Week of June 4, 2023

The, Like, REALLY Big Cycles


Rocks on Earth’s surface eventually erode into sediments, which often get transported to water bodies. At the bottom of these water bodies they get compacted and pushed deeper underground where they are buried with greater heat and pressure. The rock material then returns to the surface either when melted into magma which flows from volcanoes as lava, or when the rocks it has joined return to the surface with tectonic activity. This is the rock cycle.


Liquid water from earth’s surface evaporates when heated by the sun, and condenses into clouds in the atmosphere. It then falls back down to earth as snow or rain, which evaporates directly or flows into other water bodies to evaporate and begin the cycle again. This is the water cycle.


Nitrogen gas, which comprises about 78% of Earth’s atmosphere, is “fixed,” or converted into a form usable to plants largely by bacteria and blue-green algae. When those plants die and decompose or are eaten by animals which later die, the remains are broken down by microorganisms into ammonia and ammonium. Nitrifying microorganisms then convert ammonia into nitrates, which can be converted into plant tissue or used by denitrifying bacteria, which produce atmospheric nitrogen gas. This is the nitrogen cycle.


Oxygen is used by humans and animals which breathe in oxygen and breathe out carbon dioxide, (which is also produced by the burning of fossil fuels). Carbon dioxide is then taken in by green plants on land and algae and made into carbohydrates, the byproduct of which is oxygen. This is the oxygen cycle.


The angle of Earth’s tilt relative to its plane of orbit around the sun is called obliquity, and varies between 22.1 degrees and 24.5 degrees, with the current tilt at 23.4 degrees. More glaciers are associated with smaller tilt angles and fewer glaciers with larger tilt angles. The period of these tilt cycles are about 41,000 years.


The shape of earth’s orbit around the sun changes from a near-perfect circle to a slight ellipse, a measurement known as eccentricity. This change comes from the gravitational pull of our massive planetary neighbors Saturn and Jupiter, and the cycle of most circular to most elliptical is about 100,000 years long.


Gravitational pulls on the tides from the sun and moon cause the Earth to wobble slightly on its own axis, which affects the extremes of each season in the Northern and Southern Hemispheres. This wobble is called axial precession, and a cycle lasts around 25,771.5 years.

These last 3 cycles are called Milankovitch cycles, after Milutin Milankovitch, the Serbian scientist who proposed them.