登录
Basaltic-andesite from Costa Rica
PQL-836Basaltic-andesite
Dimensions: 28 cm
Field of View: 50.5 cm
Fine, mostly intact "breadcrust" bomb of basaltic andesite photographed on the southwest slope of Arenál (too heavy to bring home to the U.S.!) in April 1989. Hammer is 31 cm long for scale. Scanned from a 35mm slide.
JMT-RJUBasaltic-andesite, Augite (CaxMgyFez)(Mgy1Fez1)Si2O6
Dimensions: 58 mm x 57 mm x 33 mm
Field of View: 75.8 mm
Largest Crystal Size: 12 mm
Volcanic tuff containing fragments of basaltic andesite, crystals and fragments of augite, and several accretionary lapilli, the latter forming more or less like hailstones as particles of ash (powdered volcanic rock) cling to a nucleus such as a fragment of rock in a turbulent eruption plume. The lapillus just left of center (12 mm) is weathered away to show a cross-section, revealing its rock nucleus. Collected in 1994 on a Smithsonian Research Expedition to Arenál at a site known as "Castillo West" on the south shore of Laguna Arenál roughly 7 km southwest of the summit.
J2R-JU0Basaltic-andesite
Dimensions: 59 mm x 53 mm x 50 mm
Field of View: 70.3 mm
A volcanic bomb of basaltic andesite, broken in cross section and clearly illustrating the cause of its "breadcrust" nature. A mass of porous, viscous, gas-charged lava was ejected during an explosive eruption. Gases immediately diffused from the exterior of the mass as it flew through the air, creating a dense, non-porous outer layer that cooled and hardened into a shell. However, being no longer confined in the volcanic conduit, the gases trapped in the interior of the mass continued to expand, causing the shell to crack like the crust of baking bread. The still-plastic interior held the pieces together at least briefly, but the cracking process causes many (probably most) volcanic bombs to shatter in flight or upon landing. However, some survive relatively intact--see https://www.mindat.org/photo-1168743.html . Collected on a Smithsonian Research Expedition to Arenál in May 1989.
NJW-9Y8Basaltic-andesite
Dimensions: 16.3 cm x 9.6 cm x 8.6 cm
Field of View: 18.2 cm
Nearly intact "breadcrust bomb" of basaltic andesite. Caption of https://www.mindat.org/photo-1167763.html explains the mechanism of formation of breadcrust bombs. Collected on a Smithsonian Research Expedition to Arenál in 1989, 1991, 1992, or 1994.
P7D-R5KAnorthite Ca(Al2Si2O8) , Basaltic-andesite
Dimensions: 113 mm x 88 mm x 54 mm
Field of View: 56.0 mm
Largest Crystal Size: 20 mm
Crystal of colorless anorthite 2 cm wide in basaltic andesite porphyry. Collected on a Smithsonian Research Expedition to Arenál in 1991, 1992, or 1994. ID by Dr. William G. Melson, volcanologist-petrologist and expedition leader who had studied Arenál continuously since its first modern eruption in 1968.
4XH-2R1Anorthite Ca(Al2Si2O8) , Basaltic-andesite
Dimensions: 82 mm x 62 mm x 60 mm
Field of View: 35.3 mm
Largest Crystal Size: 16 mm
Rounded crystal of colorless to faintly pinkish anorthite in basaltic andesite porphyry. The crystal is surrounded by an approximately 0.5 mm reaction rim. Collected on my first Smithsonian Research Expedition to Arenál in April 1989. ID by Dr. William G. Melson, volcanologist-petrologist and expedition leader who had studied Arenál continuously since its first modern eruption in 1968.