1:45 PM - 3:15 PM
[O08-P100] Obsidian distribution observations from obsidian structure analysis on Kozu Island, Tokyo, Japan.
Keywords:Obsidian, Stereomicroscope, Cross-section, Speckles, Kozu Island
Abstract
From the Paleolithic to the Yayoi period, Obsidian was used in tools such as arrowheads. Depending on the region, the internal structure of obsidian varies. Using the naked eye, microscopes, and chemical analysis to observe such differences. we can determine region of production. Using such methods, the field of archaeology is making progress in determining the origins of excavated obsidian.
Kozu Island in Tokyo is a known production center of obsidian tools (Map 1). Obsidian from Onhase Island near Kozu is suitable for arrowhead production. Indeed, Onhase obsidian has been found in excavations in Kanto and Chubu, dating from the Paleolithic period to the early Yayoi period. There are few instances of non-Onhase obsidian having been excavated at these sites. Thus, Onhase obsidian may be unique compared to other obsidian. Comparing the cross-sections of obsidian from different sites, we evaluated each one to understand why people of these periods preferred Onhase Island obsidian and why there was a lack of demand for other obsidian sources.
Method
For the observation, we used 10 obsidian samples collected from obsidian production areas scattered around Kozu Island (Onhase Island, Sanukazaki, Kannonura Coast, and Sawajiri Coast respectively). First, we observed the cross-sections with the naked eye, looking for irregularities, colors, patterns, and phenocrysts. We grouped obsidian by production area, observed similarities, and compared it with other production areas. Next, we used a stereo microscope to examine the internal structure of the obsidian. Obsidian from each region was processed to varying levels of opacity. Furthermore, we observed variations in specific internal patterns, the quantity of air bubbles, the obsidian's shape, and the concentration of phenocryst minerals.
Result
Onhase Island 1
- Cross section was a uniform black glass. Thin parts of the obsidian appeared gray and translucent.
- Cross-section surface is slightly uneven, but is relatively smooth.
- A few phenocrystal minerals were observed, but the number was extremely small compared to other obsidian production centers, and most were less than 1 mm in size.
- A linear striation pattern was observed. This pattern was common to all Onhase obsidian.
- Seen under light using a stereo microscope, the internal structure clearly had a vertical line pattern, unlike what was previously observed with the naked eye.
-Air bubbles scattered inside the obsidian.
Sawajiri Coast 2
-The cross section is black with slightly transparent glassy parts scattered about. Although there is some unevenness, the surface is relatively parallel.
-More phenocrysts are visible compared to Onhase Island. These are 1mm in size.
- Various patterns were visible under the stereo microscope, with some having vertical line patterns.
- Some air bubbles inside, but there very few.
Kannonura 3
- Cross section was clearly divided into black parts and transparent parts.
- The unevenness of the cross section was more apparent compared to other samples. The number of phenocrysts was much higher.
-Some of the phenocrysts were about 2 mm in size- clearly distinguishable from the phenocrysts from Onhase Island and Sawajiri Coast.
- Under observation with stereo microscope, none of the cross-sections had a vertical line pattern, and a large amount of air bubbles could be seen.
Sanukazaki 4
- Cross section is black and transparent. The transparent glassy parts form a linear pattern with small irregularities, similar to those from Onhase Island.
- The spread and size of phenocrysts were similar to those from Sawajiri.
- Under a stereo microscope, a vertical line pattern could be observed, and the distribution of bubbles was similar to those found in the Kannonura obsidian.
Conclusions
- Obsidian from different areas typically have similar characteristics. Kozu obsidian can be roughly divided into two types:Onhase and Sawajiri type, and the Kannonura and Sanukazaki type.
- The Onhase and Sawajiri type have fewer phenocrysts and smaller irregularities, making them more susceptible to impact during processing. They are therefore considered suitable for stone tools.
- The Kannonura and Sanukazaki type are composed of a mixture of black glassy ore and transparent glassy ore, with a higher amount of phenocrysts compared to the other group. The cross section has many irregularities. During processing, this obsidian will not create smooth cross sections under impact. Since it is necessary it use a smooth cross section surface to make stone tools, the Kannonura and Sanukazaki type is therefore unsuitable.
