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EXPLORE: Convection Currents and Plate Tectonics
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| Site: | Mountain Heights Academy OpenCourseWare |
| Course: | Earth Systems Q2 v2012 |
| Book: | EXPLORE: Convection Currents and Plate Tectonics |
| Printed by: | Guest user |
| Date: | Wednesday, 11 April 2018, 12:11 AM |
1 READ: Convection Currents
Earth's convection currents. Image courtesy of USGS.
Image is in the public domain.
The mobile rock beneath the rigid plates is believed to be moving in a circular manner somewhat like a pot of thick soup when heated to boiling. The heated soup rises to the surface, spreads and begins to cool, and then sinks back to the bottom of the pot where it is reheated and rises again. This cycle is repeated over and over to generate what scientists call a convection current or convection cell. While convection currents can be observed easily in a pot of boiling soup, the idea of such a process stirring up the Earth's interior is much more difficult to grasp. While we know that convective motion in the Earth is much, much slower than that of boiling soup, many unanswered questions remain: How many convection cells exist? Where and how do they originate? What is their structure?
Convection cannot take place without a source of heat. Heat within the Earth comes from two main sources: radioactive decay and residual heat. Radioactive decay involves the loss of particles from the nucleus of an isotope (the parent) to form an isotope of a new element (the daughter). The radioactive decay of naturally occurring chemical elements -- most notably uranium, thorium, and potassium -- releases energy in the form of heat, which slowly migrates toward the Earth's surface. Residual heat is gravitational energy left over from the formation of the Earth -- 4.6 billion years ago -- by the "falling together" and compression of cosmic debris under the force of gravity. How and why the escape of interior heat becomes concentrated in certain regions to form convection cells remains a mystery.
Prevailing explanations about what drives plate tectonics have always emphasized mantle convection, and most earth scientists believed that seafloor spreading was the primary mechanism. Cold, denser material convects downward and hotter, lighter material rises because of gravity; this movement of material is an essential part of convection.
In addition to the convective forces, some geologists argue that the intrusion of magma into the spreading ridge provides an additional force (called "ridge push") to propel and maintain plate movement. Additionally, subduction processes are considered to be second force that pushes the lithospheric plates around the Earth (this is known as "slab pull"). The gravity-controlled sinking of a cold, denser oceanic slab into the subduction zone (called "slab pull") -- dragging the rest of the plate along with it -- is now also considered to be a driving force of plate tectonics.
We know that forces at work deep within the Earth's interior drive plate motion, but we may never fully understand the details. At present, none of the proposed mechanisms can explain all the facets of plate movement; because these forces are buried so deeply, no mechanism can be tested directly and proven beyond reasonable doubt. The fact that the tectonic plates have moved in the past and are still moving today is beyond dispute, but the details of why and how they move will continue to challenge scientists far into the future.
Source: http://pubs.usgs.gov/gip/dynamic/unanswered.html (public domain)
2 VIEW: Convecton Currents & Plate Movement
This lecture relates the movement of convection currents in Earth's mantle to the movement of Earth's tectonic plates. The upward current in a convection cell cycle is associated with divergent plate boundaries, as heated magma rises and spreads out beneath the lithospheric plates. The downward current is associated with convergent plate boundaries and subduction, as the now cooled rocks become more dense and sink back down towards the mantle/outer core boundary. The following screencast relates convection currents to tectonic movement, and introduces the concepts of ridge push and slab pull.3 VIEW: Divergent Boundaries & Convection Currents
The following video shows how divergent plate boundaries are associated with the heating and rising part of the convection cell.4 VIEW: Convergent Boundaries & Convection Currents
The following narrated animation explains how convergent plate boundaries are related to the cooling/sinking part of the convection currents in Earth's mantle.
5 READ: Transform Boundaries & Convection Currents
San Andreas Fault, CA. Photo courtesy of Ben+Sam/Flickr.
Transform boundaries are often located near divergent and convergent plate boundaries. Transform faults are very numerous along the mid-ocean ridges, as well as at subduction zones. For example, the San Andreas fault (perhaps one of the most famous transform boundaries) was created as a result of subduction of the Juan de Fuca plate under the North American plate.
Therefore, transform boundaries and their associated plate movement can be related to both the upward and/or downward part of the convection currents in Earth's mantle. Because they are associated with both parts of the convection current cycle, they can also be related to both ridge push and slab pull.