Review for Science 10 Provincial Exam - Earth Forces I. PLO 10G1: compare a variety of techniques used to learn about the Earth; seismology, remote sensing, volcanology, geological field work (mapping, drilling, and examining of rocks and structures.) Recognize the Earth’s layers. Techniques for studying the Earth: In spite of the recent Hollywood movie, The Core, which tells a story of a team of ‘terra’nauts travelling to the center of the Earth to restart the movement of the outer core, we physically can’t go into the Earth. We have to study it from the surface. There are a variety of ways to do this. 1. Seismology is the study of earthquake waves: (Please note - Grade 10 students do not need to know all about seismic waves, although they may be interested in the information. They do need to know that seismic waves were used to interpret the Earth’s layers.) There are two varieties of earthquake waves, surface waves and body waves. Body waves pass through the Earth’s layers and are used for interpreting the interior structure of the earth. There are two kinds of body waves, P & S waves. For animations go to http://www.geology.sdsu.edu/visualgeology/geology101/seis_frames.htm P (primary) waves: travel through solid, liquid and gas materials – they travel faster than S waves (6 km/s). These are compression waves, like sound waves.

S (secondary) waves travel only through solids – they travel more slowly (3.5 km/s). These waves arrive later and are more destructive. Surface waves travel more slowly than body waves but are extremely destructive as they travel along the surface of the Earth and have a greater amplitude.

Notes by Anne Laite, 2003

Notes by Anne Laite, 2003

For more information go to: http://www.exploratorium.edu/faultline/earthquakescience/eqscience4.html Since S and P waves travel at different speeds and through different materials, seimologists can study the pattern of waves generated by an earthquake as they travel through the Earth and form conclusions about the earth’s layers. If you sit on the edge of a pool on a summers day, dangling your feet in the water, the image of your feet seems distorted by the light waves refracting as they go from the air into the water. To the individual gazing into the pool, it appears as though their feet aren’t attached properly to their legs. As light waves refract going from the atmosphere into the water, seismic waves refract going through different layers of the Earth. Both P and S waves speed up as they travel through the bottom of the crust into the denser lithosphere. They slow down at the asthenosphere and then increase their speed through the denser mantle rock. At the outer core, S waves disappear altogether which has resulted in our concluding that the outer core is liquid. Refraction of P waves delineates the inner core. Seismic waves are measured on a variety of seismograph machines and rated by their amplitude and magnitude. The most common earthquake scale used is the Richter scale which is a measure of the magnitude or energy released during an earthquake. The Richter scale is a logarithmic scale which means the increase in each number on the scale indicates that the wave amplitude increases by a factor of 10 and an energy factor of 30. Each year there are about one million minor (less than 5 richter scale) earthquakes; 800 moderate (5 - 5.9 richter scale) earthquakes; 300 strong (6 - 6.9 richter scale) earthquakes; 20 major (7 - 7.9 richter scale) and 1 or 2 great (8 or higher richter scale) earthquakes. Seismic activity or earthquakes are the most common marker of a tectonic plate boundary. They can only occur in brittle crust and are most common near the surface of the crust. It is only in subduction zones that one finds deep (300 to 700 km depth) earthquakes because of the subduction of ocean plates. Useful websites: http://www.exploratorium.edu/faultline/earthquakescience/eqscience5.html

Notes by Anne Laite, 2003

The Earth was initially molten and separated into layers as it cooled. The densest part of the Earth is the core, the least dense is the continental crust. Crust: 3 – 50 km - the ocean crust is denser than the continental crust

Ñ Mantle: 2900 km - rock is much denser than in the crust Ñ Outer core: 2200 km - iron and nickel - liquid Ñ Inner core: 1300 km (radius) - iron and nickel - solid

