Download Lecture 2.1 - Divergent plate boundaries

Geodynamics
Kinematics of plate tectonics
Lecture 2.1 - Divergent plate boundaries
Lecturer: David Whipp
[email protected]
Geodynamics
www.helsinki.fi/yliopisto
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Goal of this mini-lecture
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Present divergent (accreting) plate margins
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Plate boundaries
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Three different principal types of plate boundaries
•
•
•
Divergent (accreting)
Convergent (subduction)
Transform
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Plate boundaries
•
Three different principal types of plate boundaries
•
•
•
Divergent (accreting)
Convergent (subduction)
Transform
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Plate boundaries - Divergent (accreting)
•
Fig. 1.4, Turcotte and Schubert, 2014
Plate boundary where new plates form and with divergent motion
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Plate boundaries - Divergent (accreting)
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Fig. 1.4, Turcotte and Schubert, 2014
Plate boundary where new plates form and with divergent motion
Thermal buoyancy produces topographic highs near spreading ridges, which
decrease away from the ridge
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Plate boundaries - Divergent (accreting)
Fig. 1.4, Turcotte and Schubert, 2014
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•
Plate boundary where new plates form and with divergent motion
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Typical spreading rates: 4 cm per year
Thermal buoyancy produces topographic highs near spreading ridges, which
decrease away from the ridge
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Plate boundaries - Divergent (accreting)
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Plate accretion occurs via decompression
melting, where the nearly isothermal
asthenosphere ascends beneath a
spreading center and crosses the solidus
for the basaltic component
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Assume a simple linear solidus
T (K) = 1500 + 0.12p (in MPa)
Fig. 1.5, Turcotte and Schubert, 2014
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where 𝑇(𝐾) is the temperature in Kelvins
and 𝑝 is pressure in MPa
If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3,
𝑔 = 10 m s-2 and constant temperature, at what depth 𝑦 does the
asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦.
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Plate boundaries - Divergent (accreting)
•
Plate accretion occurs via decompression
melting, where the nearly isothermal
asthenosphere ascends beneath a
spreading center and crosses the solidus
for the basaltic component
•
Assume a simple linear solidus
T (K) = 1500 + 0.12p
Fig. 1.5, Turcotte and Schubert, 2014
•
where 𝑇(𝐾) is the temperature in Kelvins
and 𝑝 is pressure in MPa
If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3,
𝑔 = 10 m s-2 and constant temperature, at what depth 𝑦 does the
asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦.
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Plate boundaries - Divergent (accreting)
•
Plate accretion occurs via decompression
melting, where the nearly isothermal
asthenosphere ascends beneath a
spreading center and crosses the solidus
for the basaltic component
•
Assume a simple linear solidus
T (K) = 1500 + 0.12p
Fig. 1.5, Turcotte and Schubert, 2014
•
•
where 𝑇(𝐾) is the temperature in Kelvins
and 𝑝 is pressure in MPa
If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3,
If𝑔 we
assume
the constant
density oftemperature
the rising asthenosphere
𝜌 = depth
3300 kg
m-3,
the
= 10
m s-2 and
of 1600 K, at is
what
𝑦 does
𝑔 = 10 m s-2 and
constant
temperature,
at what
asthenosphere
begin
to melt?
Note: Pressure
𝑝 =depth
𝜌𝑔𝑦.
𝑦 does the
asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦.
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Plate boundaries - Divergent (accreting)
•
Plate accretion occurs via decompression
melting, where the nearly isothermal
asthenosphere ascends beneath a
spreading center and crosses the solidus
for the basaltic component
•
Assume a simple linear solidus
T (K) = 1500 + 0.12p
Fig. 1.5, Turcotte and Schubert, 2014
•
•
where 𝑇(𝐾) is the temperature in Kelvins
and 𝑝 is pressure in MPa
If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3,
If𝑔 we
assume
the constant
density oftemperature
the rising asthenosphere
𝜌 = depth
3300 kg
m-3,
the
= 10
m s-2 and
of 1600 K, at is
what
𝑦 does
𝑔 = 10 m s-2 and
constant
temperature,
at what
asthenosphere
begin
to melt?
Note: Pressure
𝑝 =depth
𝜌𝑔𝑦.
𝑦 does the
asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦.
~25 km depth
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Plate boundaries - Divergent (accreting)
FRP
FRP
Fig. 1.4, Turcotte and Schubert, 2014
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Divergent plate movement here is a form of gravitational sliding driven in part
by the ridge push force 𝐹RP owing to the high elevations of the ridge relative
to the majority of the oceanic plate
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Age of the oceanic crust
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Test your might
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You can find a short quiz about this lecture at https://elomake.helsinki.fi/lomakkeet/62991/lomake.html
•
•
Please take the quiz to help me know what you have learned
Your answers are anonymous and will not count in your
course grade
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