READ: Gallery — Geology

The features and physical processes of the Earth.

The Andes are the longest continental mountain range in the world, stretching about 7,000 km or 4,300 miles from the tip of Chile in the South to Ecuador, Columbia and Venezuela in the north. The highest peak in the Andes is Mt. Aconcagua (6,962 m/22,841 ft.) in Argentina. The Andes range contains numerous high plateau regions and helps feed  the Amazon River, the world's largest river by volume.

In this image of folded strata in the Andes north of Quito, Ecuador, the geologic processes that formed the mountains are clearly visible. When the oceanic crust of the Nazca plate "subducted" under the South American plate, the lighter continental crust crumpled and continued to push upwards, eventually forming the snow-capped Andes.

Plate tectonics can also make mountains. Earth's biggest mountain range, the Himalayas formed when the India land mass (atop the Indian plate) slammed into the Eurasian plate. This slow motion collision happened about 50 million years ago, thrusting earth and stone skyward and forming many of the planet's highest mountains, including Mount Everest (8,848 m/29,029 ft.). Seen here are the Karakoram Mountains, a part of the Himalayas that spans the borders between Pakistan, India and China and includes K2 (8,611 m/28,251 ft.), the second highest peak in the world.

In the early days of Earth, a process called differentiation separated our planet into distinct layers. Density was the key here. Heavier, denser metals like iron and nickel formed the Earth's core while the extremely tightly-packed rocky mantle (some of it molten) surrounded the core. The layer of lighter rock at the Earth's surface is called the crust. It's important to remember that the formation process and ongoing activity created areas where different materials mix, such as veins of metal that reach to the Earth's surface and volcanoes that pump molten lava from the Earth's mantle, sending ash and carbon dioxide into the atmosphere, Earth's outer, gaseous layer.

The Earth's iron-nickel inner core is hotter than the surface of the Sun but intense pressure makes it solid metal. Molten metal in the outer core generates the Earth's magnetic field and the intense heat sends convection currents into the rocky mantle. These currents, or seismic waves, move through the mantle, causing the movement of the Earth's tectonic plates. The crust, a relatively thin layer around the Earth (think of an apple skin), is composed primarily of basalt (in the oceanic crust) and granite (in the continental crust).

In this image of the southern tip of South America, the snow-capped Andes Mountains are clearly visible along the western side of the continent. These mountains were formed when the heavieroceanic crust of the Nazca plate slipped under the South American plate, pushing the lighter continental crust upwards. As oceanic crust is pressed downward into the Earth's mantle, the "subduction zone" becomes a geologically active region, prone to both volcanoes and earthquakes.

Wegener’s idea that the continents had moved was not accepted by the scientific community until more evidence was found. Geologist and naval officer Harry Hess used sonar readings of the ocean floor, some taken during World War II, to demonstrate "seafloor spreading," helping to cement the modern theory of plate tectonics. Now scientists know that the Earth’s crust consists of several interacting plates that fit together like a jigsaw puzzle. There can be intense pressure where two or more plates meet and these "plate boundaries" are responsible for most of the earthquakes and volcanoes on Earth.

This volcano in the Kuril Islands, northeast of Japan, is one of the many active volcanoes in the "Ring of Fire." Shown here in the early stages of a June 12, 2009 eruption, Sarychev Peak is one of many volcanoes along the edge of the Pacific plate.  Mount St. Helens, which had a large eruption on May 18, 1980, and Mount Pinatubo in the Philippines, which exploded in June of 1991 are also in the "Ring of Fire."