Tectonic Plates: The crust, continental (20-70 km thick) or ocean (4-7 km thick) and the upper portion of the mantle are solid and form the lithosphere (75 - 125 km thick including the crust). The lithosphere is cold and brittle and can fracture during an earthquake. The lithosphere is divided into pieces called tectonic plates. These are like broken ice fragments that float on the surface of water. Tectonic plates float on the hot, plastic asthenosphere which is too hot to ever fracture, although it can be stretched apart. Earthquakes cannot occur in the asthenosphere as it is hot enough to bend rather than break under pressure. The asthenosphere is the “slip” layer the tectonic plates slide on. 2. Satellite imagery can be used to study oceans, volcanoes, fault lines and other features on the Earth. This site, developed using a NASA grant has a great introduction to Remote Sensing. Go through the two familiarization modules – What is Remote Sensing? 1 and 2. http://www.mcps.k12.md.us/departments/eventscience/rs.ebs.html

Notes by Anne Laite, 2003

Mount Rainier Nasa’s Earth Observatory sends out weekly satellite images of the Earth along with news stories explaining how the information is useful to scientists. Check the site and if you find it interesting, sign up on their email list and receive their weekly satellite images on your email. The latest from NASA's Earth Observatory is at: http://earthobservatory.nasa.gov/Study/ 3. Geological Field Work: mapping, drilling and examining earth structures Geologists working in the field collect information that helps us to better understand the earth. Maps show surface features. Drilling produces core samples that give information about the layers of earth below us. The core box (next photo) shows tubular pieces of rock which are removed from rock layers within the crust, using diamond tipped drills. After the rock is removed from the Earth’s crust, geologists make detailed maps of the core, paying particular attention to areas of interest. These might be highly mineralized zones containing gold, copper or sulphide minerals, or petroleum rich zones or even faulted areas which would indicate the strength of the rock (necessary for building dams). The core ‘maps’ are correlated, one to another, to build three dimensional images of the rocks within the Earths crust. A geologist developing a gold property is able to use a computer to build a 3 dimensional map of a potential deposit, one of the many steps necessary for designing a gold mine.

4. Volcanology: the study of volcanoes, another marker of tectonic plate boundaries

Notes by Anne Laite, 2003

Volcanoes are evidence for moving tectonic plates. They are found at different boundaries and at hotspots. They can also show the rate of plate movement. Examples:

1. The Cascade volcanoes (Mt. Garibaldi, Mt. Meagher, Mt. Baker, Mt St Helen’s) formed at the subduction boundary of the Juan de Fuca Plate and the North America Plate

2. Underwater volcanoes and pillow basalt structures form at the divergent boundary of the Juan de Fuca Plate and the Pacific Plate. Here, new ocean floor is rising from the mantle and cooling, pushing the ocean plates out in either direction.

Great websites: http://www.nrcan.gc.ca/gsc/pacific/vancouver/volcanoes/volcanoes_e.html The USGS has a remote sensing site for volcanoes: http://volcanoes.usgs.gov/About/What/Monitor/RemoteSensing/RemoteSensing.html

Earthquake and volcano distribution mark the edges of tectonic plates. II. PLO 10G2: Use fossil evidence to illustrate how life forms change over time. Refer to the Geological Time Scale in the Data Booklet. Fossils provide evidence for the change in life forms over time:

Paleontologists study traces of ancient life. Fossil evidence illustrates how life forms change over time. These recreations of horse ancestors taken from fossil evidence show how the general form and hooves of horses have changed from the Eocene to recent times.

Notes by Anne Laite, 2003

Equus Merychippus Mesohippus Eohippus

(Recent) (Miocene) (Oligocene) (Eocene)

