Mount Pleasant Caldera - Geology - Eruptive History

Eruptive History

The eruptive history of the Mount Pleasant Caldera can be divided into three stages of activity: the exocaldera sequence, the intracaldera sequence, and the late caldera-fill sequence. These can further be subdivided into strata based on their depth below the sourrounding rock.

The Intercaldera Sequence comprises formations that crop out from overlying flows in triangularly shaped area, and includes thick volcanic ash (tuff), thick breccia layers, and intermediate to felsic igneous rocks that tend to intrude the above layers and are typically located along caldera margin faults. The Exocaldera Sequence contains ash flow tuffs, mafic lavas, alluvial redbeds, and porphyritic felsic lavas that are distributed across five different layers. The late Caldera-Fill Sequence contains rocks that are similar to those of the outflows of the other, older layers, and comprises two formations and two relatively minor intrusive lava flows. The volcanic flows are generally mafic. The stratigraphic subdivision is supported by geochemical and mineralogical analyses, which indicate that the basaltic rocks are mantle-derieved and have, unusually, relatively intraplate (or hotspot) type chemical affinities. The multiple andesite flows were probably derived from basaltic magma by the crystallization of the magma material. The relatively rare, more felsic flow units, are thought to have originated from high-end crystallization inside the magma chamber. The various stages of fractionation are continually interrupted by eruptions, and have allowed scientists to recognize seven stages of caldera development. The genesis of the caldera is related to a period of lithospheric thinning that followed the Acadian Orogeny in the northern Appalachians.

The relative position of the Exocaldera and Intracaldera sequences is based on several observations:

• The upper part of the Rothea formation (Exocaldera) contains about 1% biotite. The only intracaldera rocks with this much biotite is volcanic strata within sedimentary breccia of and a tuff unit near the Scoullar Mountain formation (Intracaldera).
• Andesitic strata occurs only in two flow units: the South Oromocto Andesite of the Exocaldera sequence, and the Scoullar Mountain formation of the Intracaldera sequence.
• The exocaldera Carrow formation contains clasts from the intracaldera Seelys formation.
• The exocaldera Bailey Rock Rhyolite intrudes and overlies the Carrow formation, but is intruded by the intracaldera McDougall Brook Granite formation.