I recently saw that the Ohio Department of Natural Resources (ODNR) had a page about geologic hazards. Thus, this post will focus mostly on Ohio geohazards, which make up most of the total geohazards. They define a geologic hazard as follows:
“A geologic hazard or "geohazard" is a
geologic condition, either manmade or natural, that poses a potential danger to
life and property.”
They note that geohazard avoidance
or mitigation involves investigating and understanding the geology behind the
hazard. Thus, geologic mapping can be utilized as geohazard mapping. Certain
rock types are prone to certain hazards.
“For example, maps that show the Columbus Limestone
exposed at the surface also depict areas subject to karst development, and maps
that show shale-rich Ordovician bedrock present on steep slopes in southwest
Ohio also indicate landslide-prone areas.”
Shore Erosion
In Ohio, shore erosion is all
about Lake Erie shore erosion, but in coastal states it is oceanic shore
erosion that is problematic. That shore erosion is influenced by sedimentation
rates, which may be altered by the construction of dams that hold back
sediment, increasing the likelihood of erosion. It is also influenced by
individual storms. The Ohio page about lakeshore erosion lists the following
factors or variables: erodibility of material, soil slope and composition,
water level fluctuations, nearshore lakebed shoals and slopes, storm wave
energy and duration, precipitation, groundwater and soil conditions, ice cover,
shoreline orientation, beach composition, width and slope, and shore protection
structures. Thus, there are many factors that must be considered. Ongoing
mapping programs are important for assessing shore erosion. For Lake Erie, they
note that:
“Erosion within the nearshore is often unnoticed since
it occurs under the surface of the water, but it can be a significant cause of
major erosive events along Ohio's coast.”
Coastal erosion is a huge problem
around the world where it occurs.
Earthquakes
Earthquake hazards can be
mitigated with seismology networks where seismographs are set up to monitor
earth movement. This information is vital where earthquakes are common and can
be used to predict dangerous earthquakes and to assess epicenter location and
depth, which are important for determining future impacts and for defining
which faults have slipped. Where earthquakes are likely to occur, there needs
to be extensive knowledge of the faults and the likelihood of seismic events.
Karst Terrain: Limestone Sinkholes
Karst terrain is where karstic
limestone or dolomite outcrops or is near the surface. This can result in
sinkholes that can constrain construction and present dangers to road
construction. Groundwater hazards can also be present where groundwater
aquifers are in karstic carbonates. In those cases, groundwater can travel much
faster and farther than in other kinds of aquifers. Thus, contamination can
also travel much faster and farther as well. Normally, soil can filter
contaminants and chemically neutralize them over time, but with karst voids,
those contaminants often enter groundwater without being filtered and
chemically or biologically neutralized.
Landslides
Landslides typically occur where
slopes are steep. ODNR notes:
“Landslides occur when a hillside of earth materials
destabilizes and moves downslope, which can happen rapidly or slowly over
hours, weeks, or even years.”
As a geologist, I learned about
processes like land “creep” and slump erosion. In terms of rocks, shales are
more erodible as well as being more subject to landslides than other rocks. The
map below shows the regions in Ohio associated with the highest landslide
risks. These include regions with steep slopes, areas where shale outcrops, and
areas where glacial tills and clays are exposed to lakeshore erosion processes.
Radon
Radon is a geohazard where certain
rocks outcrop or occur close to the surface and can outgas radon into
basements. Organic black shales, light-colored
volcanic rocks, granites, sedimentary rocks that contain phosphate, and
metamorphic rocks derived from these rocks are the major sources of radon. I
wrote a post about radon in January 2025. Below is an EPA map of
areas where radon exposure is a concern.
Mine Subsidence and Other Hazards of Abandoned Mine Lands
Mine subsidence is a problem where
there are lots of abandoned underground mines, such as in Eastern Ohio and
nearby Appalachian states like West Virginia and Pennsylvania. These can
present as large sinkholes. In Ohio, I-70 has been closed at times due to mine
subsidence under the freeway. There are quite a few hazards associated with
abandoned mine lands, including mine subsidence, landslides of tailings piles,
erosion and sedimentation clogging stream channels and culverts, highwalls left
over from surface mining, acid mine drainage, and mine openings. Entering mine
openings can expose one to “poisonous or explosive gases, oxygen
deficiencies, flooded sections, unstable roofs, hard-to-see vertical shafts,
venomous insects and snakes and disorienting mazes of mine workings.” When
I was young, we used to swim at a place we called Bear Mountain (or Bare Ass
Mountain to some), and we used to jump off of a very high highwall. It was a
bit dangerous as you had to leap out enough to avoid scraping on the rock,
which is how a few people hurt themselves.
There are two basic types of mine
subsidence: pit subsidence and sag subsidence, which are described below:
“There are two types of subsidence: (1) pit, also called
sinkhole or pothole, and (2) sag, or trough. Pit subsidence is characterized by
an abrupt sinking of the ground surface, resulting in a circular, steep-sided,
craterlike feature that has an inward drainage pattern. It is associated with
roof collapse of mines that have total overburden – overlying, unconsolidated
material and rock – of less than 165 feet, weak roof rock of shale or mudstone,
and a ratio of unconsolidated-material thickness to rock thickness that is less
than 1:2:1. Pit subsidence does not occur where the thickness of the
unconsolidated overburden is more than 90 feet. Sag subsidence occurs as a
gentle, gradual settling of the ground surface and is associated with roof
collapse, pillar crushing, or pillar crunching of deeper mines (overburden of
more than 75 feet). Sag subsidence may fill with water if the surface of the
subsidence intersects the water table. Pit subsidence features generally do not
hold water because the pit drains in to the underlying mine.”
Geomagnetism
The U.S. Geological Survey
monitors geomagnetism, which may present hazards to power transformers. USGS
notes:
“Geomagnetic storms generate electrical currents that
can damage transformers and cause cascading power failures. By accessing
near-real-time information, engineers can adjust operations and avoid potential
blackouts or other disruptions.”
They use magnetotellurics to map
the electrical conductivity of rocks. The map below utilizes nearly 1800 data
points across the U.S.
Volcanoes
Areas where there are active
volcanoes, such as Hawaii, are major geohazard zones. Geologists study and try
to predict when eruptions will occur and how they will occur, whether
explosively with massive outgassing or with more gentle lava flows with less
pressure. Understanding geology is very important for predicting and mitigating
such events. Related hydrothermal phenomena like geysers can also be hazardous.
Glacier Collapses and Glacial Lake Collapses
As occurred recently in Nepal, glacier collapses can be immensely hazardous. In addition to that, the collapse of glacial lakes can also lead to catastrophic downstream flooding as well.
References:
Geologic
Hazards. Ohio Department of Natural Resources. Geologic
Hazards | Ohio Department of Natural Resources
Geologic
Hazards Science Center. Colorado School of Mines. U.S. Geological Survey. Geologic
Hazards Science Center | U.S. Geological Survey
Ohio Karst:
Geofacts. ODNR. GF31_Aden_2024.pdf
Mine
Subsidence: Geofacts. No. 12. 2010. Mine
Subsidence
Recently
completed geophysical survey will help protect critical infrastructure from
geomagnetic storms and space weather. U.S. Geological Survey. News Release. August
8, 2024. Recently
completed geophysical survey will help protect critical infrastructure from
geomagnetic storms and space weather | U.S. Geological Survey































