Andesitic lava flow, from the 1719 eruption of Mt. Iwate. -- This nearly two and one-half mile long lava flow was extruded through the east flank of Mt. Iwate. The surface is typical blocky, or aa, type of lava. Note that some vegetation has begun to grow on the lava surface. -- Other volcanic landforms near Mt. Iwate include bubbling hot springs, fumaroles and small geysers, especially in the Hachimantai Plateau to the north.
Alluvial flatlands, floodplain of Kitakami River, Hiraizumi. -- Flat land is scarce in Japan and is mostly the result of stream action, though shoreline processes have produced some flat areas along the coast. Here, as its gradient, velocity and consequently its ability to transport sediment decreased, the Kitakami River has deposited sediment eroded from nearby mountain ranges. Most alluvial floodplains like this in Japan are small and linear in shape. Higher flat terraces paralleling the stream may be present if old floodplain surfaces have been uplifted and dissected by a rejuvenated and newly downcutting stream. -- Flat land is important in Japan because it is both the best agricultural land and the best site for cities and towns.
Mountainous Kitakami Highlands, mostly Paleozoic sedimentary rocks, at Tanohata. -- The Kitakami Highlands are typical of many of the mountains of Japan. The landscape is mostly slopes with narrow ridge tops and only a few small flat valley bottoms. One is visible in the right side of this scene. These mountains are not volcanic but rather consist of sedimentary rocks deposited in horizontal layers which were later deformed by crustal movements related to plate convergence.
Rice fields, Kanto Plain (Japan's largest), near Narita Airport. -- The Kanto Plain is the largest of Japan's coastal lowland plains with an area of about 8,000 square miles (this is about equal to a square 70 miles on a side). The slide shows the multiple uses of flat land in Japan. Rice fields, gardens, roads, villages and towns and an international airport all compete for part of this lowland area.
Incised valley and upland terrace, Tanohata. -- If stream erosion and lowering of a land surface exceeds the rate of uplift of that surface, topographic relief decreases. This reduces stream gradients and velocity and causes streams to begin shifting laterally creating floodplains and eventually broad, nearly flat surfaces. If uplift is subsequently renewed, relief and stream gradients to the sea increase, causing the streams to erode downward once again. This can produce stream valleys incised into remnant terraces of the old flat surface, as seen here. -- Similar topographic features are produced when flat, wave-eroded surfaces along a coastline are uplifted and then cut into by downcutting streams flowing to the new, lower shoreline.
Rice fields, Mt. Iwate, west of Morioka. -- Here, competing uses for flat alluvial plains for agriculture and urban expansion are evident. Originally this was all agricultural, but now the growing suburbs of Morioka are encroaching on the land. The volcanic cone of Mt. Iwate rises above the plain.
Weathered granodiorite, Shirasakatage. -- In mountainous terrain weathered rock material, a major component of soil, is rapidly carried away downslope. The result is soil that is thin, poor and rocky. The eroded material may be deposited along a river farther downstream producing a floodplain, or it may be carried all the way to the sea. Because most Japanese rivers and streams are short and fast moving, much of the material goes to the sea. Agriculture is difficult in the mountainous parts of Japan because of steep slopes and poor soils, and much of this land remains covered by forest.
Coastal features, straight Pacific coast: wave-eroded sea cliffs, wave-cut terrace, Mizushirazaki. -- Japan has about 18,000 miles of coastline whose configuration differs from place to place depending upon the interaction between shoreline erosive processes and earth crustal movement. At Mizushirazaki, the trend of rocks and geological structures is nearly parallel with the coast resulting in a fairly straight coastline. (Compare this coastline with that shown in slide 1.19.) Waves are actively eroding and undercutting the base of sea cliffs causing them to collapse into the water. As the coastline retreats in this manner, a flat wave-cut bench is produced offshore a few feet below sea level. The flat surface covered with trees above the cliffs is an old wave-cut bench that has been uplifted above sea level by crustal movement. -- Notice the small beaches that have formed from sediment deposition in areas sheltered behind rock promontories.
Flat land used for cities, Tokyo viewed from the top of Tokyo Tower. -- Flat land is scarce and very valuable in Japan. It is the most productive agricultural land, and also the easiest land upon which to build. Consequently. there is great competition and tension between development and agricultural interests. As the population increases and more people move from rural to urban areas, Japanese cities continue to expand and an increasing proportion of flat land is lost to agricultural and other uses.
Steep gradient, downcutting stream. The Sacred Bridge at Toshogu Shrine, Nikko. -- As rivers downcut and reduce their gradient, they tend to eliminate rapids and waterfalls. Because of more-or-less constant uplift in Japan, rivers typically have not been able to accomplish this. This river has rapids and falls indicating a steep gradient. There are no flat surfaces adjacent to the stream to indicate that it is shifting laterally. This is a small, fast-flowing, actively downcutting, short river typical of most of the rivers of Japan.
