Lesson 8 - Shaping the Lithosphere
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To do:
- Check the schedule for this week's reading & upcoming assignments
- Read the lecture and assigned reading in the text
- Participate in discussions
- Take the Week 7 Quiz
By the end of this lesson you should be able to:
- Describe the physical structure of the Earth
- Apply an analogy to the age of the Earth and major events in the development of the biosphere
- Discuss the mechanism that drives plate tectonics
- Review the rock cycle in terms of the processes that form the three major rock types
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Volcanoes
You will recall that last week we introduced the inner workings of the earth: the composition of the earth, plate tectonics and the rock cycle. This week we are going to expand upon those topics and discuss in detail the mechanisms which shape the lithosphere.
The location of volcanoes are concentrated around the Pacific rim, the east African rift, and the Atlantic ridge – especially on Iceland. There are also some stray volcanoes in the middle of the Pacific and Atlantic oceans. The majority of the earth’s surface is devoid of volcanic activity. Why do volcanoes cluster in these regions?
Volcanism is directly related to the theory of plate tectonics. Volcanoes are found only at particular types of plate boundaries and rarely in the middle of ocean plates. In the following discussion we will summarize the three circumstances which explain the formation of nearly every volcano on earth. First, though, let’s go over the two types of volcanoes.
In an earlier lesson, we described two types of earth crust: The thick, relatively buoyant and light weight silica rich continental crust, and the heavy, thin ocean crust made of iron and magnesium-silicate rocks. Two types of magma correspond to these two types of crust (liquid rock is called magma when it is below the surface of the earth – above the surface it is called lava). Volcanoes on the ocean floor are made of the same material that makes up the ocean floor – iron or magnesium silicate minerals. This magma is very runny, or not viscous. It forms broad shield volcanoes such as the island of Hawaii which erupts large quantities of lava. On the other hand, volcanoes that form from silica rich magma tend to be steep and tall, such as Mount St. Helens in Washington. Silica rich magma is also very thick, or viscous, and tends to cause very violent, ash-rich eruptions without a lot of lava.
Volcanism Where Plates Collide
There are two types of plate collisions: Ocean-continent, and ocean-ocean. The plate that is subducted melts partway down (partial melting), and the melted material rises up through the crust and forms a volcano. The melted material contains bits of the sea floor that were pulled down with it and water, so it is more silica rich and buoyant than pure ocean crust. As the magma rises it melts additional crust around it, making it even more silica rich – especially when it rises through continental crust. As discussed above, this silica rich magma is much more viscous than its mafic (magma that is rich in iron and magnesium, and has less than 50% silica) cousin. The rising magma often forms plugs in the neck of the volcano, leading to pressure buildup within the magma chamber, and eventually a highly explosive eruption. Oftentimes, the magma does not get out of the chamber, and ends up cooling slowly just below the surface, forming granite or some other type of intrusive igneous rock.
Where plates collide, two types of volcanoes are formed. When an ocean plate collides with a continental plate, volcanic mountains are formed on the continental plate. Examples of active volcanic mountains today are the Cascade Range in Washington, and the Andes in South America. Sometimes, following a plug of silica-rich magma in a volcano is a volume of more runny mafic magma from deep in the earth. When this mafic magma is erupted, it spreads rapidly, covering large areas. The flood basalts of the Colombian Plateau in Oregon-Washington covered the region 12 million years ago.
Where two ocean plates collide, volcanoes are formed in an ‘island arc’. Island arcs are characterized by their linear structure, mirroring the plate boundary. Japan, the Philippines, and New Zealand are examples of island arcs.
Volcanism Where Plates Diverge
Earlier, we noted that there was a series of volcanoes in eastern Africa, and we also noted several near the mid-Atlantic ridge, and on the island of Iceland. These are examples of volcanoes which form where plates diverge. The driving force behind plates moving apart is the upwelling of hot magma in the mantle which spreads laterally, driving plate motion. Sometimes, some of that hot magma melts the crust that overlies it, causing the crust to thin, and volcanoes form.
The east African rift is a zone of crustal thinning and movement, where the African continent is in the process of splitting in two. This is an example of volcanism at the formation of a plate boundary. Here, the continental crust is much more thin that normal because of an upwelling of hot magma from deep in the earth’s crust. This is the beginning of a new plate boundary. The volcanoes of the east African rift are a combination of the silica rich, explosive type, and the silica-poor mafic type.
The island of Iceland sits atop the mid-Atlantic ridge, and is an example of a series of volcanoes which form where ocean plates diverge. The volcanoes that comprise Iceland are all mafic, so they have formed an island with shallowly sloping mountains or shield volcanoes. Shield volcanoes are large, gently sloping summits formed from successive eruptions of mafic lava.
Hot Spot Volcanoes in the Middle of Ocean Plates
Sometimes, for reasons that are still not well understood, an upwelling of hot magma forms a ‘hot spot’ in the middle of an ocean plate. The hot spot creates a shield volcano. Often, the hot spot will remain in the same place, while plate moves over it, creating a series of islands. One example of this is the Hawaiian island chain. The oldest island, Niihau is located northwest of the youngest island, Hawaii. The island chain documents the motion of the Pacific plate as it passed over the hotspot.
Volcanic Hazards
As volcanoes modify the surface of the earth, they also enrich it, drawing people to live in volcanically active areas in order to cultivate the soil. The ash from volcanic eruptions provides vital nutrients to enrich and renew the soil, and soil formed from decomposed basalt (volcanic rock) is very fertile. Consequently, volcanic areas such as Hawaii, the Philippines the Andes of South America and eastern Africa all have large populations living at the bases of active volcanoes. Predicting and understanding the hazards that these people face is of great importance to earth scientists and emergency management personnel.
