Mountains and Volcanoes

Mountains Mountains are over 300 m in height and have sloping sides. Orogeny is the process of mountain building Takes tens of millions of years; usually produces long linear structures, known as orogenic belts Two main processes that form mountains: 1) Deformation: continental collisions resulting in folding and faulting. 2) Volcanic Activity: opening in crust which allows magma to escape from below

Types of Mountains Types are based according to their origin Fault-block: tension, normal faulting Folded: compression, reverse faulting Dome: magma pushing up on Earth’s crust Volcanic: Shield and stratovolcano Complex: mixture of most of the above

Fault-Block Mountains Form at faults (plates slipping by). Edges of plates catch and push, which generates pressure. Pressure can cause earthquakes, or push parts of plate upward to form mountains.

Tilted fault-block range: Sierra Nevada from east, Steep side of block fault; Ansel Adams photo

Horst and Graben Alternating normal faults lead to a characteristic pattern called a “horst and graben” system. An area under tension will often have multiple mountain ranges as a result.

Folded Mountains Form at convergent boundaries (continental-continental). One plate plunges into the mantle, while the other folds under pressure. ex) Rocky Mountains and Himalayas. http://www.geography.info/images/coco.gif

Rocky Mountains, BC. North American plate collides with Juan de Fuca plate The Himalayas, Asia. Eurasian plate collides With Indian-Australian plate

Dome Mountains Form when magma from mantle rises and interacts with parts of the crust that won’t crack. Magma pushes section of crust up to form a dome. ex) Mount Royal, Quebec http://www.montrealbb.ca/img/mont_royal.jpg

Volcanic Mountains Three types: a) Shield volcanoes b) Stratovolcanoes c) Cinder cones

Shield Volcanoes Found anywhere in a plate, not just edges. Form above hot spots in the mantle. Magma collects in large pools and eventually melts the rock above it and pours out through a hole in the crust. Mauna Loa, Hawaii

Shield Volcanoes Magma that flows out is called lava. Lava is runny and flows like rivers. Hardens to form basalt rock. Hardens more quickly if occurs in ocean and forms cones. Shield volcanoes do not explode.

Mauna Loa in Background Kilaeua is Behind Mauna Loa Mauna Kea

Stratovolcanoes Volcanoes that explode and blow ash and rock everywhere! Forms where two plates collide, one plate slides under the other (subduction). The descending plate heats up and melts, magma rises and escapes through a hole in the top plate.

Stratovolcanoes Magma is thick and sticky. Water from descending plate heats up and forms steam. Steam increases pressure in volcano, which causes it to explode. ex) Mount St. Helens, Washington Mount St Helens, 1980

Cinder Cones Built from lava fragments called cinders. The lava fragments are ejected from a single vent and accumulate around the vent when they fall back to earth.

Status of Volcanoes Active – currently erupting or has erupted within the last 200 years Dormant – has not erupted recently (within the past few thousand years) but is considered likely to do so in the future Extinct – has not erupted for a very long time (tens of thousands of years) and is considered unlikely to do. Truly extinct volcanoes are no longer fueled by a magma source.

Complex Mountains Continental-continental collision Tend to have a little of everything: volcanoes, folds, and faults

Orogenic Belt Long tracts of highly deformed rock Parallel strips of rock exhibiting similar characteristics along the length of the belt

ANATOMY OF AN OROGENIC BELT

Anatomy of an Orgogenic Belt Oceanic Plate: plate containing the ocean floor Accretionary Prism: sediment collected at a subduction zone Igneous Arc: collection of igenous rock where lava is cooling Foreland: land that develops next to a mountain/volcano Craton: stable part of a tectonic plate found near the middle of the plate