Coastal features, deeply embayed Pacific coast, Yamada. -- The coastline at Yamada is irregular with large bays separated by mountainous promontories that jut into the sea. (Compare this coastline with the straight one shown in slide 1.18.) This kind of coastline can be produced where rocks and geological structures trend across the coastline so that shoreline erosion of weaker rocks produces bays while more resistant rocks are left as headlands and promontories. It can also result from submergence of a mountainous land surface where the flooded valleys form bays and the mountains stand above the sea.
Beach at Noda. -- Where there are indentations or bays along coastlines, wave action is reduced. In these more sheltered environments sediment which has been carried along the coast or brought to the bay by rivers from the land, may accumulate forming bars and beaches. If such deposition continues for a period of time, the bay may gradually fill, producing a level plain in the former coastal indentation. There are many such coastal plains along the Japanese shoreline, most of them small and isolated. -- Here, at Noda, barriers have been placed just offshore to further reduce wave action and promote deposition and beach formation and retention.
Mt. Iwate, a composite or strato-volcano, Backbone Range, elevation 6,734 feet. -- Contributing to the mountainous terrain of Japan are about 200 volcanoes constituting about 6 percent of the Japanese land area. Sixty of these have been active at since the 7th century, sometimes with disastrous results. Volcanoes are formed at boundaries of converging earth plates where one plate descends deep enough beneath the other plate to start melting. The molten rock material, called magma below the earth's surface and lava above it, is relatively light and rises to the surface where it erupts. Depending upon the composition of the lava, eruptions range from relatively quiet extrusion of lava to hazardous explosive ejection of ash and burning gases. -- Some volcanoes, such as Mt. Iwate, erupt lava of different composition at different times. These are called composite or strato-volcanoes and have typical concave shapes with steeper slopes at the top.
Caldera occupied by Lake Tazawa, seen from Mt. Komagatake, Backbone Range. -- Lake Tazawa, the deepest lake in Japan, is nearly circular in shape and about three and one-half miles across. It is nearly 1,400 feet deep and occupies a large depression of volcanic origin, called a caldera. This depression has steeply sloping sides and a mostly flat bottom. The caldera formed when huge amounts of volcanic material were erupted leading to the collapse of the resulting volcano into the vacated, empty chamber below. At least one small cone, formed by later volcanic activity, rises above the lake floor, but not to the lake surface. There are other calderas in Japan, including Lake Towada north of Morioka. Crater Lake in Oregon is a similar feature in the United States.
Flat land used for agriculture, rice fields and harvest, Yahaba. -- Flat land has always been important in Japan for agriculture simply because much of the country is too steep for cultivation. It is also the most fertile land. The lowest land, usually near a river or stream, is easiest to irrigate has often been used for growing rice. Higher terraces left by rejuvenation and renewed downcutting by a stream, tend to be sandy and drier and are used for grains, orchards and tea. -- This illustration shows an alluvial plain near the Kitakami River.
Flat land used for new suburbs west of Morioka. -- This illustrates how a burgeoning population and demand for housing leads to suburban sprawl onto available flat land. The ridges remain forested while the flat alluvial plain is occupied by new homes. Previously this was agricultural land.
Bayhead bar beach and coastal village of Hiraiga. -- Along this mountainous coastline, flat land is scarce. The homes and buildings of the small village of Hiraiga are crammed into nearly every piece of reasonably flat land above the high tide line. In much of Japan, rugged mountains separate the small areas of flat land upon which villages could be built causing the villages to be isolated from one another. The scarcity of flat land also leads to concentration of the Japanese population into those areas flat enough to build upon.
Vegetable garden, rice field, Yahaba. -- Vegetables and grains grow well on the slightly elevated terrace surfaces while rice grows on the lower surfaces that can be more easily irrigated.
Wave-cut cliffs, sea stacks, Pacific Ocean at Kitayamazaki, Rikuchu Coast National Park. -- This coastline has undergone at least two episodes of crustal uplift. At the present time, wave erosion is forming a wave-cut bench just below the water surface and offshore from the cliffs (the sea stack in the foreground and the isolated smaller rocks showing above the water are remnants on this bench). The level surface notched into the cliffs about halfway up is an older wave-cut bench that has been uplifted to its present position by crustal movement. Evidence of a still earlier episode of uplift is found in the flat surface at the top of the cliffs which is an even older and more extensive wave-cut bench. Note that this top surface is not horizontal but slopes downward toward the ocean in the background. Because it had to be horizontal when formed just below sea level, this indicates that the uplift that raised it was not uniform, but was greater in the foreground.