The Earth is made up of layers of rock and minerals. The surface of the Earth is called the crust. Just below the crust is the upper mantle. The upper mantle, like the crust, is relatively hard and solid. The crust and upper mantle together are called the lithosphere.
While the lithosphere doesn't flow like lava, it can change. This happens when gigantic plates of rock, called tectonic plates, move and shift. Tectonic plates can collide, separate, or slide along one another. When this occurs, the Earth's surface experiences earthquakes, volcanoes, and other major events.
Orogeny: Mountains Created by Plate Tectonics
Orogeny (or-ROJ-eny), or orogenesis, is the building of continental mountains by plate-tectonic processes that squeeze the lithosphere. It may also refer to a specific episode of orogeny during the geologic past. Even though tall mountain peaks from ancient orogenies may erode away, the exposed roots of those ancient mountains show the same orogenic structures that are detected beneath modern mountain ranges.
Plate Tectonics and Orogeny
In classical plate tectonics, plates interact in exactly three different ways: they push together (converge), pull apart, or slide past each other. Orogeny is limited to convergent plate interactions-in other words, orogeny occurs when tectonic plates collide. The long regions of deformed rocks created by orogenies are called orogenic belts, or orogens.
In actuality, plate tectonics is not at all that simple. Large areas of the continents can deform in blends of convergent and transform motion, or in diffused ways that do not give distinct borders between plates. Orogens can be bent and altered by later events, or severed by plate breakups. The discovery and analysis of orogens is an important part of historical geology and a way to explore plate-tectonic interactions of the past that do not occur today.
Orogenic belts can form from the collision of an oceanic and continental plate or the collision of two continental plates. There are quite a few ongoing orogenies and several ancient ones that have left long-lasting impressions on the Earth's surface.
- The Mediterranean Ridge is the result of the African plate subducting (sliding) underneath the Eurasian plate and other smaller microplates. If it continues, it will eventually form extremely high mountains in the Mediterranean.
- The Andean Orogeny has been occurring for the past 200 million years, although the Andes have only arisen in the past 65 million years. The orogeny is the result of the Nazca plate subducting underneath the South American plate.
- The Himalayan Orogeny started as the Indian subcontinent began moving towards the Asian plate 71 million years ago. The collision between the plates, which is still ongoing, has created the largest landform of the past 500 million years-the combined Tibetan Plateau and Himalayan Mountain range. These landforms, along with the Sierra Nevada range of North America, may have induced a global cooling around 40 million years ago. As more rock is lifted to the surface, more carbon dioxide is sequestered from the atmosphere to chemically weather it, thus decreasing Earth's natural greenhouse effect.
Major Ancient Orogenies
- The Alleghanian Orogeny (325 million years ago) was the most recent of several major orogenies to help form the Appalachian Mountains. It was the result of a collision between ancestral North America and Africa and resulted in the supercontinent of Pangea.
- The Alpine Orogeny began in the Late Cenozoic and created mountain chains on the African, Eurasian and Arabian plates. Although the orogeny ceased in Europe within the past few million years, the Alps continue to grow.