Orogeny & epeirogeny

Volcanoes, Moving Land and Mountain Formation

May 7, 2012, 7:07 pm
Content Cover Image

Origami orogeny. A "mountain fold" in origami where the paper is creased upward, creating a tent-like form.

Processes intrinsic to Earth and its atmosphere also alter climate. They include orogeny (the process of mountain formation), epeirogeny (the formation and distribution of land masses), volcanism, albedo (reflectance of solar energy), and atmospheric composition. Often these factors operate in conjunction with one another. For example, volcanic activity may affect both Earth’s albedo and its atmospheric composition.


Orogeny (Greek, mountain building) is the process in which tectonic movements of Earth’s crust or volcanic activities form mountains. Mountains, especially those with a north–south orientation (e.g., the Sierra Nevada, Rocky, Appalachian, Andes, and Ural mountains), disrupt global atmospheric circulation patterns that generally move east– west because of Earth’s rotation. Uplifting of mountains also newly exposes rock that undergoes chemical weathering and absorbs CO2. Moreover, higher elevations accumulate ice and snow that increases Earth’s albedo. For these reasons, times of relatively rapid mountain building—say, from 40 million years ago, when the Himalayas and Sierra Nevada first arose, until today, as these ranges continue to uplift—are usually cooler periods.


Epeirogeny (Greek, continent formation) is the formation of continents and ocean basins through tectonic deformations of Earth's crust., Global distribution of land masses determines the amplitude of glacial-interglacial cycles and, at the extreme, may foster a Snowball Earth. Mid-ocean ridges, where most of the new plate material is produced, release large amounts of energy and greenhouse gases. Sea levels rise and fall as new plate materials modify the shape of ocean basins.


In comparison to orogeny and epeirogeny, which proceed over many millions of years, volcanoes can have an explosive effect on climate. For example, when Mount Pinatubo in the Philippines erupted on June 12–15, 1991 after over four centuries of inactivity, it spewed vast amounts of sulfur dioxide and fine particles into the upper atmosphere. These materials quickly spread over most of the world and formed a haze of aerosols in the stratosphere that reflected enough sunlight to lower global temperatures by as much as 0.5°C for nearly 2 years. Volcanic eruptions as large as Pinatubo occur only every century or so, but smaller, yet significant, eruptions such as Mount St. Helens and El Chichón occur every decade. All of these eruptions release carbon dioxide (CO2), a greenhouse gas that contributes to global warming, as well as sulfur dioxide and fine particles that reflect sunlight and contribute to global cooling. The amount of CO2 released from volcanoes, however, averages about 10,000-fold less than that from the modern-day burning of fossil fuels. Therefore, on balance, volcanic eruptions cool, rather than warm, the planet.


Atmospheric aerosols. Measurements of aerosol scattering at a wavelength of 1.02 um by the SAGE (Stratospheric Aerosol and Gas Experiment) II satellite. (A) The atmosphere was relatively aerosol-free before the eruptions of Mount Pinatubo in the Philippines. (B) Aerosols in the tropics increased by almost a factor of 100 immediately following the eruption. (C) Aerosols had spread into Earth's mid-latitudes three months later. (D) Volcanic aerosols decreased slowly over several years.


Estimated aerosols from volcanic eruptions. Amount of aerosols (difference in solar radiation at the top of the stratosphere with greater stratospheric loading of aerosols) estimated from sulfur deposited in the GISP2 ice core from Greenland. These data indicate the timing and size of major volcanic eruptions, although the locations of the eruptions are only sometimes known from historical accounts. After Zielinski 2000; with additional volcano names from Kurbatov et al. 2006.


©2010 Sinauer Associates and UC Regents



Bloom, A. (2012). Volcanoes, Moving Land and Mountain Formation. Retrieved from http://www.eoearth.org/view/article/161656


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