The Toba depression in north central Sumatra is a complex of several overlapping calderas resulting from three major ignimbrite‐forming eruptions. Within the depression, the upland masses of Samosir and northern Uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment, hitherto interpreted to be two parts of a single resurgent dome. This study has demonstrated that the welded tuffs of Samosir and Uluan have different magnetic polarities and therefore at least two different ignimbrites are present; the Samosir/Uluan massif may consist of parts of two resurgent domes. The first ignimbrite eruption occurred at 0.84 Ma and produced a very thick (>400 m), densely welded unit having a reversed polarity. Anisotropy of magnetic susceptibility (AMS) flow direction and lithic size data indicate that the source lies in the southern part of the Toba depression, and the thick deposit of Uluan is thought to have ponded in a 40‐km‐wide caldera. The second ignimbrite is normally magnetized. AMS flow direction data indicate two separate source vents, one to the north in the Haranggaol caldera, and another to the south. The thick deposit at Samosir is thought to have ponded in the southern caldera. Coarse sediments then accumulated over Samosir and northern Uluan and were capped by lacustrine deposits. A renewed episode of resurgence then uplifted Samosir Island and possibly the northern part of Uluan. At approximately 0.075 Ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the Toba depression. This ignimbrite is mostly nonwelded and normally magnetized. Part of the Uluan dome was destroyed by collapse of the Sibandung caldera and Latung graben and concomitant with renewed subsidence of the Haranggaol and Porsea calderas.

We present a systematic study of the absorption, heating behavior, and microstructure evolution of porous copper powder metal compacts subjected to 2.45 GHz microwave radiation and explain our observations using known physical mechanisms. Using a single-mode microwave system, we place the compacts in pure electric (E) or magnetic (H) fields and compare the heating trends. We also investigate the effect of particle size on the same. The observed trends and the differences between E- and H-field heating are reflected in the dramatic changes in the conductivity, permittivity, and permeability of the samples. These property changes are effected by the microstructure evolution during heating in the two types of fields. We also find that the observed dependence of the initial microwave heating on particle size is suggestive of single-particle behavior.

Paleomagnetic results have been obtained from the Late Cretaceous‐early Tertiary igneous complexes of the north‐central Montana alkalic province. Data from 94 sites in Eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82.0°N, 170.2°E (A95 = 3.5°; k = 18.6), while 36 sites in Paleocene intrusions yield a paleomagnetic pole at 81.8°N, 181.4°E (A95 = 5.4°; k = 20.2). These poles differ by only 1.6° and are not significantly different statistically. The 130‐site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander (APW) during the period of magnetization of the igneous centers. Postmagnetization structural complications in these rocks are minimal. The presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration. The northcentral Montana data together with other early Tertiary, Cretaceous, and mid‐Tertiary paleomagnetic results require modification of our earlier APW chronology [Diehl et al., 1980]. It now appears that APW relative to North America since the Early Cretaceous consists of a polar still‐stand during much of the Cretaceous (120–75 m.y. B.P.), a period of rapid movement in latest Cretaceous time (75–65 m.y. B.P.), and a period of slow polar movement thereafter. The onset of this period of rapid APW correlates well with a major change in plate motions at ∼80 m.y. and the beginning of the Laramide Orogeny.

Floodplain sediments deposited along the lower course of the Morava River (eastern Czech Republic), were studied in the Strážnické Pomoraví region to describe the alluvial history of the river over the last millennium. The sediments exposed in up to 5 m high erosional river banks were analysed using mineral magnetic, geochemical and chemical approaches. The age model of the sedimentary sequences was constructed from radiocarbon dates in association with 206Pb/207Pb and POP (DDT, PCB) analysis and 137Cs activity data. The Cu-trien method was used for stratigraphically correlating these deposits based on the variation of expandable clay minerals in the sediments. The resulting stratigraphic pattern reveals the alluvial history of the currently active river channel system since the end of the first millennium AD. Fine overbank clayey sediments deposited during the `Mediaeval Warm Period' were eroded from cultivated fields newly formed during Mediaeval colonization between 1250 and 1450. These fine deposits are overlain by coarser floodplain sediments of the `Little Ice Age', indicating a change in the sediment source since the sixteenth century AD, and a substantial increase in the sediment load in the second half of twentieth century. The Strážnické Pomoraví floodplain deposits represent a valuable palaeoenvironmental archive of the last millennium, containing records of fluvial processes considerably altered by human activities.