Physiographically, Iran lies within the Alpine-Himalayan mountain system and is composed of a vast central plateau rimmed by mountain ranges and limited lowland regions. Iran is subject to numerous and often severe earthquakes and volcanic eruptions. The Iranian Plateau (alt. c.4,000 ft/1,200 m), which extends beyond the low ranges of E Iran into Afghanistan, is a region of interior drainage. It consists of a number of arid basins of salt and sand, such as those of Dasht-e Kavir and Dasht-e Lut, and some marshlands, such as the area around Hamun-i-Helmand along the Afghanistan border. The plateau is surrounded by high folded and volcanic mountain chains including the Kopet Mts. in the northwest, the Elburz Mts. (rising to 18,934 ft/5,771 m at Mt. Damavand, Iran's highest point) in the north, and the complex Zagros Mts. in the west. Lake Urmia, the country's largest inland body of water, is in the Zagros of NW Iran. Narrow coastal plains are found along the shores of the Persian Gulf, Gulf of Oman, and the Caspian Sea; at the head of the Persian Gulf is the Iranian section of the Mesopotamian lowlands. Of the few perennial rivers in Iran, only the Karun in the west is navigable for large craft; other major rivers are the Karkheh and the Sefid Rud.
The climate of Iran is continental, with hot summers and cold, rainy winters; the mountain regions of the north and west have a subtropical climate. Temperature and precipitation vary with elevation, as winds bring heavy moisture from the Persian Gulf. The Caspian region receives over 40 in. (102 cm) of rain annually. Precipitation occurs mainly in the winter and decreases from northwest to southeast. Much of the precipitation in the mountains is in the form of snow, and meltwater is vital for Iran's water supply. The central portion of the plateau and the southern coastal plain (Makran) receive less than 5 in. (12.7 cm) of rain annually.