The Earth -- Introduction

Class Notes - Mountain Building

  1. Aleutian-type Subduction

    When two oceanic plates converge, one will be subducted beneath the other. Partial melting and the upward migration of the resulting andesitic magma produces Island Arcs that often develop mountainous terrains as they are eroded.

  2. Andean-Type Subduction

    When an oceanic plate meets a continental plate, the oceanic plate (denser) is subducted beneath the continental plate.

    Passive Margins are the first stage in the development of Andean-type mountains. During this stage a thick wedge of shallow-water sediments accumulate adjacent to a continental mass (such as off the east coast of North America at the present time. Turbidity currents may transport sediment out into deeper water.

    Active Margins may eventually replace passive margins and the oceanic plate begins to be subducted beneath the continental plate. Sediment derived from the land and from the downgoing slab is plastered against the landward side of the trench to form an acretionary wedge. Partial melting and the production of andesitic magma results in active volcanos at the surface.

      The Sierra Nevada and the Coast Ranges are good examples of inactive Andean-type orogeny. At the time of formation, the Pacific plate was being subducted beneath the North American continent. The Sierra Nevada batholith of granitic rocks represents the core of the continental volcanic arc that formed during subduction. The chaotic rocks of the Franciscan Formation of California represent an accretionary wedge of mixed origin that accumulated in the trench.

  3. Continental Collisions occur when both plates carry continental crust. Continental crust is too bouyant to be subducted.

    1. the Himalayas

      About 45 million years ago India collided with the Eurasian plate. Watch the following animation showing the break-up of Pangea. Note that about 200 million years ago there was a large "wedge-shapped" ocean - the Tethys - at the equator on the right hand side of the image. What happens to the Tethys? Follow the assembly of the southern margin of Europe and Asia.

      Reload your page to start the animation

      After collision, the subducted oceanic plate probably decouples from the rigid continental plate and continues its descent into the asthenosphere.

      India continues to migrate north a few centimeters per year. Should a subduction zone develop south of India, the growth of the Himalayas would cease.

    2. the Urals

      Plate tectonics provided an understanding as to how a mountain range could form within a continental interior. The Urals formed when proto Europe collided with proto Asia and the two were sutured or welded together.

    3. the Applachians

      Mountain ranges now in Greenland and Scandanavia formed at the same time as the Applachians. The orogeny lasted about 300 million years (the Paleozoic). The animation above begins with the breakup of a supercontinent. Welding of the supercontinent produced the Applachians and Caledonians.

      Orogenesis and Continental Acretion

        Small crustal fragments collide and merge with continental margins -- the Assembly of California.

        These fragments may have been island arcs or microcontinents. When oceanic plates move they transport the microcontinents or island arcs. The upper portions of these thickened zones may be trhust from the descending plate as relatively thin sheets onto the adjacent continental block. This material increases the width of the continents. These accreted crustal blocks are called terrane.

      The Origin and Evolution of Continental Crust

        1. Early Evolution of the Continents - proposes that almost all continental crust formed early in the history of the Earth and has been recycled many times.

        2. Gradual Evolutiion of the Continental Crust - proposes that continental crust have formed over time with a gradual increase in continental material.

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      Copyright by John C. Butler, July 29, 1995