Download ES 104 key points about tectonics 1. The oceanic ridge system is the

ES 104 key points about tectonics
1. The oceanic ridge system is the most prominent topographic feature on Earth’s surface
a. It is a product of high heat flow lifting the lithosphere at the upwelling of a convection
cell. The rift valley in the central area is a minor feature.
b. Magma is created at oceanic ridges from the underlying mantle
i. As the plates diverge, mantle material rises
ii. As it is subjected to less pressure, it partially melts
1. Most silicic components melt first resulting in a magma more rich in
silica than the mantle it came from
2. But it is magma that is low in silica compared to other magma
a. Low silica magma makes rock that is dense: basalt
b. Low silica magma has quiet eruptions
i. Hot so it flows easily
ii. Gas can escape because there isn’t much silica to trap it
c. The dense rock created results in a layer that lays low on the underlying asthenosphere
i. The amount of water we have on Earth floods it
ii. So we have oceanic crust created at the oceanic ridge system
d. Also called spreading centers
i. Described as divergent margin of tectonic plates
ii. Crust is created at these divergent plate margins: constructive margins
2. Oceanic trenches occur at margins where oceanic plate is convergent with another plate
a. The oceanic plate is being forced/pulled under the other plate
b. There is a trench because the lithosphere is rigid and doesn’t bend easily
i. The trench is the bent-down part of the lithosphere
ii. The oceanic plate is being subducted into the mantle
iii. These are destructive margins: crust is destroyed
c. Magma is created here by adding water from subducted plate
i. Partially melts mantle material without change in temperature
ii. Rises to base of crust, that has a higher-silica content than the magma
d. This magma is basaltic, as it is from partial melting of the mantle
i. It is hot and fluid
ii. When it gets to the base of the crust, it causes some of the crust to partially
melt
1. It mixes with the crustal magma
2. This new magma is more rich in silica yet
a. High silica magma is cooler
b. High silica magma does not allow gas to escape easily
c. Result is magma that doesn’t flow easily: results in explosive
volcanism
iii. Rock formed is lower density than the oceanic crust: so it is more buoyant
e. Greater buoyancy means that in convergent margins,
i. the continental crust remains as crust, the oceanic crust is subducted
ii. this allows the continental crust to age, and thicken
1. thicken by addition of more magma at convergent margins
2. thicken by older rock gets bigger mantle root bonded to it over time of
cooling
3. thicken by crumpling of leading edge at convergent margins
iii. This leaves the oceanic crust in a constantly-recycled condition
1. It is young, so it doesn’t age or thicken by those other mechanisms
2. Because it is not thick, it doesn’t have deep ‘roots’ to sit high on
asthenosphere, leaving it lower than thicker continental crust
f. There is an arc of volcanic peaks on the overlying plate at subduction zones
i. Where the overlying plate is oceanic
1. There is an arc of islands
2. The volcanism is somewhat lower in silica than where overlying plate is
continental
ii. Where the overlying plate is continental
1. The arc is a volcanic mountain range
2. The magma is high-silica, and the volcanic peaks have violent eruptions
3. Non-magmatic plate margins
a. Convergence of two continental plates
i. Subduction ceases when oceanic plate is consumed and continent runs into
continent
ii. Buoyant continental plates do not subduct
iii. Uplift of high mountain ranges due to inertia of plate convergence
1. May be driven by upwelling convection pushing plate from other side
2. May be driven by continuing currents within asthenosphere
b. Transform boundaries
i. Conservative margins where crust is not created or consumed
ii. Site of earthquakes, but no volcanism