Magmatic flow structures, such as this ladder dike in Cathedral Peak Granodiorite (~86 – 88 Ma), develop during convective flow in partially crystallized bodies of magma (plutons). This dike appears on slabs near the Tuolumne River in Yosemite. The concentric stacking of layers forms as a roughly cylindrical flow conduit changes its axial position over time (with respect to the surrounding melt/crystal mush). In the case of this photo, the flow conduit has moved away from the observer, meaning that the concentric layers in the foreground are older than those in the background. The orientation of the axis of the conduit is down and away from the observer. Layers are composed of alternating bands of light- and dark-colored minerals that are segregated by mechanical flow-sorting and gravitational settling of the heavier, dark minerals. Large orthoclase (Potassium feldspar) phenocrysts and megacrysts (i.e. Tuolumne knobs) were entrained in the conduit and can be seen within the concentric layering structure. It is thought that these phenocrysts began to crystallize much earlier than the finer-grained minerals and therefore had more time to grow to such large sizes. Note red pen for scale. Cathedral Peak Granodiorite is a member of the Tuolumne Intrusive Suite. In reference to the map, Tuolumne Meadows lies at the northern exposed portion of Johnson Granite Porphyry (~84 Ma). Click here to see a description of specific rock types. Ma = million years old; a crystallization age.
|Photo Location:||North_America: United_States: California: Yosemite_National_Park: _Park_Overview_|