trilobite fossil Index fossils are traces of organisms that indicate a specific age. They are usually widespread, and they probably had hard bones or shells or exoskeletons, making them likely to be preserved as fossils. They are useful as age indicators because index fossil species did not exist for long periods of time on the geologic time scale. Trilobites are useful index fossils. They are found in many Cambrian rocks, and they are the predominant fossil organism found in rocks from shallow marine environments until the end of the Permian. Paleontologists who study trilobites recognize a great number of different species. Individual species may represent as little as a million years on the geologic time scale. Individuals of those particular species will accurately place the rocks they are found in within a specific window of geologic time. Even for the novice, trilobites are useful. If you find a trilobite, you can be sure that it came from a very ancient rock. Great websites to visit: http://www.ucmp.berkeley.edu/ http://www.fossilmuseum.net/Time%20Machine/Geologic_Time_Machine.htm http://www.palaeos.com/Palaeo/lagerstatten.html III. PLO 10G3: Compare techniques used for establishing geological time scales; relative and absolute dating, law of superposition, cross-cutting rule and half-life. 1. The Geologic Time scale is based on changes in life on earth and reflects the abundance and variety of fossils found in the rock record. The Cenozoic means recent life, the Mesozoic means middle life and the Paleozoic means ancient life. The PreCambrian Era represents the greatest amount of time on the geologic time scale but usually takes up the smallest space on most charts because of the small amount of lifeforms preserved in the rock record throughout that era.

Notes by Anne Laite, 2003

2. Relative and Absolute dating Relative dating is comparing the ages of life forms, using a variety of strategies. In simple terms, you might say, “I am younger than my teacher” or “my teacher is older than me.” This is a relative age determination. Absolute dating uses radioactive isotopes, which decay at a known rate to determine absolutely when a rock formed or an event occurred. For more information see your review sheets on Radioactivity. In simple terms, an absolute date refers to a number. “I am 6 years old,” said John. Radioactive isotopes are unstable because of extra neutrons within their nuclei. They randomly decay over time. Although isotopes are far too small to watch as they decay, we can measure the time it takes for half of a sample of a “parent” isotope to decay to a “daughter” isotope. This is called the half-life. Half-life times vary for different isotopes and some are more appropriate than others for age dating rocks. It takes careful decision making to choose the right isotope for testing and geologists usually rely on additional information such as the relative age of the rocks in question. Great websites to visit: http://www.talkorigins.org/origins/faqs-youngearth.html http://vearthquake.calstatela.edu/VirtualDating/ 3. A variety of laws and rules help geologists interpret the order of events in a sequence of rocks. The Law of Superposition says that in layers of rock, the oldest layer was deposited first, and will be at the bottom of a stack of layers. In the Grand Canyon, rocks at the bottom of the canyon are up to 2.2 billion years old, (2,200 million) whereas rocks at the top of the canyon may only be 250 million years old. The Grand Canyon represents a long sequence of the Earths history. At the bottom of the canyon the first fossils are simple stromatolites, or algal mats (think pond scum). Farther up in the sequence of rocks there are trilobites and ammonoids and traces of ancient organisms that have been extinct for a long, long time. Near the top of the canyon the rocks contain traces of fish, clams, reptiles and amphibians. There are even caves worn into the upper layers of the canyon where you can find the ancient scattered bones of the American Mammoth, a mere 11 million years old. The oldest traces of people in the canyon are dated at 1200 years old.

