2013/03/21

Large boulders and pedestals on uplifted coastal terraces in Kikai Island, southwestern Japan / 喜界島,海岸段丘面上の巨礫と台座岩


Large boulders and pedestals on uplifted coastal terraces in Kikai Island, southwestern Japan / 喜界島,海岸段丘面上の巨礫と台座岩

   Kikai Island (Kikaijima) is famous for the modern coral reef studies undersea. However, the ancient coral reef, which currently composes coastal rocky landforms, is also another important topic for geomorphologists.
   Numerous large limestone boulders, coming either from ocean as tsunami deposits or from mountain-side cliffs as fallen block, are distributed upon the coral-limestone surface of uplifted coastal terraces in eastern Kikai Island. Being fixed on the uplifted surface, the boulders act as "umbrella" to save the bedrock limestone beneath them from dissolution by raindrop. The surface of the terraces broadly lowers due to the rainfall-induced dissolution, and the protected portions beneath the boulders remains as pedestal.
   The boulders in the picture are located on relatively young surface of uplifted coastal terrace, whose duration of dissolution is approximately 1,500 years. Small pedestals with a height of several tens of centimeters, as well as dissolution pits on the surrounding terrace surface, are being formed . Higher pedestals, often exceeds 1 m at 6,000 years after emergence, are observed on higher terraces inland. The pedestals therefore allow us to obtain the surface lowering rate of the uplifted coastal terraces, which means the dissolution rate of the coral-reef limestone in Kikai Island. (See also the report by Hayakawa et al. in thisissue)


 サンゴ礁石灰岩からなる喜界島の完新世海成段丘面上には,同じくサンゴ礁石灰岩からなる巨礫が多く転がっている。津波で打ち上げられたか,あるいは陸側の段丘崖から落ちてきたか――いずれにせよ,定置かつ離水してから後,巨礫は「傘」となって,直下の基盤岩に雨滴との接触の機会を失わせる。一方,雨に晒された周りの地表面は次第に溶解し,その高度を減じていく。雨宿りの結果取り残された基盤岩は,巨礫を支える台座岩(ペデスタル)として現前する。
 写真は喜界島東岸の比較的新しい離水面に載る巨礫で,台座岩の形成が始まってから約1500年とまだ時間が浅い。そのため「台座」と呼ぶには未熟かもしれないが,周辺の溶食凹地とともに,巨礫の下部には数10 cm程度の台座岩が形成されつつあるのが見える。陸側の,より高く隆起した段丘面では,古いものほど台座岩は高く,たとえば6000年前の離水面でのそれは2 m近くにも及ぶ。喜界島における地表面の低下速度,すなわち石灰岩の溶解速度は,こうした台座岩の高さとその溶解継続時間とから求めることができる。
 なお,撮影地の背後には,サンゴの石垣で有名な阿伝集落がある。また,一段高い段丘面に載る別の巨礫と台座岩がある末吉神社も近い(本号掲載論文参照)。(2009年1月撮影)


(表紙写真:地形30 (3), 2009) 

Large boulders and pedestals on uplifted coastal terraces in Kikai Island, southwestern Japan / 喜界島,海岸段丘面上の巨礫と台座岩
(百之台から撮影地近辺を望む)