A mountain range or hill range is a series of mountains or hills arranged in a line and connected by high ground. A mountain system or mountain belt is a group of mountain ranges with similarity in form, structure, and alignment that have arisen from the same cause, usually an orogeny. Mountain ranges are formed by a variety of geological processes, but most of the significant ones on Earth are the result of plate tectonics. Mountain ranges are also found on many planetary mass objects in the Solar System and are likely a feature of most terrestrial planets. Mountain ranges are usually segmented by highlands or mountain passes and valleys. Individual mountains within the same mountain range do not necessarily have the same geologic structure or petrology. They may be a mix of different orogenic expressions and terranes, for example thrust sheets, uplifted blocks, fold mountains, and volcanic landforms resulting in a variety of rock types. Most geologically young mountain ranges on the Earth's land surface are associated with either the Pacific Ring of Fire or the Alpide belt. The Pacific Ring of Fire includes the Andes of South America, extends through the North American Cordillera, the Aleutian Range, on through Kamchatka Peninsula, Japan, Taiwan, the Philippines, Papua New Guinea, to New Zealand. The Andes is long and is often considered the world's longest mountain system. The Alpide belt stretches 15,000 km across southern Eurasia, from Java in Maritime Southeast Asia to the Iberian Peninsula in Western Europe, including the ranges of the Himalayas, Karakoram, Hindu Kush, Alborz, Caucasus, and the Alps. The Himalayas contain the highest mountains in the world, including Mount Everest, which is high. Mountain ranges outside these two systems include the Arctic Cordillera, Appalachians, Great Dividing Range, East Siberians, Altais, Scandinavians, Qinling, Western Ghats, Vindhyas, Byrrangas, and the Annamite Range. If the definition of a mountain range is stretched to include underwater mountains, then the Ocean Ridge forms the longest continuous mountain system on Earth, with a length of .