I am teaching a secondary school course on my state's local history. The textbook begins with the history of the first humans, yet I think it is unfair not to start back much earlier, introducing the geologic history.

Students finished a lesson on the slow changes that took place over the past 4.5 billion years on Earth, gradually leading to the formation of our state. I want the students to understand these changes haven't stopped, just we maybe can't see everything happening, as it happens slowly.

A later lesson will focus on human changes, but I want to first cover natural changes:

  • Earth's magnetic pole is moving.
  • Moon slowly getting further away.
  • Days are slowly getting longer.
  • Tectonic plate slowly moving 1 inch west-southwest.
  • Wind and rain slowly shaping landscape.

Without having a geology background, I'm quite concerned I overlooked some important changes. There are some locally specific features I identified (such as a slowly-splitting rift valley), but are there any other major changes I ought to include?

  • $\begingroup$ Not directly geology, but related to history before the first humans: changing composition of the atmosphere, apparition (and extinction) of various forms of life, including mass extinction events, with their multiple causes (mostly volcanism, but also meteor strikes and more...). $\endgroup$
    – jcaron
    Jul 15 at 11:11
  • $\begingroup$ I heavily changed the title to make it simpler and clear. Hopefully I captured the intent. $\endgroup$
    – f.thorpe
    Jul 16 at 5:00
  • $\begingroup$ People may have more specific ideas if you mention which state you are in. $\endgroup$
    – jcaron
    Jul 16 at 11:40
  • $\begingroup$ @f.thorpe thank you $\endgroup$
    – Village
    Jul 16 at 14:28
  • $\begingroup$ @jcaron in the US Southwest, but was sure there are general answers that apply everywhere $\endgroup$
    – Village
    Jul 16 at 14:29

The effects of Earth's past climate variations on the landscape, especially glaciation, cannot be underestimated.

The multiple glacial advances and retreats over the last 4 million years or so have been a major (and sometimes the principal) force that shaped the current landscape. We're actually living in one of the temporary glacial minima - there will be more advances and retreats in the future.

Some important effects include:

And that's just what I could think of in 5 minutes.

Not only that, major glaciations in the distant past have affected the geological record, such as the Snowball Earth episodes ~ 700 Ma ago that nearly eradicated a billion years of stratigraphy.

  • 1
    $\begingroup$ I remember a field trip ~45 years ago where the teacher pointed to large boulders and explained that they were pushed there by glaciers. Geologists could tell this because their composition was unlike the local bedrock. $\endgroup$
    – Barmar
    Jul 15 at 14:27
  • 2
    $\begingroup$ @barmar Those are called "glacial erratics". $\endgroup$
    – Spencer
    Jul 15 at 14:46
  • $\begingroup$ Related, and links into the human impact, but perhaps not appropriate for schools: deepseanews.com/2012/06/… $\endgroup$
    – Tim
    Jul 15 at 19:54
  • 1
    $\begingroup$ As anyone who's tried growing anything in glacial materials will tell you: they're the worst, though not as bad as playground sand. That "good" midwestern soil is a direct result of Pleistocene herbivore droppings when the average size animal size was 200+lbs. $\endgroup$ Jul 16 at 19:28
  • $\begingroup$ @KnobScratcher Perhaps, but till can grow grass for those herbivores to graze in; otherwise they wouldn't have been there to poop in.. $\endgroup$
    – Spencer
    Jul 16 at 20:14

You could mention the fact that the solid Earth (i.e., not the atmosphere) is slowly cooling down, as the two main sources of heat, primordial and radioactive, are slowly decaying. Because of this, the planet may become tectonically dead at some point.

Gradual change over long periods of time is known as uniformitarianism, and has been opposed to catastrophism in the 19th century. From a history of sciences point of view, it could be worth mentioning the dispute between the two theories (and the fact that the evolution of Earth is actually driven by a bit of both).

  • 4
    $\begingroup$ (do not use with climate-change denialists) $\endgroup$
    – user253751
    Jul 15 at 12:15
  • $\begingroup$ Assuming that the Earth is not hit by another Thea-level asteroid strike. $\endgroup$
    – chiggsy
    Jul 17 at 0:21

Major geologic changes occur during natural events. Events like wildfire, floods, earthquakes, drought, volcanic eruptions, tsunamis, etc. can severely alter the landscape and allow new geologic processes to take place. For instance, after a landscape is hit by wildfire, it is easily subject to flash flooding and you get soil loss. This was not really "slow" on a geologic time scale, it was circumstantial.

This biggest event of all is a meteor impact, which can cause tsunamis, fires, etc. and also introduce new material to Earth.

Perhaps overlooked on your list is the affect that life has on geology: from microorganisms and worms in the soil creating humus to larger animals that burrow and dig, to vegetation that can transform local soil, moisture, and temperature... plants and animals do make a difference.

Finally, I know you said you will talk about people's influence later, but make sure they know we are in a new geologic epoch, the anthropocene. And, while human influence on climate is very important, we are possibly causing a bigger geologic effect with plastic waste.

  1. Carbon cycling through the mantle is a hugely important geologic process. Carbon gets into the mantle when subducted continental arc materials bearing rich, organic runoff are carried to depth, then exuded through volcanism at plate boundaries. That rich organic runoff is created by rain which is deeply influenced by...

  2. Oceanic currents which largely control planet-wide weather. These currents are influenced by plate tectonics, Milankovich Cycles, as well as global temperature.

Technically, #2 is an "oceanographic" process, but it's influence on crustal geologic processes is profound. Processes like the flipping of the poles, the distancing moon, wind erosion are interesting but play little part influencing the bigger geological picture.

Your later lesson might include how human-caused global warming is affecting global ocean currents and disturbing stable processes reflected in the geochemistry at plate boundaries. You might tell your kids how 25 million years from now, a terrestrial-squid geology grad student may study the relationship between the sudden disappearance of carbon-rich andesites from the western South American plate and the anthropocene.

Incidentally, if you'd like to show your students some riveting "future-geology", I highly recommend the series "The Future is Wild", in which global geology (and evolution) is observed 5, 100, and 200 million years into the future.


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