Lecture 14: Plate Tectonics, cont’d

- Transform Margins
o When plates slide past each other
o E.g., San Andreas fault (called right lateral)
o Offsets between ridge segments in mid ocean ridge system caused by transform faults

- Hot Spots
- These are areas of upwelling where the magma upwells in plumes that originate deep in the lower mantle (ca. 670 km below the surface). These plumes stay fixed, while the lithospheric plates move above them.
- Ca. 50 to 100 hotspots, figure in book shows the locations.
- We are most familiar with the example of the HI island chain.

- Overview: production is matched by destruction
o When destruction of old ocean plate, deep trenches
- Part of sedimentary layer rides atop the ocean plate incorporated into overriding plate
o Lithospheric plates that have both ocean and continent: e.g. No. America. Made up of continent and oceanic lithosphere to east- continues to the mid ocean ridge.
- Passive continental margin
- Broad, gently seaward-dipping continental shelf
o Continental lithosphere is welded onto the oceanic beneath the continental shelf
- Active continental margin
- Ocean - continent boundary is convergent margin therefore a subduction site. Pacific coast of No-West American

What drives Plate Tectonics?
Geothermal heat flux – heat escapes to the surface mostly through mantle convection
Origin of heat in Earth’s interior:
1. Radioactive decay
2. Residual heat from Earth’s formation

- Radioactive decay: unstable isotopes that spontaneously disintegrate (decay) to a more stable form of a different element. Heat is released
- Carbon-14 – half-life is 5730 years.
- Potassium, uranium and thorium.
E.g. 40K has half-life of 1.24 billion year

- Residual heat: associated with heating events during Earth’s formation.
- Accretion of planetesimals that collided and merged forming large, primitive planet.
- Larger collisions probably caused widespread melting of Earth’s upper mantle
- One collision, with a Mars-sized object is believed to have melted the whole surface of Earth and sent bits and pieces out into orbit which then coalesced into the moon.
- Segregation of Earth into less dense mantle and crust and denser core released gravitational energy in the form of heat.
- Convection of the outer core and mantle has been transferring this heat to the surface ever since.

Convection in the mantle
At temperatures and pressures within the mantle the solid rocks are ductile – heated locally, these materials expand, become less dense and rise buoyantly, though very slowly.
Cooler denser material sinks to replace the buoyant material
In this way, the mantle convects.

Forces acting on Plates:
Don’t worry about this section – I gave more detail in lecture than you need to consider

Recycling of the Lithosphere
- Igneous rocks of the oceanic lithosphere are extruded volcanically at mid-ocean ridges and die on average some 80 million yrs later when subducted and incorporated into the asthenosphere. Oldest known oceanic rocks are about 200 my old
- Oldest known continental rocks formed nearly 4 billion yrs ago. These reside in old, now tectonically dormant regions of the continental interiors known as Cratons.
o These very old continental blocks form the nucleus on which new material over the past 3 to 4 by leading to the growth of continents
- Weathering and erosion
o Once formed, igneous rocks subject to variety of processes that can alter their chemical composition and weaken their structural integrity, typically leading to complete disintegration or dissolution (dissolving away) of original rock
- On sea floor, oceanic lithosphere altered as hydrothermal solutions circulate the ocean crust (water goes into the cracks on the ocean ridges and is heated then expelled through vents on the axis of the ridge. First changed chemically. But these processes don’t tend to destroy the rocks.
- Igneous rocks exposed on the land are subject to a variety of physical, biological and chemical forces that tend to degrade the rock, referred to as weathering processes.
- Weathering transforms solid rock into small particles and dissolved material
- Physical weathering:
o Rocks expand and fracture as weight of overlying material is removed through erosion
o In temperate latitudes (between the tropics and the high latitudes), water seeps into fractures in spring, summer and fall. Expands when it freezes during the winter, cracking the rock.
o At high altitudes and latitudes rocks ground up as glaciers advance and retreat
o Biophysical mechanisms of weathering including action of plant roots wedging rock apart
- Chemical weathering
o Minerals tend to dissolve when exposed to rainwater and acidic soil waters (generated by bacteria, fungi or plant root discharges)
o Products of chemical reactions include dissolved materials and relatively insoluble clays that form in the soils
- Transport of products of weathering is called erosion.