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```Geodynamics
Kinematics of plate tectonics
Lecture 2.1 - Divergent plate boundaries
Lecturer: David Whipp
[email protected]
Geodynamics
www.helsinki.fi/yliopisto
1
Goal of this mini-lecture
•
Present divergent (accreting) plate margins
2
Plate boundaries
•
Three different principal types of plate boundaries
•
•
•
Divergent (accreting)
Convergent (subduction)
Transform
3
Plate boundaries
•
Three different principal types of plate boundaries
•
•
•
Divergent (accreting)
Convergent (subduction)
Transform
4
Plate boundaries - Divergent (accreting)
•
Fig. 1.4, Turcotte and Schubert, 2014
Plate boundary where new plates form and with divergent motion
5
Plate boundaries - Divergent (accreting)
•
•
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
5
Plate boundaries - Divergent (accreting)
Fig. 1.4, Turcotte and Schubert, 2014
•
•
Plate boundary where new plates form and with divergent motion
•
Typical spreading rates: 4 cm per year
Thermal buoyancy produces topographic highs near spreading ridges, which
decrease away from the ridge
5
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 (in MPa)
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.
6
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.
7
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.
8
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
9
Plate boundaries - Divergent (accreting)
FRP
FRP
Fig. 1.4, Turcotte and Schubert, 2014
•
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
10
Age of the oceanic crust
11