Main Body

Chapter 11

Topic 11 – The New Global Tectonics

“The plates in dynamic mosaic through history both fresh and archaic like bold engineers for some two billion years have kept Earth from becoming prosaic.” – Jack Oliver

Before the 1960s, theories on how mountains ranges like Asia’s Himalayas and North America’s Rocky Mountains had been built to such staggering heights were largely based on ideas about how the Earth’s crust must have folded and contorted during contraction as it cooled after its original formation. In the early 1960s evidence for movement at a continental scale in Earth’s outermost crust began to fall into place. Now measurements by high precision laser and global positioning systems (GPS) confirm that the lithospheric crust of the Earth is comprised of a series of interlocking plates that move, deform and dynamically interact through the process of tectonics.

Simple diagram of an oceanic-continental convergent plate boundary and the features on the surface such as oceanic trench and volcanic arc.
An oceanic-continental convergent plate boundary

Earlier in these readings we defined the solid Earth as being composed of distinct layers, the outermost layer a relatively thin, brittle material representing the lithospheric crust. Immediately below this layer heat and pressure support a more plastic, deformable layer of rock, near its melting point called the Asthenosphere. Recall that there are two general types of tectonic plates (based on mineral composition) that make up the upper surface of the lithospheric crust:

• oceanic crust (Density ~3.0 g/cm3)
• continental crust (Density ~2.7g/cm3)

See the overview of plate tectonics here:

https://www.youtube.com/watch?v=zbtAXW-2nz0
https://www.youtube.com/watch?v=kwfNGatxUJI&

In the early 1900s some scientists hypothesized that the continents had moved significantly in the geologic past. This was based on a few lines of evidence:

• Similarity in geology and apparent age of mountain ranges now on separate continents
• Apparent fit of continental margins like a jigsaw puzzle
• Similar fossil remains found on now widely separated continental locations
• Record of significantly different climate in the geologic past for some continental locations, indicating they had moved from different latitudes

Although this evidence demanded some consideration at the time, it was not conclusive on its own. One of the defining moments in driving plate tectonics forward as a viable theory was the systematic mapping of major ocean-floor ridges (for example, the mid-Atlantic ocean ridge). These ocean ridges represented locations where the oceanic crust was separating and diverging in opposite directions as the underlying tectonic plate was exposed to extensional stress, allowing magma (liquid rock) to come to the surface and form new crust. Magnetic surveys and age dating of the rocks around spreading ocean ridges confirmed that the rocks on either side of the ridges had matching magnetic fields and were similar age and therefore had likely formed in the same location, later drifting apart. It was also found that the accumulation of ocean-floor sediment increased with increased distance from ocean ridges, indicating the further away from the ridge you travelled, the older the crust became.

A mid ocean ridge showing how the magnetic minerals within surface basalt indicate polarity and point to either of the poles proving that Earth undergoes magnetic reversals.
Polarity within the rock originating from a mid-ocean ridge

If new crust is being created in one location, it indicates that it must be recycled at another. Where two tectonic plates actively meet (converge), the higher density crust sinks (subducts) beneath the less-dense crust, eventually sinking into the mantle where it can be recycled as new rock. As the crust sinks into the mantle it acts to heat and melt the continental crust above, generating surface volcanic activity. Where tectonic plates of similar density converge, they compress and deform, thrusting the crust vertically into mountain ranges (obduction), where the material is exposed to extreme weather conditions and rapidly weathered into sediment.

Transform, divergent and convergent plate boundaries on a global scale showing the processes of subduction, island arc creation, mantle plumes and volcanic arcs.
Plate boundaries and the regional landforms they create.

While we understand the relationship between tectonic plates somewhat well, the driving mechanism for plate movement is less well understood. For a long time scientists have hypothesized that large-scale convection currents move heat from Earth’s core, through the mantle towards the lithospheric crust. Upon encountering the crust, it is likely that these currents have the ability to invoke some drag on the underside of the crust; however, it appears this is not enough force to generate the apparent movement of the plates. Instead a second mechanism called ‘slab pull’ linked with the movement of the plates themselves may be more significant. As new material is erupted at a divergent boundary, it cools and becomes more dense, sinking into the mantle. This sinking acts to exert a small pull on the divergent boundary. Furthermore, as a tectonic plate subducts into the mantle, the sinking of this plate likely exerts a large pulling force on the rest of the still buoyant plate, dragging it towards the subduction zone.

When two neighbouring plates move at different rates of speed, a shear zone forms between them to accommodate the different lateral movement. This is called a transform boundary and may generate moderately large earthquakes, such as those along the San Andreas fault system in California.

Where volcanic activity occurs far from any apparent plate boundaries, it appears that an upwelling of material in the mantle at that point location may be responsible. These ‘hotspot’ locations in the mantle are responsible for volcanic activity in relatively remote locations, like the Hawaiian Islands.

A table showing the different types of boundaries, their movement, plates involved, geomorphology, seismicity and volcanism characteristics

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