Nature

Jogashima: shaping of an island, commercially and geologically

Cities and towns are influenced by geography as well as by the people who run and inhabit them, much like the roles nature and nurture play in human development. Geography is in turn shaped by geology, where tectonic plates and the elements slowly make their mark over millions of years. It’s easy to forget the geological attributes of the big cities we live in when asphalt and concrete cover everything, but if we downsize a few notches, the earth, in the most literal sense, usually lays in plain sight – Jogashima, a small coastal fishing village in Kanagawa Prefecture, is a case in point.

Miura Municipality is a peninsula that lies between Sagami Bay and Tokyo Bay. An abundance of fertile land, access to the ocean and a massive market to serve in Tokyo shifted policy makers’ attention to commercial fishing and agriculture. Over time, Miura Municipality developed competitive strengths in tuna fishing and its daikon and watermelon farms continue to be key suppliers to Tokyo’s fruit and vegetable markets.

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Painting of Miura Municipality’s key industries

Things look even simpler if you downsize further.

Jogashima is a small island off the southernmost tip of Miura Municipality. Without much land and being even closer to the Pacific Ocean, many residents are reliant on commercial fishing for a living. Misaki Port, Japan’s second largest tuna port, is just a short boat ride from Jogashima. Fresh and strange (specialty tuna restaurants serve unique versions of chiirashi that contain all key parts of a tuna, including cured organs and entrails) seafood  attracts tourists to the island, especially since the municipality started offering economical travel packages to visitors.

Other tourist activities on the island are also centred around water – the scuba and snorkelling school caters to active tourists interested in observing underwater life, whereas the coast gives recreational fishermen uninterrupted access to the sea.

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Sardines drying on a netted rack in Misaki Port

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Recreational fishermen patiently wait for their catch on a cliff in Jogashima

The most impressive feature of Jogashima is undoubtedly the rock formations that stretch along the western side of the island.

Arriving tourists are greeted by stretches of wave-cut benches.  Wave-cut benches are flat rock platforms at the base of sea cliffs. There are a number of formation stages and the process starts when waves erode the lower section of a cliff face to form a hollow.  The structural weakness that ensues, as the hollow enlarges to form a cave, places so much pressure on the remaining cliff overhang that it eventually topples, leaving behind a flat rock platform that was only spared from the destructive force of waves because it was below sea level. Depending on rock type and position, wave-cut benches have different erosion patterns and sediment composition. Structural reliefs of wave-cut benches provide visual cues that simplify the categorization process – sediment layers usually have different colors depending on constituent minerals, hard and soft layers erode at different rates to create relief patterns and water flow induced patterns provide further clues to the type of environment that shaped the rocks.

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Differential erosion of siltstone and scoria tuff layers of a wave-cut bench in Jogashima

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Wave-cut bench with ripple relief pattern caused by hydraulic action

More questions naturally come to mind: Why are sedimentary rocks in Jogashima of volcanic origin even though there are no volcanoes on the island? Why are wave-cut platforms so different in appearance despite close proximity? Are changes still occurring? What factors are driving the changes and are there any other interesting observations that have been made?

Broadly speaking, there are three categories of rocks: igneous, metamorphic and sedimentary. Igneous rocks are the only rocks that originate from volcanoes directly but all rocks can have volcanic origin – for example, igneous rocks gradually erode over time and become sediment that forms sedimentary rock, whereas metamorphic rocks result from one or more types of sedimentary or igneous rocks that undergo physical or chemical changes under intense pressure or heat.

The loose gravel in Jogashima are extrusive igneous rocks that are ejected during volcanic explosions and the sedimentary rocks on the island are also of volcanic origin. This creates a mini mystery – there are no volcanoes on Jogashima, how did the rocks get there?

Although it may seem otherwise, landforms are dynamic structures that slowly undergo substantial changes over millions of years. The changes are either caused by erosion or the movement of tectonic plates, which together with the earth’s crust, is known as the lithosphere.

2D images often depict the world’s twelve tectonic plates as simple jigsaw pieces that fit together neatly, but in reality this could not be further from the truth. Tectonic plates rest on the asthenosphere, the layer of partially molten rock directly under the lithosphere. Temperature differences within the asthenosphere create convection currents that propel the tectonic plates in various directions – either leading them to converge, diverge or transform by sliding past each other.

Tectonic plates continually create and destroy land as they move. Divergent plates are responsible for the formation of new lithosphere as oceanic or continental plates move away from each other. Transform plates do not create or destroy landforms but are the source of many earthquakes. Convergent plate boundaries are characterised by subduction zones where denser tectonic plates subduct under more buoyant plates. They are regarded as the most destructive plate boundaries because of the catastrophic natural disasters it causes and the lithosphere that is destroyed during subduction.

