Twice a year my running partner and I, both of us geoscientists, teach geology for Texas Parks and Wildlife Becoming an Outdoors Woman program. I created the course in 2014 by condensing an entire semester of freshman level physical geology into a 3.5 hour workshop (~1 hour lecture, ~1 hour lab, and ~1 hour spent in the field). While it’s impossible to absorb and retain everything that’s packed into a half day workshop, we try to encourage the participants to hold onto one thing from the broad spectrum of geology that they like. I think there’s something for everyone. It may be gem minerals for beading jewelry, fossil hunting with their kids, geologic time for relative aging, igneous rocks for their new counter-tops, or glacier formation for their bucket list cruise to Alaska. I call it “geologic speed dating.”
I’ve condensed the workshop even more here. This is not comprehensive by any means, definitely not too sciencey, but simply intended to be a short refresher (or precursor for the Downtown Houston geologic walking tours that I’ll publish soon). I want participants/enthusiasts to at least appreciate geologic processes, rocks and minerals, and/or fossils when they’re out hiking, traveling, and even shopping. Geologic beauty is everywhere!
Geologic Time
We start the class with a brief introduction to the geologic time scale, which can be overwhelming if you’ve never seen it before, however important in knowing how to look for ages of rocks in museums and state park guides. So I emphasize the importance of how to read the timescale. I cut the time scale below/far left (newest at top/yellow to oldest at bottom/grey) into parts and displayed them from left (newest) to right (oldest) for easier viewing on-screen here. Think of it as a stack of magazines you’d get throughout the year with January at the bottom to December at the top. Color is generally universal on geologic maps. Cretaceous is almost always going to be green. And the number in the right column(s) is age in millions of years. So Cretaceous is ~145-65 million years ago. Everything else is probably more than you’d care about…
I prefer more of a pictorial timescale like the one below. This one is more fun to read, shows dominant critters during each time period, and lists a couple large extinction events. And there’s a dog at the very top. Dogs make everything more fun, even science.
Fun Facts:
- The first multi-celled organisms occurred 570 million years ago
- “Age of Reptiles” (dinosaurs) was 245-66.4 million years ago
- Of the 5 mass extinction events, the worst was 250 million years ago (96% marine and 70% land species died off)
- The most recent extinction event was 66.4 million years ago (dinosaurs)
Another version of the geologic time scale is circular, which shows more of a true scale and also includes fun facts about each period/era.
The Rock Cycle
If I were to simplify all geologic processes into one square, it would be the block diagram below. There’s weathering, which is the breaking down of rocks, soils, and minerals through contact with the earth’s atmosphere, biota, and waters. Also erosion, which is the removal of material by water, ice, snow, wind, waves, and gravity. Sedimentary rocks are formed by the accumulation and deposition of sediment, burial, and compaction of material in large and small basins. Metamorphic rocks are formed by the deformation and metamorphism of rocks and soft sediment (heat, pressure, and chemical processes to alter mineralogy, texture, and chemical composition). Igneous rocks are formed by the melting and the crystallization/solidification of molten rock material/magma. It’s A LOT to take in, but this is our dynamic earth in a simplified box.
Igneous Rocks
Igneous rocks are formed by the melting and the crystallization/solidification of molten rock material/magma. To further categorize (but still keeping it simple) igneous rocks, they can be in one of two categories:
Intrusive (meaning below the surface)
- Slow cooling allows larger crystals (visible to naked eye) time to form
- Examples of intrusive igneous rocks are granite and gabbro
Extrusive (above the surface)
- Fast cooling will only allow smaller crystals (or glass) to form
- Examples of extrusive igneous rocks are rhyolite, basalt, and pumice
Metamorphic Rocks
Metamorphic rocks, material deformed by heat, pressure, and/or chemical processes to alter mineralogy, texture, and chemical composition, are a lot more complicated than igneous rocks. But simplified, with each increase in temperature and/or pressure yields a higher grade of metamorphism. Meaning mud heated and pressurized will be come slate, and with increased temperature and pressure will become phyllite, then schist, then gneiss (as the minerals condense and separate into layers/foliation). Limestone metamorphosed will become marble. Generally.
Sedimentary Rocks
Sedimentary rocks are formed by the accumulation and deposition of sediment, burial, and compaction of material in large and small basins. Sedimentary rocks are extremely broad, and quite possibly my favorite category. There are books and books and semester courses on sedimentary rocks alone. I will digress before I even get started and keep it super simple by rock:
- Glacial environments: conglomerate
- Arid/desert environments: gypsum, halite, sandstone
- Fluvial/river environments: sandstone, mudstone
- Beach environments: sandstone, limestone, siltstone
- Offshore environments: claystone, mudstone, sandstone
Also, grain size determines type of sedimentary rock. So claystone/mudstone is very fine grained, siltstone is fine grained, sandstone is coarse grained, and conglomerate contains large grains to boulders.
Resources
If you’re still with me, thank you for sticking with it. The rest is just fun stuff to explore on your own. I highly suggest perusing through the rock images on geology.com (screenshot below) by each category. There are a lot of fun pictures and fun facts on that site.
Two online mapping sources free to the public and easy to use are interactive geologic maps:
- Texas tx.usgs.gov/texasgeology
- United States https://mrdata.usgs.gov/geology/state/map-us.html
This is where the color of units generally correspond to the colors in the geologic time scale. The unit names may seem foreign, but you can click on them individually for pop-ups providing additional information about the type of rock, geologic age, and fossils found.
Pinterest contains a lot of home schooling pinners and geology enthusiasts that collect geologic knowledge into boards. I have a Geology Rocks! board that you’re welcome to follow. I pin maps, minerals, teaching adventures, fun hiking trips, articles, and diagrams. Start one of your own!
Geology is everywhere! I’ve found fossils, minerals, and cool rocks at wineries, I’ve walked on a glacier in Alaska, jumped on basalt columns in Northern Ireland, marveled at the diversity of sand all over the world, and have photographed world class mineral displays at Houston Museum of Natural Science. I don’t stop and smell the roses (allergies), but I do walk with my head down looking for rocks everywhere I go. Do you?
Coming this year to the blog is a new series, Skyline Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings, and will highlight the stones in, around, and on buildings in groups of about 10. Each tour will be less than a mile and will take less than an hour to walk. Links to the walking tours (~1 per month) will be updated here as they are published. Stay tuned and check back for more!
Published Walking Tours
Skyline Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings (City Hall Loop)
Tunnel Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings (Red Loop)
Skyline Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings (Hyatt Loop)
Tunnel Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings (Yellow Tunnel)
Skyline Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings (Jones Loop)
Tunnel Geology: A Self-Guided Walking Tour of Downtown Houston’s Buildings (Teal Tunnel)
More Geologic Adventures on the Blog:
British Isles Cruise on Royal Princess (Part 2: England, Ireland, Northern Ireland)
Alaskan Cruise: Juneau Part 1 (Mendenhall Glacier)
Alaskan Cruise: Scenic Cruising Tracy Arm Fjord
TPWD Becoming an Outdoors Woman Program
Becoming an Outdoors Woman as an Instructor
Becoming an Outdoors Woman Workshop: Spring 2016
Visiting Big Bend National Park: Something for Everyone
A Return to Big Bend National Park: Lodge Edition
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