The Theory of Laterally Spreading Batholiths

Batholiths have been commonly pictured as enormous masses extending to great depths. Field studies, however, are showing that many of these "bottomless" batholiths rest in places on visible floors of older rocks. There is evidence of much lateral movement by the magma which has formed sheetlike, tack-shaped or tonguelike masses of much less volume than commonly supposed. Experiments were made by the junior author to determine the mechanics of intrusion without the complicating factor of lateral compression. They show that liquids forced upward through artificial strata do not progress straight upward, but follow inclined pathways, and that, if conditions permit, they spread out laterally. Tension cracks formed on the surface above an intrusion are not ordinarily penetrated by the liquid, which, if it does reach the surface, pours out along the margin of the area uplifted. Batholiths develop characteristically in the hearts of strongly folded belts during late stages of the folding. The forces involved are a combination of the mountain-building compressive stresses and the magma's own force of intrusion. Both forces independently tend to develop wedge-shaped blocks. Beneath the middle portion of the wedge the deformation extends deepest, relief of pressure is greatest, and potential magmas may become actual magmas. From the deep root zone, magmas work upward along inclined pathways following the lines of least resistance. The upfolding of the overburden, with potential cavities beneath, facilitates lateral spreading of the magma as it approaches the surface. Reasons are given why the roof is not commonly broken through.