Plate tectonics: Plate boundaries and igneous process 3.2.2
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|(a)||the generation of maficmagma by partial melting which results from upwelling of the mantle at
divergent plate boundaries and intraplate hot spot settings
|To include: geotherm, solidus, liquidus and adiabatic conditions.
||the generation of intermediate and silicic magmas at convergent plate boundaries where crustal material is carried downward resulting in partial melting
To include magma mixing.
||the processes of intrusion which cause a body of magma to ascend through the
crust and how these affect the country rock
|To include formation of diapirs and transgressive sills.
||(i) the characteristics of major and minor intrusive bodies and the settings under which they form
(ii) the use of geodetic and geophysical data to identify the subsurface intrusion of magma
|To include: chilled and baked margins and metamorphic aureoles.
To include: harmonic tremor, 3D visualisation of seismic data, tiltmeter and GPS observations.
||(i) how changes in the properties of magma can affect buoyancy forces such that the magma can make its way to the surface producing a volcanic eruption
(ii) practical investigations to model the properties of magma and how changes to conditions can affect buoyancy forces
|To include: exsolution of volatiles, crystal content, recharge and groundwater changes.
||the diagnostic geological characteristics of dykes, sills and lava flows
||how the composition (percentage silica) and temperature of the erupting lava controls its viscosity and its ability to exsolve volatiles
||To include a qualitative understanding of the effect of OH− ions on silicate polymerisation.
||how the composition and physical characteristics of the erupted material control the volcanic landforms produced by both explosive and effusive activity
||the nature of volcanic hazards and their relation to the composition and properties of the source magma.
||To include the plotting and interpretation of isopachyte maps.|
This topic allows learners to consider igneous processes and their relationship with plate tectonics. Partial melting is revisited here and learners can consider how decompression and upwelling allow mafic magma to be generated, this setting can be compared to a convergent plate boundary where contamination by the crust may partial crustal melting with differing products. Learners could consider in detail how magma ascends through the crust and could look at many reconstructions and varied diagrams about how this could work, they may consider what evidence is behind the predictions, in comparison the evidence of intrusions on the surface can be evaluated as to what it reveals about subsurface processes. Learners may find it easier to relate eruption styles to composition and physical properties of magma by considering a wide range of case studies across different plate tectonic settings.
Common misconceptions or difficulties students may have:
There may be challenges with learners visualising the process of partial melting, modelling by a practical experiment may be helpful, once some melting has occurred in the mantle or crust the idea that only small pockets need to melt not a vast area, a gradual collection of pockets of partial melt accumulating and rising due to slight changes of density.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course:
The content in this topic is developed further in module 5.3.1 igneous petrology where learners quantify the evolution of partial melting and how the composition of a melt may change, alongside this learners consider the sequence when melts begin to crystallise and the features that form. Phase diagrams are introduced which can be used to track specific starting mineral compositions.
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