Giant White Clams and Plates

Plate Tectonics

The Earth's surface consists of plates. Plates are large, rigid slabs of solid rock. There are seven major crustal plates, subdivided into smaller plates. The plates are approximately 80 kilometers thick and are in constant motion with each other at rates varying from 10 to 130 millimeters per year.

Most of the important geological mountains, rift valleys, volcanoes, earthquakes and faulting are due to different types of interaction at different plate boundaries.

The world's earthquakes are not randomly distributed over the Earth's surface. They tend to be concentrated in narrow zones. The ocean floor is continuously pulled apart along mid-ocean ridges. Hot volcanic material rises from the Earth's mantle to fill the gap, continuously forming new oceanic crusts. The mid-ocean ridges themselves are broken by offsets know as transform faults.

Sagami Bay from the Shinkai 2000

Discovery of Hot Springs

The ocean floor is home to many unique communities of plants and animals. Most of these marine ecosystems are located near the water's surface. The sun's energy is strongest at the water surface and penetrates only about 300m below the surface of the water.

In 1977, scientists discovered hot springs at a depth of 2.5km, on the Galapagos Rift. Hot springs were found along the active mid-oceanic ridges, where magma, at temperatures of over 1,000 Centigrade, erupted to form new oceanic crust. Then, abundant and unusual sea life, including giant tube worms, huge clams and mussels, that thrived around the hot springs were also discovered.

Unusual Sea Life

Since 1977, other hot springs and associated sea life were found at a number of sites along the mid-oceanic ridges. Many on these were found on the East Pacific Rise. We now know that hydrogen sulfide-oxidizing bacteria, which live symbiotically with larger organisms, form the base of this ecosystem food chain.

Ecosystem Food Chain Model on the Deep-Sea Floor
( 1999.7 『海から生まれた神奈川』 Yokosuka City Museum )

　 The hydrogen sulfide (H2S - the gas that smells like rotten eggs) is needed by this type of bacteria to live. H2s is contained in the volcanic gasses that spew out the hot springs. Most of the sulfur comes from the Earth's interior while a small portion is produced by chemical reaction of the sulfate (SO4) present in the sea water. Thus, the energy source that sustains this deep ocean ecosystem is not sunlight but rather the energy from chemical reaction.

But the story is continually unfolding. In the late 1980s, scientists documented the existence of a dim glow at some of the hot geothermal vents. These are targets of intensive current research. The occurrence of natural light on the dark seafloor has great significance because it implies that photosynthesis may be possible even at deep-sea geothermal vents. Thus, the base of the deep-sea ecosystem food chain may comprise both chemosynthetic as well as a small portion of photosynthetic bacteria.

Reference from USGS National Earthquake Information Center's Web Site

Scene of surface stripping

In the Ikego Case

Researching

In 1986, scientists found the sea life fossils on the hilltops behind the Ikego Artifacts Museum. Even more exciting, however, was the discovery that deep-see life communities, including Giant White Clams. Giant White Clams ones, have strong relationships to the moving plates.

Japanese scientists from many fields of research gathered in Ikego to study these fossils in detail. The data obtained clarified aspects of the ancient environment, the geological structure and the formative process of Miura Peninsula and also Japanese archipelago.

We couldn't leave these hills as they were because of the proposed construction. Scientists, therefore, strived hard to preserve a number of excellent scientific samples. They even hardened the stratum surface and stripped it from the cliff-side slowly and deliberately, as shown.

Click on photograph to magnify.

Exploring the Ikego Giant Clams

This section of stratums contained Giant White Clam fossils (photo right) is exhibited on the Ikego Artifacts Museum wall.

Basically, older stratum lie lower but sometimes change their locations. The communities of this stratum inhabited 1,000m below the ancient Sagami Bay four million years ago. Due to a landslip, however, they became piled up on the deep sea floor. After years of uplifting by plate movement, the stratum rose above the sea.

Giant White Clams inhabit along the cracks of the earth's crust on the deep-sea floor even now. Therefore, they are called "living fossils".