Geology topics

E.E. Larson

Publications and source records attributed to E.E. Larson.

5 recordsLinked to original sources

USGS2012

The Steens Mountain (Oregon) geomagnetic polarity transition: 3. Its regional significance

Study of the variations of direction and intensity of the geomagnetic field as recorded by the Miocene lava flows on Steens Mountain, southeastern Oregon, has resulted in a detailed description of total field behavior during a reversal in polarity. In addition to information about the polarity reversal itself, the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15.5 Ma. In order to test the feasibility of using this record as a means of correlation in this part of the western United States, comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the Steens Basalt or rocks of equivalent age. Despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes, convincing similarities among some of the records are evident. The Steens Basalt paleomagnetic record does, indeed, have potential as a correlation tool during this time of widespread basaltic volcanism. Additionally, paleomagnetic data from flows that were sampled in detail yield a middle Miocene paleomagnetic pole at 88.3°N, 209.0° (α 95 = 6.3°) for the High Lava Plains of Oregon. This pole position is statistically indistinguishable from the earth's rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted. Data from selected sites within the coeval part of the Columbia River Basalt Group yield a paleomagnetic pole at 88.7°N, 171.6°E (α 95 = 4.0°). The Columbia River Basalt Group pole is statistically indistinguishable from either the rotational axis or from the High Lava Plains pole. These findings indicate no post‐20 Ma differential rotation between south‐eastern Washington and south‐central Oregon, in contrast to previous interpretations.

Journal of Geophysical Research B: Solid Earth↗

USGS1992

Pliocene and Pleistocene geologic and climatic evolution in the San Luis Valley of south-central Colorado

Sediments of the Alamosa Formation spanning the upper part of the Gauss and most of the Matuyama Chrons were recovered by coring in the high (2300 m) San Luis Valley of south-central Colorado. The study site is located at the northern end of the Rio Grande rift. Lithologic changes in the core sediments provide evidence of events leading to integration of the San Luis drainage basin into the Rio Grande. The section, which includes the Huckleberry Ridge Ash (2.02 Ma) and spans the entire Matuyama Chron, contains pollen, and invertebrate and vertebrate fossils. Stable isotope analyses of inorganic and biogenic carbonate taken over most of the core indicate substantially warmer temperatures than occur today in the San Luis Valley. At the end of the Olduvai Subchron, summer precipitation decreased, summer pan evaporation increased, and temperatures increased slightly compared to the earlier climate represented in the core. By the end of the Jaramillo Subchron, however, cold/wet and warm/dry cycles become evident and continue into the cold/wet regime associated with the deep-sea oxygen-isotope Stage 22 glaciation previously determined from outcrops at the same locality. Correspondence between the Hansen Bluff climatic record and the deep-sea oxygen-isotope record (oxygen-isotope stages from about 110-18) is apparent, indicating that climate at Hansen Bluff was responding to global climatic changes.

Colorado↗

USGS1976

Thermomagnetic analysis of meteorites, 3. C3 and C4 chondrites

Thermomagnetic analysis was made on samples of all known C3 and C4 chondrites in a controlled oxygen atmosphere. Considerable variation was noted in the occurrence of magnetic minerals, comparable to the variation observed earlier in the C2 chondrites. Magnetite was found as the only major magnetic phase in samples of only three C3 chondrites (2–4 wt.%) and the Karoonda C4 chondrite (7.7 wt.%). The magnetite content of these three C3 chondrites is only about one-third that observed in the C1 and C2 chondrites which were found to contain magnetite as the only magnetic phase. Five C3 chondrites were observed to undergo chemical change during heating, producing magnetite: this behavior is characteristic of troilite oxidation. Upper limits on initial magnetite content of about 1–9% were established for these meteorites. Samples of the remaining five C3 chondrites and the Coolidge C4 chondrite were found to contain both magnetite and metallic iron. In two samples, iron containing ≤2% Ni was observed, while in the other four, the iron contained 6–8 wt.% Ni. In addition to containing both magnetite and iron metal, three of these samples reacted during heating to form additional magnetite. Variations in the magnetic mineralogy and, hence by inference bulk mineralogy, of C3 and C4 chondrites indicate a more complex genesis than is evident from whole-rock elemental abundance patterns.

Earth and Planetary Science Letters↗

USGS1975

Thermomagnetic analysis of meteorites, 2. C2 chondrites

Samples of all eighteen of the known C2 chondrites have been analyzed thermomagnetically. For eleven of these, initial Fe 3 O 4 content is low (generally <1%) and the J s -T curves are irreversible. The heating curves show variable greater (up to 10 times) than it is initially. This behavior is attributed to the production of magnetite from a thermally unstable phase — apparently FeS. Four of the remaining seven C2 chondrites contain Fe 3 O 4 as the only significant magnetic phase: initial magnetite contents range from 4 to 13%. The remaining three C2 chondrites contain iron or nickel-iron in addition to Fe 3 O 4 . These seven C2 chondrites show little evidence of the breakdown of a thermally unstable phase.

Earth and Planetary Science Letters↗

USGS1975

Origin of magnetite and pyrrhotite in carbonaceous chondrites

CARBONACEOUS chondrites, although comprising only about 2% of known meteorites, are extremely interesting for scientific investigation. Their mineral constitution, and the correspondence between their bulk chemical composition and the solar abundance of condensable elements, indicate that minimum chemical fractionation and thermal alteration have occurred. The mineral phases observed in these primitive chondrites are sufficiently unique, with respect to other meteorite classes, to have elicited considerable speculation about the physical environment in which they formed 1–7 .

Nature↗