Ocean-Continent Convergence – Andes and Rockies

At a Glance

• Plates Oceanic meets continental
• What subducts The oceanic plate
• Landforms Trench, fold mountains, volcanic arc
• Main example Andes: Nazca plate under South America
• Hazards Great earthquakes and explosive volcanoes
• Exam link UPSC Prelims, Geography: Geomorphic Movements and Plate Tectonics

Where Does It Fit?

This is the eighth topic of the category on geomorphic movements and plate tectonics. The previous topic described the collision of two oceanic plates, and this one covers an oceanic plate meeting a continent. The next topic covers the collision of two continents. Previous topic: Ocean-Ocean Convergence and Island Arcs. Next topic: Continent-Continent Convergence – Himalayas and Tibetan Plateau.

What Is Ocean-Continent Convergence?

In short: a heavy ocean plate dives under a lighter continent, crumpling its edge into mountains.

When an oceanic plate meets a continental plate, the oceanic plate is denser and thinner, so it is forced under the continent in a process of subduction. The line of descent is marked by an oceanic trench just off the coast. The sinking plate melts partially at depth, and the magma rises through the continental crust, forming a chain of volcanoes on the continent called a continental volcanic arc. At the same time, the compressive force deforms and thickens the edge of the continent, producing folded and faulted mountains. Sediments scraped from the descending plate pile up as an accretionary wedge between the trench and the coast. The difference from the island arc process is explained in Ocean-Ocean Convergence and Island Arcs.

How Were the Andes Formed?

The Andes are the longest continental mountain range above water, extending for about 7,000 km along the western coast of South America. They arose where the oceanic Nazca plate subducts beneath the South American plate at a speed of about 7 to 8 cm a year. The offshore trench is the Peru-Chile Trench, also called the Atacama Trench, with a depth of about 8,000 m. The highest peak is Aconcagua in Argentina, at about 6,961 m, which is the highest peak outside Asia. The range includes many active volcanoes, such as Ojos del Salado on the Chile-Argentina border, the highest active volcano in the world at about 6,893 m, and the Andean volcanic belt is part of the Ring of Fire. Powerful earthquakes occur: the 1960 Valdivia earthquake in Chile, of magnitude 9.5, is the largest ever recorded.

What About the Rockies and Other Western Ranges of North America?

The Rocky Mountains extend for about 4,800 km from Canada to New Mexico, with Mount Elbert in Colorado, about 4,401 m, the highest peak. They arose during the Laramide orogeny about 80 to 55 million years ago, when the oceanic Farallon plate was subducted at a very shallow angle beneath North America, which squeezed and lifted the interior far from the coast. The Farallon plate has since nearly disappeared, and its remnants are the Juan de Fuca and Cocos plates. The Cascade Range of the Pacific Northwest is a present-day volcanic arc formed by the subduction of the Juan de Fuca plate, and includes Mount St Helens, which erupted in 1980. The Sierra Nevada and the Coast Ranges also belong to this system.

What Are the Common Features of Continental Arcs?

• Trench: a deep trench offshore, though not as deep as in the Pacific island arcs because large amounts of sediment fill it.
• Volcanoes: explosive, as the rising magma is rich in silica and gas, producing andesite and rhyolite.
• Fold mountains: thick crust and high peaks formed by compression.
• Earthquakes: shallow near the trench and deeper under the continent, along the Wadati-Benioff zone.
• Metals: hydrothermal fluids concentrate copper, gold and silver, which is why Chile and Peru are among the world’s major copper producers.

As an analytical point, mountain-building at ocean-continent boundaries is not always uniform: the angle of subduction decides whether the mountain-building is near the coast, as in the Andes, or well inland, as in the Rockies. The third kind of convergence, between two continents, is explained in Continent-Continent Convergence – Himalayas and Tibetan Plateau.

Why Is This Topic Important?

• Examples Andes and Rockies frequently asked
• Hazards Volcano and earthquake belts
• Contrast Differs from island arcs

What Came Next?

The next topic describes continent-continent convergence, the Himalayas and the Tibetan Plateau. Next topic: Continent-Continent Convergence – Himalayas and Tibetan Plateau.

Key People and Terms

• Continental arc Volcanic chain on a continent
• Nazca plate Oceanic plate off South America
• Accretionary wedge Scraped sediments at a trench
• Laramide orogeny Mountain building in the western US
• Aconcagua Highest peak of the Americas
• Andesite Intermediate volcanic rock

Exam Corner

Mains Angle

Questions compare types of convergence. These points add depth.

• Contrast Island arc versus continental arc
• Hazards Explosive volcanoes, great quakes
• Resources Copper in Chile and Peru