Marie D. Jackson

The laccolith-stock controversy: New results from the southern Henry Mountains, Utah

Domes of sedimentary strata at Mount Holmes, Mount Ellsworth, and Mount Hillers in the southern Henry Mountains record successive stages in the growth of shallow (3 to 4 km deep) magma chambers. Whether the intrusions under these domes are laccoliths or stocks has been the subject of controversy. According to G. K. Gilbert, the central intrusions are direct analogues of much smaller, floored intrusions, exposed on the flanks of the domes, that grew from sills by lifting and bending of a largely concordant overburden. According to C. B. Hunt, the central intrusions are cylindrical stocks, sheathed with a zone of shattered sedimentary rocks, and the small flanking sills and laccoliths grew laterally as tongue-shaped masses from the discordant sides of these stocks. New geologic mapping demonstrates that the sedimentary overburden, now partially eroded from the domes, was uplifted about 1.2 km at Mount Holmes, 1.8 km at Mount Ellsworth, and at least 2.5 km at Mount Hillers. The radii of the domes are similar, between 5 and 7 km. The strata over the domes have a doubly hinged shape, consisting of a concave-upward lower hinge and a concave-downward upper hinge. A limb of approximately constant dip joins these two hinges and dips 20° at Mount Holmes, 50° to 55° at Mount Ellsworth, and 75° to 85° at Mount Hillers. The distal portion of each dome is composed of a gently dipping peripheral limb 3 to 4 km long, presumably underlain by sills and minor laccoliths. Although geologic cross sections and regional aeromagnetic data for the three domes are consistent with floored, laccolithic central intrusions, these data alone do not rule out the possibility of a stock at depth. At Mount Hillers, paleomagnetic vectors indicate that tongue-shaped sills and thin laccoliths overlying the central intrusion were emplaced horizontally and were rotated during doming through about 80° of dip. This sequence of events is not consistent with the emplacement of a stock and subsequent or contemporaneous lateral growth of sills and minor laccoliths. Growth in diameter of a stock from about 300 m at Mount Holmes to nearly 3 km at Mount Hillers, as Hunt suggested, should have been accompanied by considerable radial shortening of the sedimentary strata and a style of folding which is not observed. Geologic and geophysical data and mechanical models support a laccolithic origin for the central magma chambers underlying the domes.

Unlocking the secrets of Al-tobermorite in Roman seawater concrete

Abstract Ancient Roman syntheses of Al-tobermorite in a 2000-year-old concrete block submerged in the Bay of Pozzuoli (Baianus Sinus), near Naples, have unique aluminum-rich and silica-poor compositions relative to hydrothermal geological occurrences. In relict lime clasts, the crystals have calcium contents that are similar to ideal tobermorite, 33 to 35 wt%, but the low-silica contents, 39 to 40 wt%, reflect Al3+ substitution for Si4+ in Q2(1Al), Q3(1Al), and Q3(2 Al) tetrahedral chain and branching sites. The Al-tobermorite has a double silicate chain structure with long chain lengths in the b [020] crystallographic direction, and wide interlayer spacing, 11.49 Å. Na+ and K+ partially balance Al3+ substitution for Si4+. Poorly crystalline calcium-aluminum-silicate-hydrate (C-A-S-H) cementitious binder in the dissolved perimeter of relict lime clasts has Ca/(Si+Al) = 0.79, nearly identical to the Al-tobermorite, but nanoscale heterogeneities with aluminum in both tetrahedral and octahedral coordination. The concrete is about 45 vol% glassy zeolitic tuff and 55 vol% hydrated lime-volcanic ash mortar; lime formed <10 wt% of the mix. Trace element studies confirm that the pyroclastic rock comes from Flegrean Fields volcanic district, as described in ancient Roman texts. An adiabatic thermal model of the 10 m2 by 5.7 m thick Baianus Sinus breakwater from heat evolved through hydration of lime and formation of C-A-S-H suggests maximum temperatures of 85 to 97 °C. Cooling to seawater temperatures occurred in two years. These elevated temperatures and the mineralizing effects of seawater and alkali- and alumina-rich volcanic ash appear to be critical to Al-tobermorite crystallization. The long-term stability of the Al-tobermorite provides a valuable context to improve future syntheses in innovative concretes with advanced properties using volcanic pozzolans.

Peridotite and Gabbroic Structures in the Monte Maggiore Massif, Alpine Corsica

Penetratively deformed peridotites from the Monte Maggiore spinel/plagioclase lherzolite massif in Alpine Corsica are intruded by gabbroic dikes ranging in mineralogy from troctolites to Fe and Ti gabbros. The dikes cross-cut diffuse plagioclase segregations and veinlets which may represent trapped mafic melt within the residual peridotite. Elliptical shaped dunite pods with refractory compositions are often bounded by lace-like networks of plagioclase (

Cement Microstructures and Durability in Ancient Roman Seawater Concretes

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New Proposed Drilling at Surtsey Volcano, Iceland

) which invade neighboring undepleted lherzolite. Lherzolite mineral compositions range from olivine

Extension and contraction of faulted marker planes

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