China Hat Geological Site

China Hat Geological Site. . Noticeable for their distinct shapes, China Hat and nearby China Cap are rhyolite domes that intruded and pierced the basalt of the Blackfoot Lava Field. The basaltic phase of this volcanic province was active in middle Pleistocene around 500,000 years ago, whereas the rhyolitic dome-building phase is younger. The combination of these two compositional types in a single area is known as bimodal volcanism. , The dome of molten rock was formed from melted rock within the crust of the Earth. As rock melted, it became less dense than the surrounding rock and rose toward the surface of the Earth. Molten rock reached the surface, forming domes at China Hat, China Cap, and North Cone. Locally on the sides and tops of these cones were eruptions and flows of pumice, a gas-rich spongy rhyolite. , When the forces driving the basalt to the surface subsided, molten lava chambers high in the crust drained back toward the region where it originated. As these chambers emptied, they collapsed producing depressions at the land surface as a can be seen by the broken surface of the Blackfoot Lava Field around China Hat. , This area and other parts of the lava field contain sub-parallel northward trending faults in the basalt lave flows. These reflect extension in an east-west direction, perhaps related to intrusion of the domes. The faults create an intriguing landscape west of here, consisting of "grabens" (German of grave). which are elongated depressions created when blocks of crust dropped down in response to extension. Steep cliffs in the lava and linear piles of lava blocks mark the fault locations.