Introduction to CrowdMag and the Iron Dike
For geologists, magnetics play an important role in understanding the behavior of rocks. Rocks containing magnetic minerals like magnetite (iron oxide) can give off strong magnetic signals which can be used to determine their exact location, even if they are hidden deep beneath the Earth’s surface. All smartphones used today have the capacity to sense magnetic signals using their built-in magnetometers. Crowdmag, an app created by scientists at NOAA, is used to capture those magnetic signals, as well as current latitude and longitude, and turn them into quantifiable data that can be used to locate rocks that are giving off strong magnetic signals.
Over a billion years ago, before the Rocky Mountains were born, and even before there was life on land, molten rock bubbled up inside the Earth’s crust and pushed a column of hot magma up into an extensive crack in the surrounding granite. The magma then solidified into an iron-rich wall of rock, now called the Iron Dike, about three meters wide that extends almost 40 miles through Colorado and Wyoming today. In some places, the Iron Dike disappears beneath the surface of the ground but still continues underground. One of the areas where it isn’t visible at the surface for many miles is in the mountains near Lyons, Colorado.
In geology, the Principle of Lateral Continuity states that, unless proven otherwise, geological features tend to be laterally continuous. This principle is typically applied to sedimentary layers, such as two rock bodies separated now by erosion by a river, for example. In the case of the igneous Iron Dike, the principle suggests that the dike is likely continuous between the scattered outcrops at the surface. This means that even though the mountain-building episode that gave rise to the Rocky Mountains and the subsequent erosion of the uplifted material has resulted in some of the Iron Dike being underground, it is reasonable to trace it through on maps even though geologists can’t see it as continuous in outcrop. However, using Crowdmag’s ability to read magnetic data, the Iron Dike can now be mapped extremely accurately instead of having to rely on assumptions based on geologic principles.
Geologic map with Iron Dike plotted in purple. The black box shows the approximate area of our chosen field sites. The geologist who made this map used the Principle of Continuity to draw a continuous line (geologic unit label Ygb) for the dike even though it can only be seen in scattered outcrop.
A map of the two field sites used to find the Iron Dike.
Testing CrowdMag at the Scenic Overlook Site
Finding the Iron Dike in person was easy! CrowdMag’s sensors are fairly sensitive (although the signal from smartphone magnetometers also include some random noise) and were able to pick up evidence of the Iron Dike under both the Scenic Overlook near Allenspark, Colorado, and Overland Road near Jamestown, Colorado. The first step to finding the Iron Dike was using CrowdMag to locate the Iron Dike at the Scenic Overlook where the Iron Dike can be seen in the exposed roadcut on the south side of the highway. My CIRES RECCS summer mentors, Rick and Manoj, had been able to find the Iron Dike using other methods a few years before so we knew the Iron Dike was there but we needed to know if CrowdMag could detect it. We started by firing up the CrowdMag app on several phones (a variety of Samsung and iPhones) at the same time and walking in step together perpendicular to where we knew the Iron Dike crossed underneath us. This would give us a theoretically perfect idea of how wide the Iron Dike actually was and its exact position under the Scenic Overlook.
As we were leaving we ran into the owner of the property adjacent to where we were taking data. We learned that he was a retired climatology professor who knew all about the Iron Dike! He let us go up on his property to where the former owners of the property had built a mine into the Iron Dike. He told us a story about this being a “salted” mine, where no minerals of value could be found but some dishonest people sprinkled a little gold around to entice buyers to take the useless mine off their hands. He had inherited the property long after but still found the history interesting and passed it along to us! We were able to read some very strong magnetic signals up there right on top of the Iron Dike as we explored the abandoned mine.
We took all the data back to the lab and transferred all of the data sets into Microsoft Excel. CrowdMag records all magnetic field data for the horizontal component, the vertical component, and the total intensity. We tried to use the same time frame to synchronize the data but this didn’t work very well, perhaps because we each walked a slightly different path as we did some bushwhacking on our cross-country profile. Stacking the data sets by latitude and longitude produced much more reliable results. The data sets were stacked and smoothed to help us distinguish signal from the noisey outlying data points picked up by CrowdMag’s sensors. These stacked profiles showed a steep rise and dip at the beginning of the Iron Dike, correlating with a magnet’s positive and negative side. This rise and dip is characteristic evidence of the Iron Dike.
Holly, Nir, and Manoj using CrowdMag to find the Iron Dike at Scenic Overlook near Allenspark, Colorado. (Rick took the picture!)
Iron Dike mine located on private property (accessed with the owner’s permission) off the Scenic Overlook field site. Holly’s backpack for scale. The exposed rocks of the mine are very magnetic, they contain a high concentration of magnetite, the most magnetic iron mineral. High concentrations of magnetite are often correlated with valuable minerals such as silver or gold, but not in this case.
The Overland Road Experiment and Conclusions
The next step was studying geologic maps of the area where the Iron Dike has been mapped but has not been seen because it travels underground. We chose the area near the intersection of the Peak-to-Peak Highway and Overland Road near Jamestown, Colorado. This location is a short distancesoutheast of the Scenic Overlook where we found the Iron Dike the first time. We chose this new location mostly based on accessibility, as we didn’t want to trespass on private land if we could help it. The same process was repeated there at Overland Road, using a variety of Samsung and iPhones again and walking perpendicular across the Iron Dike. Overland Road runs pretty much west to east and the dike is mapped running south to north, making data collection extremely easy. Back at the lab we stacked the data sets according to latitude and longitude again and found a clear rise and dip much further west along Overland Road than the geologic map showed.
I concluded that this may have been another previously unknown vein of the Iron Dike, or perhaps the mapped vein was actually further west than originally projected by the mapping geologist following the Principle of Continuity. More research in this area would be needed to make any definitive conclusions. This study found that CrowdMag is useful for many geologic applications despite its challenges. It was also found that using three or more phones significantly improves confidence in the results. The Iron Dike is an ideal target for citizen science projects! I believe the Iron Dike is a great way to get people involved with CrowdMag to expand its potential. The community engagement and technological growth that CrowdMag can promote could improve mapping in remote or inaccessible places and make advancements in mining, transportation, and navigation. CrowdMag’s applications are potentially limitless!
Holly, Aaron, and Andy find the Iron Dike at Overland Road.
Geologic map zoomed in on Overland Road field site. Arrows show inferred spots where the Iron Dike passes under the road and dotted line shows another potential vein of the Iron Dike or a different location of the mapped veins.
