Sciency Words: Critical Zone

Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we take a closer look at the definitions and etymologies of science or science related terms, in order to expand our scientific vocabularies together.  In today’s post, our Sciency Word is:

CRITICAL ZONE

So I could make this post about climate change, but I won’t.  Certain people would just get angry with me in the comments, and I’m not in the mood for that today.  So instead, I’m going to talk about Earth’s “critical zone” as something that could be relevant to astrobiological research.

Definition of Critical Zone: In Earth sciences, the critical zone is the surface and near surface environment of our planet—in other words, it’s the part of our planet where life lives.  The proper scientific study of Earth’s critical zone will require an interdisciplinary approach, combining geology, biology, hydrology, and other related fields.

Etymology of Critical Zone: The term was first introduced in 1998 by American sedimentary geologist Gail Ashley.  She called it the critical zone because, in her words, “it’s critical for life” and also because it is “critical to know more about it.”  (Source: see the Eos article in the links below.)

Here’s a fun fact that I like to share at parties: there are somewhere between four and five thousand different minerals found here on Earth.  Four to five thousand!  Other planets in our Solar System are known to have only a few hundred, at best.

Why is Earth so mineralogically diverse?  Plate tectonics, for one thing.  Liquid water plays an important role as well.  Some minerals can only form in the presence of liquid water.  But the biggest factor, by far, is life.  Living things do lots of weird chemistry, and all that weird chemistry messed with the planet’s soil and bedrock. Earth’s biosphere affects Earth’s geology.  And Earth’s geology, in turn, affects Earth’s biosphere.  There’s a synergistic relationship between living things and non-living rocks on this planet—and that is what the concept of Earth’s critical zone is all about.

I feel this is a terribly important concept to understand as it relates to Earth.  It’s also something worth bearing in mind as we think about other worlds out there, worlds which may or may not support life of their own.

WANT TO LEARN MORE?

I’m going to recommend this article from Eos, entitled “Critical Zone Science Comes of Age.”  I think that article is a pretty good summary of how critical zone research started and how it’s going, and it includes some quotes from Gail Ashley explaining what she was thinking about when she originally coined this term.

I’m also going to recommend this brief article entitled “‘Critical Zones’ on Mars and Across the Solar System,” which attempts to adapt the concept of the critical zone for other worlds.

And if you want to read more about why Earth is so mineralogically diverse, here’s a piece from EarthDate.org entitled “Minerals Evolve, Too.”