2 アラビア楯状地と厚い堆積層の形成

陸地が侵食によって削られ、すり減らされて出来た瓦礫は、雨水で流されて堆積盆を形成しました。堆積した瓦礫が時間と共に一体となり、幾種類かの堆積岩を作り、深い層のものは上部の地層の圧力で潰され、変成されます。火山活動が次第に収まるに連れて、地中の深い場所で冷えて固まった深成岩は、花崗岩、閃緑岩､その他色々の岩石の地域となって地上に露出してきました。最終的には比較的新しい堆積岩を突き抜けて古い火成岩の露頭がある概ね平らな景観を作り出し、小さなプレートを付着しながら一つのプレートを形成する過程は、約6億4千年前で終了しました。

The land was scraped by erosion and further worn down to debris. The debris were washed away with rainwater and deposited to form the basins. The sediments coalesced over time and formed several types of the sedimentary rocks. The deep layers of them are crushed and metamorphosed by the pressure of the upper strata. As the volcanic activity gradually subsided, the intrusive rock that cooled and solidified in the deep underground and became granite, diorite and many other rocks. And some of them have been exposed to the ground. Eventually it created a generally flat landscaped including outcrops of the old igneous rocks that broke through the relatively new sedimentary rocks. The process of forming the single tectonic plate while adhering small tectonic plates was completed about 640 thousand years ago.

その後は非常に大きなマグマがその地表の下に入り込み、ゆっくり冷やされて花崗岩の塊になるに連れて引っ張りと圧縮の力がたくさんの割れ目や断層を作りました。多量の火山性熔岩が地表に流れ出し、堆積岩と混じりあい、種々雑多に異なる岩石が雑然と混入したメランジュと呼ばれる地質体を作りながら5億5千年前までにアラビア楯状地の形成は終わりました。

After that, very large magmas penetrated beneath its surface. As they slowly cooled into a granite mass, the pulling and compressing forces occurred and they created many crevices and faults. By 550 thousand years ago, the formation of the Arabian Shield had been completed. In its process, a large amount of the volcanic lava had been flowing to the surface and been mixing with the sedimentary rocks. In some places, the terranes called mélange were created with cluttered the various different rocks.

その頃から地球上の全ての海面が切れ目無く上昇し続け、アラビア楯状地は新しく多様な生物の営みで活気付いていた海中にゆっくりと沈み、タブーク周辺の西北部とイエメン南西部の何ヶ所かの陸地を残すだけになるまで海水がアラビア楯状地の奥深くまで侵入して来ました。4億7千年前までにその浅い海は、数百メートルもの厚い層の砂と泥の混ざった堆積物を残して後退して行きました。

From that time on, the sea on the Earth continued to rise without any interruptions. The Arabian Shield slowly sank into the sea, which was booming with the new and diverse creatures, and the seawater penetrated deep into the Arabian Shield. Only a few places around Tabuk in the northwestern shield as well as a few places in the southwestern Yemen remained as land. By 470 thousand years ago, the shallow sea had receded, leaving behind hundreds of meters of the thick sand and/or mud deposits.

アラビア楯状地が陸地として海面上に現れて間もなく、その西側の半分は厚い氷の層に被われ、地表に残された堆積物の多くは氷の浸食を受けました。氷河は大地を削り、地表を被っていた堆積層を岩屑、巨礫岩の河原やその他の氷河特有の残存物に変えてしまいました。

Shortly after the Arabian shield emerged as land on the surface of the sea, its western half was covered with the thick layer of the ice, and many of the marine sediments left on the surface were eroded by the ice. The glaciers have carved the grounds and transformed those marine sediments into the debris rock flours, the boulder rock fields, and the other typical glacier remains.

平らな景観は氷河で削られ、巨礫岩が雑然と転がり、谷底を砂利で被われた谷で引き裂かれてしまいました。氷河から流れ出る水をテチス海へと運ぶ河川によってその東側すなわち氷河地帯と海洋の間でも堆積層は浸食されました。アラビア楯状地を含むゴンドワナ大陸が極地からもっと暖かい緯度へ移動すると、氷河は後退して再び海水が北西部を除いてアラビア楯状地を浸水しました。

The flat landscape was carved by glacial movement and torn with the glacier valleys. The bottoms of the glacier valleys were covered with the thick gravel layers and/or the boulders rolling cluttered. The marine sediments had still left between the ice-covered west and seaside east. However, the rivers that carried the water from the glaciers to the Tethys sea had washed away those marine sediments. As the Gondwanaland, including the Arabian shield, moved from the polar regions to the warmer latitudes, the glaciers receded and the seawater again infiltrated the Arabian shield except in the northwest.

海水が引いてアラビア楯状地が再び陸上に現れた時には筆石類の化石をたくさん含む頁岩の厚い堆積層が残りました。前回との違いは、湖、河川、海岸等の近傍に緑が生えて来た事でした。陸地の塊は海水面や構造地質とは無関係に、地中深いマントルの中の動きによって隆起したり沈下したりするので、アラビア楯状地はその後も12回以上も海中に没して居り、陸上に現れている期間の二倍も海中に没していました。約3億7千5百年前にも同じ様な現象が起き、海面の高さが一定であるのにアラビア楯状地が乾いた陸地として海中から隆起しました。これらの時代の顕著な地質学的な氷河作用はカスィーム地方に残る氷礫土、トゥワイク山脈最南端に広がる多くの花崗岩の巨礫岩地層、アカダル山塊（ハジャル山脈の一部）の氷礫土や縞模様のある巨礫岩、オマーンの涸れ谷カラタの河床を刻んでいる深さ1mもの並行な溝等にその特徴を残しています。

When the seawater recedes and the Arabian shield reappeared on land, the thick marine sedimentary layer of the shale remained on the ground surface. Those shale contained a lot of fossil of the Graptolithina species. The difference from the last appearance on the land was that the greens grew near lakes, rivers, coasts, etc. The land masses rise and fall due to movement in the deep underground mantles, regardless of the sea level or the structural geology. Because of this, the Arabian Shield has been submerged more than 12 times since then. It submerged in the sea twice as long as it appeared on the land. A similar phenomenon occurred about 375 million years ago. The Arabian Shield rose from the sea as the dry land, even though the sea level remained constant. The remarkable geological periglaciation left in the Arabian Shield during the time. The tills (the glacier-specific sediments) can be seen In the Qasim Region. There are many granite boulder formations at the southernmost tip of the Jebel Tuwaiq. The tills and the striated boulders exist in the Jebel Akhdar, which is part of the Hajar mountains. The parallel grooves placed with a depth of 1 m carving the flat rock bed at the bottom of the Wadi Al Khlata, In Oman as the characteristics of the glacier action.