By Rodney Grapes
Contributing Writer
If you don't spend all your daylight hours in the subway and are able to walk around above ground and look beyond and between the buildings, you can see hills cloaked in green or appearing as bare rock faces.
What kind of rocks are these hills made of, how old are they, and why are the hills shaped the way they are?
Have you thought about where your drinking water comes from and what controls the position and course of the Han River and its tributaries? Compared with Japan, why does Seoul (and most of Korea) hardly ever experience such violent natural events?
The hilly and river landscape of Seoul, which now covers an area of some 600 square kilometers, dictated King Taejo's decision to establish his new capital here in 1394, based on the principles of geomantic harmony ― that there should be high, rugged mountains to the north (Bukhan), lower hills to the east and west, a low hill to the south (Mt. Nam), and a wide flood plain through which a major river (Han) flows.
The high mountains and low hills that surround Seoul are made of granite and gneiss, which are hard crystalline rocks.
Granite and Gneiss
Both rocks are incredibly old.
Between 1,800 and 2,500 million years ago, gneiss was formed by a tortuous process of stretching, squeezing, squashing and folding at a high temperature and pressure, deep within the Earth, to become the oldest rock in Korea.
Granite is much younger and was formed 165 million years ago by cooling and crystallization from molten liquid, or magma.
Granite is one of the main sources of the building stones in Seoul.
Polished slabs of granite form floors and line walls, and smoothed blocks of the rock make up the tough subway steps that millions of commuters walk up and down each day.
Gneiss is used to construct the many garden embankments that decorate the city.
The remainder of the Seoul metropolitan area consists of a thin cover of river sediment that is less than two million years old, mere infants in earth time.
The sediments are composed of mineral grains and rock fragments eroded from the granite and gneiss, and continue to be deposited today, especially by the Han River.
The gneiss, granite and river sediments are related to each other by a continuum of birth, destruction and rebirth.
Rivers, such as the Han, carry sediment to the West Sea where it is dispersed into deeper water by ocean currents.
Here it is slowly buried, compacted, and eventually (over several million years) turned into new rock.
With deeper burial, the rocks become deformed and are changed at high pressure and temperature into gneiss.
At higher temperatures of around 650 degrees centigrade, gneiss starts to melt and produces molten liquid that eventually forms into granite when it cools and solidifies.
The gneiss and granite are slowly uplifted, at a rate of perhaps a few millimeters per year.
Uplift produces mountains that expose granite and gneiss above the Earth's surface where they are once again slowly destroyed by water, ice and wind and carried in bits and pieces back to the sea by rivers.
The rocks that underlie Seoul have taken some 2 billion years to undergo this unending recycling process of erosion, burial, deformation, melting, cooling and uplift.
Mountains in Seoul
The granite mountains of Seoul look like domes, and their slopes are often exposed as bare rock.
These dome-like mountains are actually wrapped by layers of fractured rock, like the outer layers of an onion. Each rock layer is typically between 30 and 160 centimeters thick.
Repeated freezing and thawing during winter loosens the rock sheets and make them vulnerable to sliding. They tend to peel away from the underlying rock and intermittently slide off the dome, causing the development of the rounded, smooth outcrops of granite that we now see.
What finally triggers a slab of rock to suddenly slide off the dome might be a summer storm event or possibly an earthquake.
The result is a pile of broken rock that accumulates on the lower flanks of the granite dome.
In this way, the hills are slowly losing height by erosion. For example, the average rate of erosion of the rock sheets from the 810-meter-high Insubong granite dome is estimated to be about six centimeters per 1,000 years.
At this rate, it will take another 12.6 million years for Insubong to be reduced to the level of the Han River, which is some 50 meters above mean sea level.
This seems like a long time, but compared to the 165-million-year-old granite, it is only a few seconds in the vastness of geological time that extends back to birth of the earth 4.5 billion years ago.
Water
Because the river sediments can easily soak up water, they are the main source of fresh water in Seoul.
Water also passes comparatively slowly through cracks and fractures in the underlying gneiss and granite.
This water, known as groundwater, flows from the surrounding mountains toward the Han River at an average depth of about 12 meters below the surface.
The groundwater level is shallower in the autumn and is deepest in spring.
Fresh water is obtained from some 15,000 wells with over 41 million cubic meters of groundwater pumped annually.
As might be expected, this vital fresh water source requires careful and continuous monitoring for purity, because with over 10 million people living in the Seoul metropolitan area, the groundwater may be locally contaminated by leakages from the city water supply pipe network, sewerage pipes and subway tunnels.
Earthquakes
Seoul sometimes experiences rather small earthquakes that are not powerful enough to cause any damage, the great majority of which are never felt.
Despite this, earthquake-related fault lines are known to extend along the main north-south valley (Cheonggye Stream) running through the northern part of Seoul; across two right angle kinks in the Han River; and along the prominent 30-kilometer-long straight section of the Han River that extends through the western part of Seoul.
These earthquake fractures have the potential to move during a future large earthquake.
Based on earthquake records extending back to 1905, during which time there were no damaging earthquakes, the chances of this happening appear to be statistically very small.
But why is it that Japan is often rocked by large earthquakes but not Seoul, or most of Korea for that matter?
Japan is a chain of volcanic islands that borders the Pacific Ocean, the rock floor of which is now moving northwestwards and plunging beneath Japan's eastern coastline at a rate of several centimeters per year.
The enormous forces of friction generated by this movement causes large earthquakes that periodically affect Japan, like the most recent ones that shook Tokyo on Aug. 9 and 12. But they have little or no affect on Korea, which is part of the more stable Asian mainland. Seoul is located about 1,000 kilometers from the zone of intense and damaging earthquake activity.
The writer is a geology professor in the Department of Earth and Environmental Science at Korea University.
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무디스, 스페인·伊·포르투갈 신용등급 강등