Notes by Anne Laite, 2003

The Crosscutting rule says that any thing that disturbs other geologic features had to have happened after the layers were deposited. Here, the sill must have occurred after these layers were laid down. The top two layers might be more recent than the sill, although we would need more information to know for sure. The rules for interpreting rock strata are like the “rules” for making a sandwich. The first piece of bread put down on a bread board would be the “oldest”, the top piece would be the youngest. (Law of Superposition) You would probably not cut the sandwich into two halves until after it was made. (Crosscutting Rule) And while you are making the sandwich, if someone comes along and takes away one of the layers, perhaps a piece of cheese or a pickle, you have a missing layer. In geological terms this would represent an “unconformity” or a time gap in the rock sequence. Unconformities often represent periods of erosion where rocks have been worn away rather than deposited. IV. PLO 10G4: Identify major factors responsible for earthquakes, volcanic eruptions, mountain building, and formation of ocean ridges. Include divergent, convergent and transform fault plate boundaries, tectonic plates, mantle convection and hot spots. Tectonic Plates: (see diagram under I. PLO 10G1) The crust, continental (20-70 km thick) or ocean (4-7 km thick) and the upper portion of the mantle are solid and form the lithosphere (75 - 125 km thick including the crust). The lithosphere is cold and brittle and can fracture during an earthquake. The lithosphere is divided into pieces called tectonic plates. These are like broken ice fragments that float on the surface of water. Tectonic plates float on the hot, plastic asthenosphere which is too hot to ever fracture, although it can be stretched apart. Earthquakes cannot occur in the asthenosphere as it is hot enough to bend rather than break under pressure. The asthenosphere is the “slip” layer the tectonic plates slide on. Tectonic plates may contain continent and ocean crust or only ocean crust. For example, the North American Plate contains the continent of North America and about half of the ocean crust under the Atlantic Ocean. The Juan de Fuca Plate and the Nazca Plate contain only ocean crust. There are 7 large plates and many smaller plates. Convection Cells - the engine that drives the plates: Tectonic plates move because of convection currents* in the mantle. Rising magma pushes plates apart at spreading centers we call divergent plate boundaries. This is because magma is heated up near the core and becomes less dense and rises. Colliding or converging ocean plates are pushed down into the mantle at subduction zones. Ocean plates subduct when they collide with continental plates because they are denser. If two ocean plates collide, one of them will subduct, as well. *Convection also causes air flow around a candle flame; causes the movement of warm and cold air that we call weather; and causes movement of cold and warm water in the worlds oceans that creates ocean currents. Continental crust is less dense than ocean crust and never subducts. Continental crust is forever pushed around the surface of the Earth and consequently contains the oldest rocks on Earth, up to

Notes by Anne Laite, 2003

4 billion years old. Ocean crust is continually being recycled at convergent boundaries and is much younger.

Rising magma at divergent plate boundaries pushes the lithosphere apart. Subducting plates at convergent plate boundaries pull lithosphere behind them as they sink. Geologists call this “slab push” and “slab pull.” The boundaries of tectonic plates are marked by earthquakes, volcanoes, mountain ranges and deep ocean trenches. There are three main types of plate boundaries. Geologists distinguish Divergent plate boundaries by double parallel lines on maps. Convergent plate boundaries are marked by sawtooth lines and transform fault boundaries are marked by single lines. When considering the interaction of tectonic plates at plate boundaries, it is important to think about the type of plate boundary (convergent, divergent or transform) AND the type of crust at that plate boundary. Three combinations of crust are possible: O-O; O-C and C-C where “O” stands for ocean and “C” stands for continental crust. Ocean crust is considerably denser than continental crust and at convergent plate boundaries it will subduct under the continental crust.

1. Divergent or spreading centers (double lines) occur where rising magma is pushing plates apart. (i) Ocean – ocean , example - the Mid-Atlantic Ridge which makes up part of the worlds’ Mid-Ocean Ridge (MOR) system, the largest mountain chain on Earth, on the ocean floor. (ii) Continent – continent , example - the African Rift Valley

Notes by Anne Laite, 2003

2. Convergent plate boundaries (saw teeth – with the teeth on the side of the over-riding plate) occur where two plates are colliding together. Two continental plates will crumple and form mountain ranges because neither is dense enough to sink into the mantle. However, ocean plates will form subduction zones as they sink back into the mantle. (i) Continent – continent example, the Himalayas Mountains formed from the collision of the continent of India with the continent of Eurasia. (ii) Continent – Ocean example, the Nazca Plate is subducting under the less dense South American Plate. (iii) Ocean – Ocean example, the Pacific Plate is subducting under into the Aleutian trench.

3. Transform fault (a single line, often with arrows on either side showing the direction of plate movement) (i) Continent - continent, the San Andreas Fault in California (ii) Ocean - ocean, most transform fault boundaries are located on the seafloor because rising magma plumes at divergent plate boundaries or mid-ocean ridge do not erupt at the same time at different locations. The result are many transform fault boundaries between the divergent boundaries. (They are almost like tear faults, like the uneven tearing of an unfolded piece of paper.)

