3. PLATE TECTONICS LAST NAME (ALL IN CAPS): FIRST NAME: PLATES

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1 LAST NAME (ALL IN CAPS): FIRST NAME: PLATES 3. PLATE TECTONICS The outer layers of the Earth are divided into the lithosphere and asthenosphere. The division is based on differences in mechanical properties and in the method for the transfer of heat. The lithosphere is cooler and more rigid, while the asthenosphere is hotter and flows more easily. The key principle of plate tectonics is that the lithosphere exists as separate and distinct tectonic plates, which ride on the fluid-like asthenosphere. Plate motions range up to a typical mm/year (Mid-Atlantic Ridge; about as fast as fingernails grow), to about 160 mm/year (Nazca Plate; about as fast as hair grows). Tectonic plates consist of lithospheric mantle overlain by one or two types of crustal material: oceanic crust and continental crust. Average oceanic lithosphere is typically 100 km (62 mi) thick; its thickness is a function of its age: as time passes, it conductively cools and subjacent cooling mantle is added to its base. Because it is formed at mid-ocean ridges and spreads outwards, its thickness is therefore a function of its distance from the mid-ocean ridge where it was formed. For a typical distance that oceanic lithosphere must travel before being subducted, the thickness varies from about 6 km (4 mi) thick at mid-ocean ridges to greater than 100 km (62 mi) at subduction zones; for shorter or longer distances, the subduction zone thickness becomes smaller or larger, respectively. Continental lithosphere is typically about 200 km thick, though this varies considerably between basins, mountain ranges, and stable interiors of continents. Tectonic plates may include continental crust or oceanic crust, and most plates contain both. For example, the African Plate includes the continent and parts of the floor of the Atlantic and Indian Oceans. The distinction between oceanic crust and continental crust is based on their modes of formation. Oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes. Oceanic crust is also denser than continental crust owing to their different compositions. Oceanic crust is denser because it has heavier elements than continental crust. As a result of this density stratification, oceanic crust generally lies below sea level (for example most of the Pacific Plate), while continental crust buoyantly projects above sea level. Page 1 of 9

2 You can find alternative Plate Tectonic Map of the World at the link below: By "Eric Gaba for Wikimedia Commons", CC BY-SA 3.0, Page 2 of 9

3 PLATE BOUNDARY TYPES The location where two plates meet is called a plate boundary. Plate boundaries are commonly associated with geological events such as earthquakes and the creation of topographic features such as mountains, volcanoes, mid-ocean ridges, and oceanic trenches. The majority of the world's active volcanoes occur along plate boundaries, with the Pacific Plate's Ring of Fire being the most active and widely known today. Some volcanoes occur in the interiors of plates, and these have been variously attributed to internal plate deformation and to mantle plumes. Three types of plate boundaries exist, characterized by the way the plates move relative to each other. Transform boundaries (Conservative) occur where two lithospheric plates slide (grind) past each other along transform faults, where plates are neither created nor destroyed. The relative motion of the two plates is either sinistral (left-lateral) or dextral (right-lateral). The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion. Divergent boundaries (Constructive) occur where two plates slide away from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. Active zones of mid-ocean ridges (e.g., the Mid-Atlantic Ridge), and continentto-continent rifting (such as East African Rift; the Red Sea), are examples of divergent boundaries. Convergent boundaries (Destructive) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision. At zones of ocean-to-continent subduction (e.g. the Andes mountain range in South America, and the Cascade Mountains in Western United States), the dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate is heated it releases volatiles, mostly water from hydrous minerals, into the surrounding mantle. The addition of water lowers the melting point of the mantle material above the subducting slab, causing it to melt. The magma that results typically leads to volcanism. At zones of ocean-to-ocean subduction (e.g. Aleutian Islands; Japanese island arc), older, cooler, denser crust slips beneath less dense crust. This motion causes earthquakes and a deep trench to form. The upper mantle of the subducted plate then heats and magma rises to form volcanic islands. Closure of ocean basins can occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of granitic continental lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted. Page 3 of 9

4 THREE KINDS OF APPLIED FORCES, KINDS OF DEFORMATION, & FAULT TYPES THEY CAUSE STRESS (force) FORCES, DEFORMATION, FAULTS AND PLATE BOUDNARIES STRAIN (deformation) Resulting FAULT Corresponding PLATE BOUNDARY Compression Shortening Reverse Convergent Tension Lengthening Normal Divergent Shear Tearing and smearing Strike-slip Transform Page 4 of 9

