5. Rocks from far-off southern seas
- Theme A:
Learning about and enjoying the earth's transformation from peridotites
The northern part of the hilly terrain on the Hidaka Mountains is dotted with small limestone outcrops that have long been mined. Limestone is one of the few industrial raw materials supplied domestically, and numerous limestone mines are in operation across Japan.
This limestone came from far to the south. Hawaii is known in particular for eruptions of Kilauea Volcano; beneath its islands is a hotspot where magma wells up from the mantle through the plate and reaches the surface. The formation of the Hawaiian Islands results from local volcanic activity that has continued for more than 70 million years.
Coral reefs developed around volcanic islands that formed over the hotspot, and coral fragments accumulated on the sea floor along with dead shellfish to form limestone. The limestone on the Pacific Plate moved westward several centimeters a year and eventually reached Japan.* The chain of coral reefs, seamounts and other topographical features extending from Hawaii to Midway Atoll and to Kamchatka is referred to as the Hawaii-Emperor Seamount Chain, and the formation shows plate movement. In addition to limestone, a rock known as chert also reached Japan on the same oceanic plate. Like limestone, chert is formed from radiolarian remains and other organic matter accumulated on the ocean floor.
When the limestone and chert carried by the oceanic plate move downward beneath a continental plate in a trench, the surface layer of the oceanic plate is scraped off and accreted onto the continental plate, thereby forming an accretionary prism. In the trench where this prism is formed, limestone, chert and other rocks are mixed with sandstone and mudstone from the overriding plate to form a mélange - a mixture of different rocks without continuous bedding.
The Japanese archipelago, under which oceanic plates subduct, is considered to be based on accretionary prisms. The form of a mélange, which is considered part of an accretionary prism, can be seen at Mt. Apoi Geopark.
- *The distance between Hawaii and Japan is about 6,400 km. As the Pacific Plate moves around 10 centimeters a year, it would have taken a mind-boggling 64 million years for the coral in Hawaii to become limestone and reach Japan. The limestone found in Mt. Apoi Geopark is considered to have formed 220 million years ago based on its conodont fossil content.