From the Paleolithic to the Yayoi period, Obsidian was used in tools such as arrowheads. Depending on the region, the internal structure of obsidian varies. Using the naked eye, microscopes, and chemical analysis to observe such differences. we can determine region of production. Using such methods, the field of archaeology is making progress in determining the origins of excavated obsidian.
Kozu Island in Tokyo is a known production center of obsidian tools (Map 1). Obsidian from Onhase Island near Kozu is suitable for arrowhead production. Indeed, Onhase obsidian has been found in excavations in Kanto and Chubu, dating from the Paleolithic period to the early Yayoi period. There are few instances of non-Onhase obsidian having been excavated at these sites. Thus, Onhase obsidian may be unique compared to other obsidian. Comparing the cross-sections of obsidian from different sites, we evaluated each one to understand why people of these periods preferred Onhase Island obsidian and why there was a lack of demand for other obsidian sources.
Method
For the observation, we used 10 obsidian samples collected from obsidian production areas scattered around Kozu Island (Onhase Island, Sanukazaki, Kannonura Coast, and Sawajiri Coast respectively). First, we observed the cross-sections with the naked eye, looking for irregularities, colors, patterns, and phenocrysts. We grouped obsidian by production area, observed similarities, and compared it with other production areas. Next, we used a stereo microscope to examine the internal structure of the obsidian. Obsidian from each region was processed to varying levels of opacity. Furthermore, we observed variations in specific internal patterns, the quantity of air bubbles, the obsidian's shape, and the concentration of phenocryst minerals.
Result
Onhase Island 1
- Cross section was a uniform black glass. Thin parts of the obsidian appeared gray and translucent.
- Cross-section surface is slightly uneven, but is relatively smooth.
- A few phenocrystal minerals were observed, but the number was extremely small compared to other obsidian production centers, and most were less than 1 mm in size.
- A linear striation pattern was observed. This pattern was common to all Onhase obsidian.
- Seen under light using a stereo microscope, the internal structure clearly had a vertical line pattern, unlike what was previously observed with the naked eye.
-Air bubbles scattered inside the obsidian.
Sawajiri Coast 2
-The cross section is black with slightly transparent glassy parts scattered about. Although there is some unevenness, the surface is relatively parallel.
-More phenocrysts are visible compared to Onhase Island. These are 1mm in size.
- Various patterns were visible under the stereo microscope, with some having vertical line patterns.
- Some air bubbles inside, but there very few.
Kannonura 3
- Cross section was clearly divided into black parts and transparent parts.
- The unevenness of the cross section was more apparent compared to other samples. The number of phenocrysts was much higher.
-Some of the phenocrysts were about 2 mm in size- clearly distinguishable from the phenocrysts from Onhase Island and Sawajiri Coast.
- Under observation with stereo microscope, none of the cross-sections had a vertical line pattern, and a large amount of air bubbles could be seen.
Sanukazaki 4
- Cross section is black and transparent. The transparent glassy parts form a linear pattern with small irregularities, similar to those from Onhase Island.
- The spread and size of phenocrysts were similar to those from Sawajiri.
- Under a stereo microscope, a vertical line pattern could be observed, and the distribution of bubbles was similar to those found in the Kannonura obsidian.
Conclusions
- Obsidian from different areas typically have similar characteristics. Kozu obsidian can be roughly divided into two types:Onhase and Sawajiri type, and the Kannonura and Sanukazaki type.
- The Onhase and Sawajiri type have fewer phenocrysts and smaller irregularities, making them more susceptible to impact during processing. They are therefore considered suitable for stone tools.
- The Kannonura and Sanukazaki type are composed of a mixture of black glassy ore and transparent glassy ore, with a higher amount of phenocrysts compared to the other group. The cross section has many irregularities. During processing, this obsidian will not create smooth cross sections under impact. Since it is necessary it use a smooth cross section surface to make stone tools, the Kannonura and Sanukazaki type is therefore unsuitable.