Notes by Anne Laite, 2003

1. The Hawaiian volcanoes are caused by a hot spot in the central region of the Pacific Plate. Hot spots are in the central region of tectonic plates and are caused by rising plumes of magma. The hot spot responsible for the Hawaiian volcanic islands has been in place for about 60 million years. As the Pacific Plate moves over the hot spot, the volcanic islands lose their source of magma and cool as they move away from the stationary mantle plume. The older Hawaiian islands get smaller as erosion wears them down. New islands are formed over the hot spot.

The Hawaiian Islands (and the Galapagos Islands) have formed over hotspots in the center of ocean plates. As the Pacific Plate has moved over a hotspot a series of volcanic island has formed. The hotspot stays at the same latitude and longitude, the plate moves. V. PLO 10G5: Identify evidence that supports the theory of plate tectonics; magnetic reversals, earthquake and volcano patterns, ocean ridges and trenches and mountain building. Include continental drift theory, fossil evidence, mountain belts and paleoglaciation.

Notes by Anne Laite, 2003

The theory of Continental Drift was proposed by Alfred Wegner and suggests that all of the Earth’s continents were once joined together in a supercontinent, Pangea, and have since drifted apart. One piece of evidence used by Wegner to support his Theory of Continental Drift was the ‘jigsaw’ fit of the edges of the continents on either side of the Atlantic Ocean. This suggested to Wegener that the continents were once together. This wasn’t a new idea, but Wegener took it a few steps further and found more evidence to support his theory.

Ancient fossils, older that the break-up of Pangea & younger than the formation of Pangea

A second piece of evidence was a number of different fossils found on several different continents, all older than 225 MYO. If these fossils are plotted on todays map of the world on the continents where they were located, we have a curious scenario. Two land reptiles, a fresh-water swimming reptile and a heavy-seed fern either evolved at the same time on different continents or moved from one continent to another. It is unlikely that identical life-forms would evolve in different places at the same time. Wegener thought that when they were living, the continents they lived on were fused together in one continent he called Pangea. (from Greek root words meaning “all lands.”) A third piece of evidence was identical rock outcrops which geologists found on continents that are widely separated today. If Pangea existed, these rocks would be continuous from one continent to the other and a common depositional history for the rocks would make sense. The Appalachian Mountains on the east coast of North America are a good example of this, they continue in western Europe through British Isles. The blue rocks (diagram - darker rocks) on adjacent continents match

Notes by Anne Laite, 2003

up although they are widely separated at the present time.

Another piece of evidence uncovered by Wegener was paleoclimatic. Late Paleozoic rocks contain evidence of ancient glaciers which existed in S. America, Africa, India, Australia and Antarctica and showed ice movement away from Antarctica. There has been no evidence found which suggests glaciers existed in the northern hemisphere at this time. In fact, fossil evidence suggests the climate in the north was far too warm for glaciers to form. The current configuration of continents would lead one to believe that glaciers must have existed in equatorial regions at the same time as tropical climates existed in equatorial regions. This, of course, does not make sense. If Pangea is drawn and the glaciation is plotted, it radiates out from the south pole area and makes sense of the northern warmer climatic evidence. Although Wegener was unable to convince the science community that his theory of drifting continents was a good one, several geologists were intrigued by his glaciation evidence.

Wegeners theory was presented to the scientific community after the first world war. It was greeted with a great deal of skepticism and put aside. The biggest problem with his theory was the audacity of suggesting continents could move around the Earth without having a mechanism for moving them. The lack of mechanism resulting in his theory languishing for almost fifty years. Scientists did not accept the Theory of Continental Drift because Wegner did not have a believable mechanism for the movement of the continents. He felt they ploughed through ocean crust and perhaps pushed up mountain ranges on their leading edges. He proposed that centrifugal force from the Earth’s rotation and gravitational forces accounted for their movement. Scientists did

