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Although volcanism accounts for the extensive mid‐ocean mountain ridges and some large islands, such as those of the Philippines, Japan, and Indonesia, Earth also exhibits extensive mountain ranges which have been created by tectonic forces.

At the present time, the western side of the Americas, southern Europe, and much of southern Asia are dominated by relatively young, high fold mountains (Figure 3.22). Many of these have grown within the last 60 million years, and they are still increasing in height today.

The tallest mountains on the planet are the Himalayas, which are continuing to rise at an average rate of about 40 mm per year as the Indian tectonic plate slides north beneath the Eurasian Plate.

Like the Himalayas, all of the continental fold mountains have been created by compressional tectonic activity involving a collision between two lithospheric plates. There are three types of tectonic convergence: ocean‐continent, arc‐continent, or continent‐continent.

When oceanic and continental plates collide, marine sediments are scraped off the descending oceanic plate, squeezed and accreted to the edge of the continent, forming a range of fold mountains parallel to the coast. Magma formed by melting of the descending plate feeds numerous volcanoes. This is the case with the Andes mountains of South America.

When a volcanic island arc collides with the edge of a continental plate, sediments are also scraped off the descending oceanic plate, but the volcanic islands collide with the continent and are also accreted to the margin of the continent. This type of collision may have been responsible for the creation of the Sierra Nevada range in California.

The most dramatic type of convergence occurs when an ocean basin closes and two continental plates collide. In this case, sediments that once lay on the old ocean floor and rocks in the continental crust are folded, fractured, and changed by heat and pressure.

Over time, the mountains are weathered and eroded. As material is removed by glaciers and rivers, the weight of the crust becomes progressively less. The continental crust makes an isostatic adjustment, causing it to rise slowly – rather like removing a weight from a floating raft. Such adjustment places considerable strain on the crust, causing it to fracture along fault lines or break up into separate blocks. A block of crust may rise to form a horst, or sink between two faults to form a graben or rift valley.

Over hundreds of millions of years, weathering and erosion lower the once‐towering mountains and make them more rounded in shape. The modern‐day Appalachians and Scottish Highlands are examples of this process, remnants of a period of mountain building that ended about 250 million years ago (see Plate Tectonics).