Shallow-crustal to mid-crustal magma-emplacement models commonly rely on local or regional extension to solve the "room" problem. We argue that existing extensional models explain the emplacement of sheeted-dike complexes or internally layered plutons but are not as easily reconciled with the emplacement of elliptically shaped plutons or batholiths for the following reasons: (1) rock strengths and crustal stresses prevent large voids from existing at depth and therefore require wall-rock extension to be balanced by magma influx; (2) rates of extension are controlled by fault slip rates of millimetres to centimetres per year and thus require numerous small injections of magma; and (3) because of their sheetlike shapes and small thicknesses, these small magma injections will cool rapidly and form sheeted plutons or dike swarms. Extension may, however, facilitate emplacement of elliptically shaped plutons in concert with other mechanisms if certain kinematic constraints are satisfied.

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