GLOSSARY – B

GLOSSARY – B

by: Charles O’Dale

BALDWIN’S CURVE

Baldwin had statistically developed two equations giving relationships for the rim diameter, rim height, and total depth of meteorite craters on the Earth and craters on the Moon.

Baldwin’s first equation was
D = 0.1083d2 + 0.6917d + 0.75
where D is the logarithm of the diameter of the crater measured in feet, and d is the logarithm of the depth, also measured in feet.

His second equation was
E = –0.097D2 + 1.542D – 1.841
where E is the logarithm of the average height of the rim in feet and D is again the log diameter in feet.

The Baldwin curve, showing the relationship between the diameter and depth of terrestrial explosion craters and
craters on the Moon. Baldwin’s curve illustrates how well the Chubb (now Pingualuit), Holleford, Brent and Deep Bay meteorite craters all fit the curve (after C. S. Beals, M. J. S. Innes, and J. A. Rottenberg, The search for fossil meteorite craters–I, Current Science 29: 206).

from:  V. BEN MEEN AND THE RIDDLE OF CHUBB CRATER
HOWARD PLOTKIN and KIMBERLY T. TAIT

When solved for Chubb Crater (now Pingualuit) , the equation gives a rim height of 640 feet (195 m). Baldwin noted that this was higher than the average of 400 feet (122 m) that Meen had measured, but explained that erosion possibly decreased the original rim height, and that his formula gives slightly higher values for rim heights of craters between 100–20,000 feet (30–6,096 m) in diameter. The physical characteristics of the crater’s diameter and erosion-reduced rim thus placed it neatly with the other terrestrial meteorite craters on Baldwin’s curve.

There is no appreciable rim at Merewether, hence it is possible to use only one of Baldwin’s equations (diameter/depth). Taking the average diameter as 650 feet (198.12 metre) and the depth as 155 feet (47.244 metre) we obtain: D(2.81)=2.79.This is excellent agreement and would be even better if the value of “d” had not been reduced by the presence of sediment in the bottom. If glaciated, then the reduction of rim height also has reduced the value of “d”. The agreement with Baldwin’s formula, therefore, would indicate that this crater is of explosion origin.

BALLEN STRUCTURE
Microscopic shock-deformation feature in quartz. Oval quartz with rims of tiny vugs filled with amorphous material.

BALLISTIC EROSION AND SEDIMENTATION
(Oberbeck 1975) Emplacement of ballistically transported impact ejecta, and ejecta-surface interaction.

BASALT
A dark coloured igneous rock. commonly extrusive, composed primarily of calcic plagioclase and pyroxene; the fine-grained equivalent of gabbro.

BEDROCK
The solid rock that underlies gravel, soil, or other surficial material.

BIOTITE
See mica.

BOLIDE
Exploding fireball.
[see – METEORITE]

BRECCIA – from Italian indicating both loose gravel and stone made by cemented gravel
A clastic sedimentary rock composed of angular clasts in a consolidated matrix. Breccias can be produced in several geologic processes: tectonic breccia, volcanic breccia (eruption breccia, vent breccia), sedimentary breccia (e.g., rock fall breccia), collapse breccia (e.g., in karst areas). Breccias may be distinguished depending on the origin of the clasts, monomictic (monogenetic, monolithologic) and polymictic (polygenetic, polylithologic).

[see – BRECCIA]
[see – SHOCK METAMORPHISM]

BRECCIA DYKE
Dyke formed in the (par)autochthonous basement or in displaced megablocks of impact craters consisting of impact breccia (polymict breccias such as impact melt rock, suevite, lithic breccia or more rarely monomict breccia).

BUNTE BRECCIA
(Bunte Breccie or Bunte Brekzie or Bunte Bresche, Loc. Ries impact crater, Germany) Local term for polymict lithic breccia forming the continuous ejecta blanket at the Ries impact crater, Germany; first detailed description as “Kalkbreccie” by C. W. von Gümbel in 1870.