Stratovolcanoes, the world’s deadliest volcanoes, are formed at oceanic subduction zones. Water trapped in the oceanic plate is released as it moves past the trench and further into the mantle. Since water lowers the melting temperature of rocks, the molten rocks it comes into contact with in the asthenosphere becomes magma and rises through the lithosphere to either form a magma chamber where it slowly crystallises to form igneous intrusive rock or penetrates further up the crust where it cools down closer to land surface. Eventually, the pressure that builds up as more rock and magma is trapped beneath the surface is released through a massive volcanic explosion that ejects clastic material, lava and volcanic ash into the atmosphere. Stratovolcanoes can be formed at oceanic-continental or oceanic-oceanic plate boundaries – Mount Fuji is a stratovolcano formed from the collision of an oceanic and continental plate and the Izu-Bonin Arc is a series of oceanic volcanic islands formed from the collision of two oceanic plates.

Japan lies on four tectonic plates, namely the North American Plate, the Philippine Sea Plate, the Eurasian Plate and the Pacific Plate. An abundance of convergent plate boundaries and two oceanic plates have resulted in numerous subduction zones that stretch from the Ryukyu Islands all the way to Hokkaido. While c. 70% of Japan’s volcanoes are stratovolcanoes, the other 30% are made up of small shield volcanoes, pyroclastic plateaus, caldera volcanoes, lava fields, lava domes, pyroclastic cones, tuff rings and maars.

The volcanoes closest to Jogashima are Omuroyama, Izu-Oshima and Hakone-yama, which are classified as scoria cone, early stage basaltic stratovolcano and late stage stratovolcano respectively. Scoria and other basaltic gravel found on Jogashima were likely ejected from these volcanoes.

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Tuffaceous beds with scoria and lapilli indicate the presence of volcanoes nearby

It’s easy to imagine gravel less than three centimetres in diameter making its way to Jogashima. But what about the large stretches of volcanic sedimentary beds that lie all around the coast – how did they get to Jogashima? And why does the appearance of rocks on the shoreline differ so greatly despite close proximity? Again, the answer lies in plate tectonics.

The many types of rock that are found in Jogashima are a result of the accretionary prism formation process, where layers of ocean floor are effectively scraped off the subducting plate at the ocean trough or trench and piled against the continental plate. The composition of accretionary prisms can vary significantly depending on what is in the ocean crust as the subducting plate moves below the continental plate. Common materials in accretionary prisms include: ocean floor basalts, pelagic sediments and trench sediments. When layer upon layer of ocean crust piles on existing accretionary prisms, the pressure from compaction causes changes in the older segments and forms metamorphic rock.

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Pelagic chert nodules embedded in limestone during the sedimentary rock formation process

As demonstrated by stratovolcanoes, not all effects of subduction zones are as benign as accretionary prisms. The most destructive natural disasters caused by convergent plate tectonics are megathrust earthquakes. Strain accumulates as the oceanic plate subducts under the continental plate. A megathrust rupture occurs when the pressure from the subduction process eventually exceeds the friction that keeps the plates from sliding. At this point, the overriding plate slips above the subducting plate and releases a phenomenal amount of energy in the form of an earthquake.

Japan is a hotspot for megathrust earthquakes. The Tohoku Earthquake in 2011 and Great Kanto Earthquake of 1923 were both megathrust earthquakes that exceeded 7.5 on the Moment Magnitude Scale and the death toll for the two earthquakes were over 100,000 and 15,000 respectively. The Great Kanto Earthquake also caused major geological changes to surrounding areas. The shoreline of Jogashima was raised by more than a meter, as evidenced by sessile marine invertebrates markings (Pomatoleios kraussii) and honeycomb weathered limestone found c. 2m above the shoreline.

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Limestone with honeycomb erosion

Unfortunately, natural disasters caused by convergent plate boundaries are  unavoidable. Having said that, the impact of earthquakes and tsunamis can be minimised and governments have invested heavily in disaster prevention. Japan learnt many lessons from the Great Kanto Earthquake and implemented changes to Tokyo’s infrastructure accordingly. Fire-prone wooden buildings were replaced by fire-resistant and earthquake-proof concrete and steel structures. Its subway and train systems were constructed with  earthquakes and flooding in mind. Early warning systems were improved by the countless sensors that have been installed on and around the archipelago. Evacuation drills and safety procedures are practiced by adults and children alike. Everyone in the country is well aware of the risks associated with earthquakes and how to best improve their chances of survival when disaster strikes.

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Black Kite (Milvus migrans) perched on tilted sedimentary rock.

As dangerous and frightening as it is, locals don’t spend all their time and energy being paranoid about the next volcanic explosion or megathrust earthquake. After a few conversations, it quickly becomes apparent that the Japanese take a rather zen approach to the matter, “shouganai, there’s no point worrying about it when we’ve already done everything we can to prepare. This is our home and we’ve decided to stay. We just have to accept what comes with living in Japan”.

Cities and towns are influenced by geography as well as by the people who run and inhabit them. It turns out that geology also has an impact on the people who run and inhabit cities and towns after all.

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