In geology, a passive margin, also known as a trailing margin, is a type of continental margin that is not associated with tectonic plate boundaries or active geologic processes like subduction or mountain-building. Passive margins are characterized by relatively stable and less tectonically active regions where continents meet ocean basins. Here are some key characteristics of passive margins:

1. **Lack of Plate Boundaries:** Passive margins are not located along the boundaries of tectonic plates where significant plate interactions occur. Instead, they are found within the interior of a tectonic plate.

2. **Limited Tectonic Activity:** Compared to active margins (such as convergent or transform margins), passive margins experience less seismic activity and deformation. They are relatively stable geologically.

3. **Sedimentary Accumulation:** Passive margins are often sites of extensive sedimentary deposition. Rivers transport sediment from the continent to the adjacent ocean basin, where it accumulates to form sedimentary layers.

4. **Wide Continental Shelves:** Passive margins typically have wide continental shelves, which are gently sloping underwater extensions of the continents. These shelves are often rich in marine life and are important for fishing and oil and gas exploration.

5. **Examples:** The eastern coast of North America, the Gulf of Mexico, and the Atlantic coast of Brazil are examples of passive margins. These regions have relatively calm tectonic histories and have not experienced recent mountain-building or subduction events.

6. **Potential for Petroleum Reserves:** The sedimentary rocks that accumulate on passive margins can be a source of significant petroleum reserves. Oil and gas often migrate and accumulate in subsurface reservoirs in these sedimentary rocks.

It’s important to note that while passive margins are generally stable compared to active plate boundaries, they are not entirely devoid of geological activity. Over extremely long time scales, some passive margins can become reactivated due to changes in plate dynamics, but these events are relatively rare compared to the ongoing tectonic activity at active margins.