2006 Fiscal Year Final Research Report Summary
Effects of climatic environment on rock weathering
| Project/Area Number |
16300292
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| Research Category |
Grant-in-Aid for Scientific Research (B)
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| Allocation Type | Single-year Grants |
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| Section | 一般 |
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| Research Field |
Geography
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| Research Institution | University of Tsukuba |
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Principal Investigator |
MATSUKURA Yukinori University of Tsukuba, Graduate School of Life and Environmental Sciences, Professor, 大学院生命環境科学研究科, 教授 (80107341)
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| Co-Investigator(Kenkyū-buntansha) |
MATSUOKA Norikazu University of Tsukuba, Graduate School of Life and Environmental Sciences, Professor, 大学院生命環境科学研究科, 教授 (10209512)
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| Project Period (FY) |
2004 – 2006
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| Keywords | weathering / salt weathering / rate of weathering / field experiment / weathering environment |
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| Research Abstract |
(1) To study the effect of rock type and geomorphic setting on weathering rates, field experiments were performed using the microweight loss technique. Tablets of eight types of rock : granite, granodiorite, gabbro, limestone, andesite, rhyolite, crystalline schist and tuff with a diameter of 3.5 cm and a thickness of 1 cm, were enclosed in nylon mesh bags and placed in four distinct locations on a granodiorite hillslope in central Japan for 10 years. The main findings are that (1) limestone has distinctive weathering characteristics compared to other rocks ; (2) limestone and granodiorite in a saturated environment have high susceptibility to chemical weathering ; and (3) physical weathering is dominant on the ground and in unsaturated soil layers.
(2) Field weathering experiments using granodiorite tablets were conducted at a catchment over 10 yr. The tablets were exposed at three positions having different weathering conditions. The weight of the tablets linearly decreased with time
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and the rates were 0.022 wt% yr^<-1> for the ground surface, 0.013 wt% yr^<-1> for above the aquifer and 0.42 wt% yr^<-1> for in the aquifer. Only minor change of the tablet surface was observed after 10 yr of weathering at the ground surface and above the aquifer. In contrast, the tablets in the aquifer were remarkably weathered, and weathering of biotite was particularly noticeable while those of plagioclase, K-feldspar, quartz, and hornblende were minor/moderate. The weathering of granodiorite is inferred to proceed by initial dissolution of mineral grain boundary (chemical process) and subsequent detachment of the mineral grain (physical process). To evaluate the amount of weathering caused only by the chemical process (dissolution), a laboratory dissolution experiment was conducted on the granodiorite tablet using a flow through reactor at 20 ℃ and pH 6-7. The obtained rates, 0.0078-0.010 wt% yr^<-1>, were 〜50 times slower than the field rate in the aquifer. This result shows that the contribution of the physical process, in granodiorite weathering in the aquifer, is very large compared to the chemical process, and relative intensities of the two processes during the 10 yr weathering are estimated to be chemical : physical = 1 : 〜50.
(3) In Kikai-jima, southwestern Japan, many pedestal rocks develop on the surface of Holocene raised coral-reef terraces with known dates of emergence. Pedestals are formed just under boulders which are considered to have been transported by tsunami and settled on a reef flat before emergence. On the assumption that boulders protect underlying limestone terrace from rainfall solution, the rate of surface lowering of these limestone terraces was evaluated from the height of pedestals and the period of their formation. The result showed that the mean lowering rate during 6000 years is 205 mm/ky. Less
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