Notes by Anne Laite, 2003

not accept his ideas because of what they understood about the strength of rocks. The forces he proposed were not enough to account for the movement of continents. The theory of plate tectonics:

The theory of Plate Tectonics, developed in the 1960’s, used Wegener’s evidence along with new information gathered in the 1940’s, 50’s and 60’s. During World War II, naval commanders used sonar to map the floor of the Atlantic Ocean. Those maps revealed something very different from a simple basin shaped ocean floor. In the center of the Atlantic Ocean and along the edge of the Pacific, a mid-ocean ridge system extends around the globe. It is an underwater volcanic mountain range more than 80,000 km’s in total length and as wide as 1500 kilometers in some locations. After the war scientists were intrigued by this information and with new technologies developed during the war years, they turned their attention to exploring the world’s oceans. Wegener’s ideas became the foundation for a new understanding of the Earth.

Notes by Anne Laite, 2003

Magnetometer surveys revealed mirror image magnetic reversal patterns on the ocean floor. These areas of the ocean floor gave alternating high and low magnetic readings as magnetometer equipment was pulled over the ocean surface. Scientists used the mirror image patterns on either side of the mid-ocean ridges to interpret the outward movement of the ocean floor. Even though they did not fully realize the significance of the magnetic reversals at the time, the mirror image patterns and the age of the ocean floor (younger in the center and older at the outside edges) was enough evidence to build the theory of sea floor spreading which led to the recognition of divergent plate boundaries. Great websites: http://pubs.usgs.gov/publications/text/dynamic.html http://www.ocean.udel.edu/deepsea/level-1/geology/geology.html

The age of the sea floor as determined by magnetic anomalies shows the formation of the Atlantic Ocean from sea floor spreading at the divergent plate boundary or mid-ocean ridge in the center of the ocean floor. Oldest rock is red (outside edges of the ocean), youngest is the center green (thin) line. Once divergent plate boundaries were recognized, the rest of the theory developed quickly. Plate Tectonics theory is known as a unifying theory today, because it accounts for the location of mountain ranges, volcanoes, earthquakes, ocean trenches, rift valleys and the like. There is also a mechanism to explain the movement of plates. Convection currents such as those that describe the movement of air (resulting in wind and weather patterns) and those that describe the movement of ocean water (resulting in ocean currents) are also present in the Earth’s mantle. Extreme heat near

Notes by Anne Laite, 2003

the core causes mantle material to heat up, become less dense, and rise towards the Earth’s crust. At the crust the magma cools forming new ocean crust. It is then pushed to the side by more rising magma. The cooled crust is pushed back into the mantle where two tectonic plates collide. Although continental crust is too light to sink or subduct back into the mantle, ocean crust being denser, will subduct. The rising of magma, forming of new crust and subduction of cooled ocean crust forms a convection cell. Tectonic plates are pushed and pulled by the movement of this crust. Details of Plate Boundaries: I. Divergent plate boundaries: 1. Ocean – ocean

Volcanoes, black smokers and pillow basalt formations form mid-ocean ridges as new ocean crust is created. Shallow earthquakes are common as magma rises to the ocean crust from the mantle. The lithosphere thins out on either side of the mid-ocean ridge. The youngest rock is at the center of the ridge, the oldest ocean crust rock is on either side of the ocean. examples include the Mid-Atlantic ridge and the East Pacific Rise.

2. Continent – continent Volcanoes are common as the lithosphere thins and magma from the mantle adds new material onto the Earth’s Crust. Shallow earthquakes are common. Eventually the continental crust will spread apart as new crust is formed. The new crust is thinner and denser than continental crust and will eventually form a new ocean as seawater fills the rifted landscape. An example is the African Rift Valley. II. Convergent plate boundaries:

Notes by Anne Laite, 2003

1. Continent – continent As the continent of India collided with Eurasia, a shallow ocean between the two continents subducted below Eurasia. Once the two continents touched, the subduction stopped and the two continents crumpled together forming the highest mountains in the world. Marine fossils high in the Himalayas are remnants of that ancient ocean. Although volcanoes would have relulted from the earlier subduction in the area, all volcanic activity has ceased since the subduction ended. Shallow to moderate depth earthquakes and high, folded mountains are characteristic of this continent-continent convergent boundary. 2. Continent – ocean

