The Fun of Geologic Maps!

Excerpt from the geologic map of the western Winston-Salem area in
North Carolina, Virginia, and Tennessee. See the full map below, or
download it from the USGS  National Geologic Map Database.
Image Credit: USGS

Friday October 18th, 2013 is Geologic Map Day. No, really, such a thing does exist. It was founded by the US Geological Survey (USGS), the American Association of State Geologists, and the American Geosciences Institute in order to raise public awareness of the significant contributions geologic maps make in science, business, and public policy. To learn more, check out the Geologic Map Day website, where they have links to lots of neat stuff, such as geologic maps (of course), FAQs, and activities. Warning, this site is very US-centric. However, most countries have their own geologic branches of the government, which often provide on-line access to geologic maps. These can usually be found with a quick Google search. For example, the Geological Survey of India has links to a number of geologic maps throughout that country.

I think geologic maps are fun because they are so colourful. Unlike road maps, which link places with a network of lines, geologic maps look at areas. Each area is defined by the type of rock that is found there, and this rock type is shown on the map by a specific colour. This way, it is easy to tell at a glance which areas of a map have the same rock types.... and which ones don't. A legend is used to tell the map user what kind of rock each colour represents. Other geologic information, such as where faults are found, is represented by symbols, which are also explained in the legend. Some maps even come with cross sections, which show you a side view of the map at certain points, as if you had sliced the earth open like a cake and taken a look from the side. And some maps also have several paragraphs of text, explaining what happened in the mapped region, from a geological point of view.

Geologic map from the western Winston-Salem area in North Carolina, Virginia, and Tennessee, prepared by Rankin, Espenshade, and 
Neuman in 1972. The full map can be downloaded from the USGS  National Geologic Map Database.
Image Credit: USGS

Most people think of maps as something we make for places on the Earth. But we have been studying planets long enough that we have a fabulous assortment of maps, including geologic maps, for the other planets. The Lunar and Planetary Institute lists a bunch of links to planetary maps (and images) for the Moon, Mars, Venus, and Mercury on their Resources page.   My favourite is the Geologic Atlas of the Moon, which has links to on-line versions of every geologic map of the Moon published by the US Geological Survey.  Here you can find geologic maps for the Apollo landing sites, other regions of interest, and the entire Moon, divided up onto smaller segments.

This geologic map from 1971 (top) shows the Hadley Rille region of the Moon, where the Apollo 15 mission landed. The red lines overlaid on the map show the traverses that the astronauts undertook (determined from recent image data). This kind of information tells us that the astronauts saw a variety of geological regions on their traverse. They started out in flat mare terrain. One of their traverses skirted the ejecta of a young impact crater (olive green). Another traverse crossed the debris slopes (olive brown) of the Apennine Mountains (brown) to venture into the hills themselves. The third traverse cut through a crater field (pink), which was most likely formed by the ejecta from a much bigger crater well off the map, and then headed into the Apennine Mountains (brown) again.  In contrast, the Lunar Reconnaissance Orbiter Camera (LROC) image (bottom) does not provide this much information. The Apollo 15 traverses, shown in red, were determined from very high-resolution LROC images.
View the full map, with legend and explanations at the Lunar and Planetary Institute's Hadley Rille map page.
Explore this area of the Moon in more detail using the ActReact QuickMap Web Interface.
Examine the Apollo 15 traverses for yourself at the LROC Apollo 15 Traverse Page.
Image Credit: USGS (Map), NASA/Goddard/Arizona State University (Image and Apollo 15 traverse), and Irene Antonenko (compositing).

You can also find geologic maps for other planetary bodies in our solar system. Many haven't been studied long enough to have geologic maps made of the entire surface, but there are sections that have been mapped. Again, a quick Google search can find you lots of interesting tidbits. On a whim, I searched for "geologic map of Titan", which is one of Saturn's moons, and found an amazing little geologic map and article on the Selk crater of Titan, from The Planetary Society. Seriously, you should go check it out!

So, I hope this article has piqued your interest and inspired you to go check out some geologic maps, whether they are of Earth or any other planetary body, and celebrate Geological Map Day.