What significant factors influencing geological change are still occurring?

Question

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?

Answer 1

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:

• mass removal and depostion of rock material (it's why the US has good soil that used to be in Canada). Also, some sizeable landmasses such as Long Island consist of moraine.

• major gouges in the landscape, such as the ones that hold the Great Lakes, or U-shaped valleys in the mountains

• isostatic rebound affecting coasts to this day

• estuaries (drowned river valleys) and barrier islands that result from sea level drop during a glacial maximum, followed by sea level rise as they melt. The Black Sea is a drowned freshwater lake.

• ancient glacial lakes created by trapped meltwater or increased rainfall, leaving lake bottom sediment in the soil and fossil shorelines. Sometimes the lakes drained catastrophically, leaving weird erosional features such as the Channeled Scablands. Sometimes they never drained at all (the Great Lakes).

• aeolian (loess) deposits from dessicated areas near the ice front

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.

Answer 2

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).

Answer 3

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.

Answer 4

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.