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Short answer: open pit mining extracts ore or aggregate from a pit dug down from the surface, working level by level in steps called benches. It is used mostly for metal and aggregate deposits and for some near-surface, steeply dipping coal seams, and the pit is deepened until recovering the ore costs more than it is worth.
Definition and scope
An open pit is a surface mine in which material is removed from the top downward, leaving an excavation open to the sky for the life of the operation. Penn State’s mining engineering course (MNG 230) describes open pit mining as almost always applied to non-coal materials, mostly metal and aggregate mining, with the exception of near-surface steeply dipping coal seams. It is a mining-engineering topic that touches geology, geotechnical engineering and occupational safety, which is why it appears alongside geology and mineralogy in research and laboratory settings that analyse mine samples.
How an open pit works in practice
The same course material gives the sequence this way:
- Mining starts at the surface and proceeds level by level down into the deposit.
- The pit walls cannot be a single straight slope. They step down, and each step is called a bench. Benches give people and equipment access and a working platform at each level.
- The number of benches depends on the bench height and the ultimate pit depth.
- Slope stability weighs heavily on the number of benches, their dimensions and the angle of the pit wall, and mining engineers must ensure slope instabilities do not occur throughout the mine life.
- Reclamation is deferred until the mine closes, because returning waste to where it came from would block access to ore on the lower levels.
Why a pit stops deepening
As the pit goes deeper, the stripping ratio (waste moved per unit of ore) eventually becomes prohibitively large, and other costs rise too; haulage from the lower levels can require additional trucks to maintain production. When the cost to recover the ore exceeds its value, the operator has two main choices: begin mine closure, or begin an underground operation, which the course notes is common in metalliferous deposits. Pit depth can exceed 2,000 feet in some cases, and that point may be decades away.
Pit geometry terms and what each one controls
| Term | What it controls (per the sources cited below) |
|---|---|
| Bench | A step in the pit wall; provides access for people and equipment and a working platform at each level (Penn State MNG 230). |
| Bench height and pit depth | Together they set the number of benches (Penn State MNG 230). |
| Bench width and height (benching) | Under 30 CFR 56.3130, where benching is necessary, width and height must be based on the equipment used to clean benches or scale walls, banks and slopes. |
| Pit wall angle | Tied to slope stability alongside bench dimensions (Penn State MNG 230). |
| Stripping ratio | Rises with depth and helps determine when the pit becomes uneconomic (Penn State MNG 230). |
Where it matters: slope stability and safety
Ground control is the central safety issue in an open pit. A NIOSH-hosted conference paper, J.M. Girard, “Assessing and Monitoring Open Pit Mine Highwalls” (Proceedings of the 32nd Annual Institute on Mining Health, Safety and Research, Salt Lake City, August 5-7, 2001), states that slope stability accidents are among the leading causes of fatalities at US surface mining operations, and introduces warning signs of slope instability and common monitoring equipment.
In the United States, the Mine Safety and Health Administration standard for surface metal and nonmetal mines, 30 CFR 56.3130, requires that mining methods be used that maintain wall, bank and slope stability in places where persons work or travel in performing their assigned tasks. The rule also ties bench width and height to the equipment used to clean benches or scale walls, as noted in the table. Land restoration after mining is a separate regulatory subject; for coal, see the Surface Mining Control and Reclamation Act guide.
Common misunderstandings
- “Open pit means coal.” The Penn State course says the method is almost always used for non-coal materials, mostly metal and aggregate; coal is the exception, for near-surface steeply dipping seams.
- “The pit wall is a smooth cone.” Cone diagrams are teaching simplifications. Real pit walls are stepped into benches.
- “Land is restored as mining proceeds.” Reclamation is deferred until mine closure because backfilling early would block lower ore.
- “A pit keeps going until the ore runs out.” It stops when the cost to recover the ore exceeds its value, which can be before the orebody ends.
FAQ
What is open pit mining?
A surface mining method that removes ore or aggregate from an excavation dug down from the surface in stepped levels called benches.
What is a bench in an open pit?
A step in the pit wall that gives people and equipment access and serves as a working platform for its level.
What happens when an open pit gets too deep?
When the cost to recover the ore exceeds its value, the operator either closes the mine or starts underground mining, which is common for metalliferous deposits.
Which US rule addresses pit wall stability?
30 CFR 56.3130 for surface metal and nonmetal mines, as quoted above.
Sources
- Penn State College of Earth and Mineral Sciences, MNG 230, open pit mining lesson: courses.ems.psu.edu/mng230/node/648 (read 2026-11-08).
- Girard, J.M. (2001). Assessing and Monitoring Open Pit Mine Highwalls, NIOSH/CDC Stacks: stacks.cdc.gov/view/cdc/8509 (abstract read 2026-11-08).
- 30 CFR 56.3130, Wall, bank, and slope stability: law.cornell.edu/cfr/text/30/56.3130 (read 2026-11-08).