When ocean plates collide with continental plates,the denser ocean plate will sink or subduct beneath the continental plate. As the ocean plate sinks into the mantle, an ocean trench marks the start of subduction. The sinking plate is cold and brittle and causes shallow to moderate to deep earthquakes as it sinks lower down into the mantle. In fact, subduction zones are the only localities on Earth where deep earthquakes (300 to 700 km depth) occur. Temperature increases one degree celcius for each kilometer of depth. As ocean plates subduct, any water within the ocean crust is eventually turned into steam. The rising steam melts the crust and magma rises to the surface in these areas, forming volcanoes. The Nazca plate subducting below the South American plate is a typical subduction zone. An ocean trench marks the start of subduction. Volcanoes occur farther to the east; and earthquakes range from shallow near the trench to deepest further east where the ocean plate is deep within the mantle. Another example is the Juan de Fuca plate subducting below the North American plate. These boundaries are characterized by ocean trenches, shallow to deep earthquakes and volcanoes.

3. Ocean – ocean

Notes by Anne Laite, 2003

When two ocean plates collide, one will subduct below the other. The start of subduction is marked by an ocean trench and shallow earthquakes. As you move away from the trench, the earthquakes become progressively deeper. This is known as a Benioff Zone. Since earthquakes can only occur in cold and brittle crust and lithosphere, the progressively deeper earthquakes mark the location of the subducting plate. As the ocean plate subducts, any water in the rock turns to steam as the plate sinks. This is because the temperature increases by about 30 degrees celcius per kilometer of depth. The rock above the steam will melt forming magma which will rise and form volcanic islands. Moderate to deep earthquakes mark the sinking of the subducting plate into the mantle. If the subducting plate is large, a long trench will mark the subduction zone and a line or arc of volcanic islands will be formed. These are called volcanic island arcs. Examples include the Aleutian Island arc, the Philipine Island arc and the Japan Island arc. III. Transform Fault plate boundaries:

Transform fault plate boundaries occur where two plates move in opposite directions, horizontally along the surface of the Earth. These occur frequently along mid-ocean ridges or divergent boundaries on the ocean floor (ocean - ocean), because the magma erupting at the surface occurs at different times in different localities. As the ocean floor widens, transform faults occur as “tear faults” between the diverging plates. Transform faults also occur on the continents (continent-continent). One of the most famous is the San Andreas fault where the surface of the Earth has puckered into low hills from the plate movement. The San Andreas is well known because cities such as Los Angeles are built on the fault. http://pubs.usgs.gov/gip/earthq3/safaultgip.html VI. PLO 10G6: Assess impacts of volcanoes & earthquakes on the environment: Volcanic activity brought water and carbon dioxide into the atmosphere of the Earth soon after it was formed. The water formed rain and filled the oceans. Much of the carbon dioxide dissolved into the ocean water. Algae and early life forms used sunlight for photosynthesis and converted carbon dioxide into oxygen which slowly accumulated in the Earth’s atmosphere. The percentage of oxygen rose from 0% to our current 21%. Once there was sufficient oxygen in the Earth’s atmosphere, life forms colonized the continents. The Earth as we know it, has been shaped and reshaped by continuous tectonic activity. It is a dynamic planet, driven by convection. People are sometimes at great risk from volcanic activity and earthquakes. Understanding the geology of this planet is necessary for understanding the risks involved with living on the edge of

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tectonic plates. Knowledge leads to better building codes which can withstand earthquake stresses and save lives. Understanding volcanoes allows for evacuation of populations from at risk areas and saves lives. The following websites discuss earthquake risk and safety measures: http://quake.wr.usgs.gov/prepare/future/ http://geopubs.wr.usgs.gov/fact-sheet/fs152-99/