5 QUESTIONS Q1. For the diagram given below, predict what kind of faulting you would expect to find in Earth s lithosphere if Earth were expanding or contracting (shrinking). Q2. Refer to the middle figure in the previous page (block diagram showing the three kinds of forces, deformation, & fault types) and fill in the Table below to indicate what kind of stress and faulting characterizes each kind of plate boundary. Plate boundary Type (Applied Force Fault Type Divergent Convergent Transform Q3. Refer to the accompanying figure to answer the following questions: I) In what general direction do each of the following plates move? South American Plate: Pacific Plate: Indo-Australian Plate: Nazca Plate: Eurasian Plate: II) What is the rate of motion along the San Andreas Fault? cm/year III) Write names of four plates not listed in I) above. Page 5 of 9

6 EARTH S LITHOSPHERIC PLATES & THEIR BOUNDARIES Numbers indicate rates of plate motion in cm/year based on satellite measurements Q4. Examine the cross sections of the plate boundaries given in the figures below. I) What type of plate boundary is shown in Figure 4A? Chose and circle one. A) Divergent B) Convergent C) Transform II) What type of plate boundary is shown in Figure 4B? Chose and circle one. A) Divergent B) Convergent C) Transform III) Which Mountain in today s South America has the same origin as the volcanic arc in Figure 4B? IV) For your answer in III) above, which plate in today s South America would represent the oceanic crust? V) For your answer in III) above, which plate in today s S. America would represent the continental crust? Fig 4A Figure 4B Page 6 of 9

7 Q5. Refer to the map of earthquake activity in Eastern Pacific Ocean and South America (map below). The figure demonstrates the alignment of earthquakes along plate boundaries. On the map, use a colored pen or pencil to draw lines (as exactly as you can) that indicate where plate boundaries occur at Earth s surface. Then label the East Pacific Rise, Galapagos Rise, Chile Rise, Peru-Chile trench, and all plates. For a map showing locations and names of plates, trenches and mid-ocean ridges, use Figure accompanying Q3 above). Q6. The accompanying figure shows distribution of the Hawaiian Island chain and Emperor Seamount chain. The numbers indicate the age of each island in millions of years (m.y.), obtained from the basalt rock of which each island is composed. Important information helpful in answering the next questions: Rate=Distance/Time (round-off results to two decimal places) 100 cm = 1 m; 1000 m = 1 km (you can find more conversion factors on page xi in your lab manual); thus to convert km to cm, multiply by 100,000. I) What was the rate in centimeters per year (cm/year) and direction of plate motion in the Hawaiian region from 4.7 to 1.6 million years (m.y.) ago? Show your work in detail. Model Answer Distance, Kauai to Molokai: 300 km To change 300km into cm: 300 km X 100,000 cm = 30,000,000 cm Time required for the plate to move from Molokai to Kauai: m. y. = 3.1 m.y. = 3,100,000 years Rate = Distance / Time= 30,000,000 cm / 3,100,000 years = 9.67 cm/year Direction of plate motion: NW Page 7 of 9

8 II) What was the rate in cm/year and direction of plate motion in the Hawaiian region from 1.6 m.y. ago to the present time? Show your work in detail (see model answer for Q6 (I) above). III) Based on your answers to Q6 (I) and (II), which of the following statements is TRUE about the rate of Pacific plate movement during the past 1.6 m.y. as compared to the older rate ( m.y.) of plate motion? Circle your answer. A) The rate of Pacific plate movement became faster during the past 1.6 m.y. as compared to the older rate ( m.y.) of plate motion B) The rate of Pacific plate movement became slower during the past 1.6 m.y. as compared to the older rate ( m.y.) of plate motion C) The rate of Pacific plate movement remained the same during the past 1.6 m.y. as compared to the older rate ( m.y.) of plate motion D) There is not enough data to compare the rate of Pacific plate movement during the past 1.6 m.y. and the older rate ( m.y.) of plate motion IV) Based on your answers to Q6 (I) and (II), which of the following statements is generally TRUE about the direction of Pacific plate movement during the past 1.6 m.y. as compared to the older direction ( m.y.) of plate motion? Circle your answer. A) The Pacific plate moved counter-clockwise to a more westerly direction B) The Pacific plate moved clockwise to a more northerly direction C) The direction of Pacific plate movement did not change at all D) There is not enough data to compare the direction of Pacific plate movement during the past 1.6 m.y. and the older rate ( m.y.) of plate motion V) Based on the distribution of the Hawaiian Islands and Emperor Seamount chains (Figure 2.7, upper diagram), how did the direction of Pacific plate movement change over the past 60 million years? Circle your answer. A) The direction of Pacific plate movement has changed from generally northerly motion during m.y. to a northwestern motion during 40 m.y. to present. B) The direction of Pacific plate movement has changed from generally westerly motion during m.y. to a northerly motion during 40 m.y. to present. C) The direction of Pacific plate movement has remained unchanged over the past 60 million years. D) It is not scientific to make conclusions about the direction of Pacific plate movement on the basis of submerged seamount chains such as the Emperor Seamount chains. Page 8 of 9

9 Numbers indicate ages of the islands in millions of years before present (m.y.) Page 9 of 9