The first, and most obvious hazard of volcanoes is lava. However, lava is generally not life threatening because most lava flows move a the speed of a few kilometers per hour, so one can avoid advancing lava, even on foot. Lava temperatures range from 500ºC to 1400ºC, so combustible materials, such as homes and forests often burn. In addition, in regions that are covered by new lava flows, the cooled lava forms rock, rendering the land unusable for cultivation.
One of the most dangerous byproducts of a volcanic eruption, especially an eruption from a silica rich volcano, is a pyroclastic flow. A pyroclastic flow is a mixture of hot gas (about 1000ºC), rocks and ash that sweep down the sides of a volcano at speeds in excess of 1000 kilometers per hour. Pyroclastic flows are extremely dangerous because they are often produced with the first stage of a large eruption, and can catch residents unaware, and they move so quickly they are impossible to escape. The majority of the damage from the 1980 eruption of Mount St. Helens in Washington was the result of a pyroclastic flow.
Another danger from volcanic eruptions is lahars or mudslides. When Mount Pinatubo erupted in the Philippines in 1991, the torrential rains that followed the eruption turned all of the ash into a heavy cement-like sludge that raced down mountain valleys obliterating small villages in their path. Read about the lahars of Mount Pinatubo at this USGS web site.
Toxic gasses are another danger in volcanically active areas. Volcanoes release gasses such as carbon dioxide, water vapor, hydrochloric and hydrofluoric acids and various sulfuric gasses. In late 1986, in Cameroon, Africa, 1700 people and many animals died when accumulated gasses trapped in the sediment of Lake Nyos, the crater of a dormant volcano, were released. Carbon dioxide is odorless and more dense than regular air, so it accumulated in a layer near the ground, suffocating any living thing that passed through it. The savage planet website provides an excellent summary of this disaster along with some good pictures of the lake.
Orogenesis, Faults and Earthquakes
Orogenesis is the term used for mountain building. A common geologist joke is that ‘subduction leads to orogeny.’ Orogenesis is often the byproduct of a continent-ocean subduction zone, where a volcanic belt is formed (see above). Orogenesis can also occur when continental plates collide with other continental plates (such as the Himalayas), or when ocean plates collide with ocean plates, forming island arc volcanoes (again, see above). However, not all mountain belts are the result of active plate collisions. The Appalachian mountains of the eastern USA are between 250 and 300 million years old. They are a scar of an old continental collision that created the super continent of Pangaea.
When plates collide, one plate is not neatly pushed beneath the other. Significant forces are exerted on the crust, causing it to bend and deform. This bending can result in ‘folded landscapes.' Often the crust of the earth will be pushed so hard that it will break along a zone of weakness or maximum pressure. This is how faults are formed. Faults generally come in clusters, as a large force is exerted over a large area, the earth will release the pressure along a fault zone.
Faults can be classified into three types, based on the forces and the direction of movement. The stresses in the earth can be resolved into three mutually perpendicular directions: vertical (Sv), maximum horizontal (Shmax) and minimum horizontal (Shmin). Faulting occurs in the direction that the forces are the weakest.
- When the vertical stress (Sv) is the greatest, then normal faulting occurs. The vertical force (gravity) basically pushes one part of the crust down to relieve stress at the zone of weakness. This puts younger rocks on top of older rocks.
- When the vertical stress is the least, then thrust, or reverse faulting occurs. The vertical force has the lowest resistance, so the tension is released by pushing older rocks on top of younger rocks.
- When the vertical stress is the intermediate stress, (SHmax is the greatest, and Shmin is the least), then strike-slip faulting occurs. In strike slip faulting the two sides of the fault do not move up or down, but rather right or left. The direction of the fault movement is denoted as ‘right lateral’ or ‘left lateral’.
When tension is suddenly released on a fault and the earth moves in one of the manners described above, an earthquake occurs. The place on the fault where the energy was released is termed the epicenter. The earth is literally covered with faults. Most faults are very small (some less than 10 meters long), and most are either inactive, or release energy in such small increments that we do not notice the resulting earthquakes. There are, however, a few large faults which build up and release large amounts of energy, resulting in catastrophic earthquakes.
The San Andreas fault runs through south-central California and veers westward and out into the ocean just north of San Francisco. The San Andreas is a major right lateral strike-slip fault. This means that the western side of it is moving north, while the eastern side is moving south. Look at this air photo of the San Andreas in Southern California. Note the river that is offset.
Earthquakes occur constantly. For a list of recent earthquakes in California, check out this USGS web site for quakes in California, and for a close-up of quakes in the Bay Area. Have you felt any of the recent quakes? Probably not, as most have less than a magnitude of 3. What do we mean when we talk about earthquake magnitude? Magnitude is traditionally measured on the Richter scale. The Richter scale is a logarithmic scale which describes the amount of energy released by a quake with a number. Each whole number increase in the Richter scale (say, from a 6.0 to a 7.0) reflects a 10-fold increase in the amount of energy released. That is to say, that a magnitude 7 quake is much, much worse than a magnitude 6. Another measurement of earthquakes is the Mercalli intensity scale, which rates a quake on a roman-numeral scale from I to XII. The Mercalli scale is based on the damage an area received. Therefore, the Loma Prieta quake, which rated a 7.1 on the Richter scale, might have received a IX or X on the Mercalli scale near its epicenter, but it received a IV where I lived in Modesto, some 200 miles from the epicenter. You may want to do some research to learn more about the characteristics of damages and the frequency expected from earthquakes on both intensity scales.
Some interesting earthquake related sites (not covered on your exam, but interesting):
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