed down so much, cooled down so much that it could never recover, the atmosphere was blown away by solar winds, oceans boiled away, etc. Mars's gravity field doesn't explain how it lost its oceans and atmosphere when Earth still has them. Mars's gravity field didn't change, it lost its magnetic field.
Jupiter's bigger moons have thick ocean frozen at the surface (frozen because of distance from the Sun) but they are smaller than Mars and have strong magnetic fields.. So size and amount of pressure inside doesn't necessarily mean a stronger magnetic field,, what all planets and terrestrial moons with strong MF have in common is an inner core that makes a small orbit around the center of the planet/moon, because of tidal effect (gravitational pull) from a big body nearby. For example, Earth's inner core is attracted by our big and heavy Moon, and it's not spinning in the dead center of Earth but has a small orbit around the center, planar to the Moon's orbit, and it stirs the soup inside as Earth spins on itself. It keeps the soup warm and generates friction thus electric currents, thus electromagnetism.
Jupiter's moons have their inner core drawing a larger orbit inside as they (moons) spin on themselves, their core is off-centered toward Jupiter.
Our Moon is actually the big core of an ancient planet that collided with our former (smaller) planet about 4.5 Billion years ago. Their liquid matter blobbed together and the smaller core bounced from the explosion as just the right speed and angle to stabilize on orbit, and the little bit of liquid matter on it cooled fast with a tide towards Earth, witch is the heavier side of the Moon that always point to Earth. With time it gathered the splash from the collision. It's a big metal core with a thin crust and this is why it rings like a bell and the craters are all shallow.
So anyway.. we're very lucky to have the Moon because otherwise Earth would be like Mars right now. Mars doesn't have a big moon so its core just spins in the center and doesn't generate enough friction. Maybe it had a moon before, with an unstable orbit destined to swing away..
Now, why does our MF tend to flip every 12-13000 years, or twice every Great Year (Precession of the Equinoxes, a 26000-year Milankovitch Cycle when the Earth's spin tilt axis wobbles 360*)?
I think it's because there are 2 critical tilt angles, during the Precession,, where the spin inertia of the iron currents, plus the angle of the inner core's orbit (planar to the Moon's orbit, not the same angle as the Earth's rotation axis),, plus the difference in speed (earth spins on itself in one day while the Moon takes 27.3 days to go around Earth),,, produce electric currents that don't agree with the magnetic field orientation and forces it to either reverse or migrate..
(to visualize this, have a pot of sauce that represents Earth, a wooden spoon that represents the inner core and an imaginary moon that goes around the pot in 27 days. Turn the pot of sauce continually to simulate earth's rotation and imagine it's tilted 23.4*,, and place the spoon (core) off-centered in the pot and tilt the spoon to simulate the difference in angles of the Earth's spin and of the moon's orbit plane. As you spin the pot, continuously, slowly orbit the spoon in the same time as your imaginary moon,, and see the currents you're generating in the sauce.. Then slowly change the tilt of the spoon to simulate Earth's precession.. It sounds fun? Do this to really get a visual)..
Now during the reversal or excursion process, the convection currents need to reorient and mix and slow down, producing less friction thus less electromagnetism, so the atmosphere gets thinner and filters out less solar and cosmic radiation. Less friction also means less heat produced inside the planet, so the volume of the planet has to shrink a little bit, and that explains the spike in seismic and volcanic activity around the Peak.
What explains how ocean levels can drop 400ft? Well if the atmosphere is thin during the day, then raw solar radiation evaporates tons and tons of ocean surface and during the night it gets very cold so all the moisture in the air has to precipitate within hours. Most of it falls back in the oceans and on land, and some of it becomes atmosphere and is eventually blown away by solar winds. That explains the biblical amounts of precipitations and erosion (Grand Canyon fills up and deserts turn into seas and the northern atmosphere is covered with very thick glaciers, big mammals flash frozen, etc).. Flash frost is also explained by rapid atmospheric density drop when a CME impacts the already thin atmosphere and temporarily blows some of it away: pressure drops fast and flash-freezes everything at the surface.
A thin atmosphere also explains the number of meteorite impacts because meteors rain down every day but usually burn with friction, blow up into small bits and only tiny bits of metal reach the ground,, but when the atmosphere is thin, meteors have much less friction so better chances to hit the ground in bigger chunks.
Those 2 critical tilt angles during the Precession are:
one as we enter Leo (the stronger magnetic Excursions (double flips) like the Gothenburg Magnetic Excursion 13000 years ago and the Leschamp Magnetic Excursion 40000 years ago, etc)
and the other one as we enter Aquarius (the mild semi-reversals (also calles Excursions) like now, or like the Lake Mungo Magnetic Excursion 26000 years ago,, etc).
This is all over ancient civilizations calendars, like the Mayan calendar, etc. For Example, the Sphinx was pointing Leo 13000 years ago, but was probably built 11000 years ago after the Younger Dryas warming, as a big immortal monument to commemorate this strong event that almost got us extinct.
Also, one important principal, heat generated inside the planet goes up slowly through the crust, then slowly raises up in the oceans, then joins the heat on the surface from solar radiations, and that heat has to go up in the atmosphere to reach the space vacuum.. The stronger the magnetic field and the thicker the oceans and the atmosphere,,, and the slower heat can reach up into space. Vice-versa, the thinner the oceans and atmosphere and the faster heat can reach up into space..
We could say that at Peak, a typical day would be like: super windy as almost unfiltered solar radiation expands the air very rapidly,, then all day would feel like a super hot sauna with tons of melting and floods, and evaporation would look like a steam engine.. Then, as the Sun starts to set the sky veils with thick condensation. Shortly after, deluges and super flash floods, turning into super ice storm breafly, then meters of snow until the air is dry and very cold, like -50C (moisture can stay in the air up to -20C but lower than that, all humidity is already deposited, so all moisture in the air has to deposit within a few hours)... Then, in the morning it starts all over again. Sometimes when there is a CME impact, it gets worse.
OK, I'll stop here, will elaborate more if there is interest.. Please give me feedback,, wherever I'm wrong and how I should rethink.. Thanks