Isthmuses, which are narrow strips of land connecting two land masses and separating two bodies of water, are formed in various ways, depending on the geology of the areas in which they occur. These geological changes to form isthmuses may involve the movement of tectonic plates, the drowning of landmass due to fault structures, volcanic activity, and the movement of waves and tides.
According to NASA's Earth Observatory, one of the most famous isthmuses, the Isthmus of Panama, was formed when two tectonic plates, the Pacific Plate and the Caribbean Plate, collided. As one plate slid under the other, the resulting heat and pressure caused underwater volcanoes to erupt and create a series of islands. At the same time, the two plates continued to move, elevating parts of the sea floor. Sediment, such as sand and soil, filled in the gaps until a complete isthmus formed.
The Isthmus of Suez was formed more recently, according to the Encyclopædia Britannica. The area was originally a single, large landmass until a geological fault caused a trough which filled with water, creating the Red Sea. Subsequent sea level oscillations caused the narrow landmass, which became the isthmus, to rise. National Geographic reports that some isthmuses are formed as sandbars by the continuous force of tides and waves. These are known as tombolos, and the most famous example is the isthmus that connects the Rock of Gibraltar with the Spanish mainland.