Week 3 posting

LECTURE 14 : PLATE TECTONICS When we begin to discuss tectonics, please focus upon the following concepts from the text. 1) plate composition​: Define the difference between ​SIALATIC PLATES​ (continents) and ​SIMATIC PLATES​ (oceanic plates). Identify the composition of each plate, for example a simatic plate is made up of mostly silica and magnesium (si+ma), which makes this plate a very dense, heavy plate relative to a sial plate (silican + aluminum). This is important as plates collide, we will see that the HEAVIER plate always is subducted beneath the lighter plate. 2) the cause of plate movement:​ Review the idea of sea-floor spreading which stimulates all plate movement. This divergent activity is caused by heat rising from the core through radioactive decay of the earth’s inner core. 3) types of movement​: Once you understand the location of plates and their direction of movement (look in the atlas p. 8-9, 21st ed/p.22-23, 22nd ed) it is easy to identify which features are located at each boundary based upon the type of movement. Use the examples in the text to fill in specific geographic features that demonstrate the following types of movement. a) DIVERGENCE: 1. type of movement? 2. examples- which plates are diverging? 3. features: ridges/rises (oceanic), rifts (continental) b) CONVERGENCE: 1. type of movement? 2. examples: which plates are colliding? 3. Features? Trenches, volcanoes earthquakes There are​ three possible types of collision​ which can occur based upon the plates which are converging (see pages 364-374, 9th ed or pages 351-377, 10th ed in discussing orogenesis).

i) oceanic-oceanic collision:​ Features created through this type of collision can be seen as the Pacific Plate collides with the Philippine Plate. See the plate tectonic map in atlas to identify these plates (p.8-9, 21st ed/p.22-23, 22nd ed). This collision creates a subduction trench​ (such as the Marianna Trench (see 21st ed atlas, page 10/22nd ed atlas, page 26 ) stimulating ​effusive volcanism (lava flows) on the plate not being subducted, creating island ​archipelagoes​ (such as the Marianna Islands) on the plate which is not subducted. Other examples of this collision can be seen throughout the Pacific—examine the Philippines, the Tonga Islands north of New Zealand—and see if you can find the plates, trenches, and specific islands created from this type of collision. ii) continental-continental collision: ​This occurs when two continental plates collide, such as the Indo-Australian plate and the Eurasian plate creating ​diastrophism​ or ​folding and faulting​ (examples through chapter 12). In this example, this collision creates the Himalayas, which are still growing in size each year, proving that collision is occurring. Remnant features of such collision are found in the Appalachians and Scandinavian Mountains (see page 6-7, 21st ed. atlas/ page 22-23, 22nd ed. atlas/p. 377 text) where we find Caledonian remnants of previous collisions. Also, from Chapter 13, you can see the Wrangellian and Franciscan ​accretions ​that have been welded onto the Pacific Northwest through collision of the Pacific Plate and the North American plate which created the Sierra Nevada and the Basin and Range topography through the Great Basin. i​ii) continental-oceanic collision​: this activity occurs when two different types of plates collide, such as the Nazca plate (oceanic) and the South American plate (continental). This collision forces the heavier plate (the oceanic plate) down into the mantle, creating explosive volcanism on the Continental plate, forming the Andes. There is also a ​subduction trench​, see the Peru Chile trench in the atlas. This type of collision creates ​large composite cone (also called stratovolcanoes) mountains​ as we can see east of Lima in Peru (see atlas page 142, 21st ed/page 170, 22nd ed). Locate the triangles in the white (high elevation) regions---and you will find the volcanic peaks. If you start at the northern portion of Peru, you should find the following as you head south along the Andes; Nevs Huascaran (22,133 ft), Nudo De Pasco (15, 118 ft), Nudo Coropuna (21,696 ft), Volcan Misti (19,101 ft) . These volcanoes are landscape evidence of ​explosive volcanis​m. See if you can find similar peaks in the Cascades in the atlas (page 114, 21st ed/page 112, 22nd ed). c) TRANSFORM MOVEMENT​: this type of movement occurs when two plates are not diverging or colliding, but sliding past each other, as seen on the San Andreas Fault, where the Pacific and the North American plate are sliding past each other (a right lateral strike slip, pages 369 to 371, 9th ed or page 352 - 356, 10th ed). This type of movement causes only earthquakes, unlike all other types of movement which cause BOTH earthquakes and volcanism. ​Transform faults​ are caused by differential rates of movement along plate boundaries. If you look in the atlas (pages 8/9, 21st ed/pages 22-23, 22nd ed) are great in showing this phenomena. Transform faults occur in conjunction with divergent movement. For example, if you look at the rise in the Gulf of Baja between the

Pacific plate and the North American plate, this divergence is also demonstrated by the Juan de Fuca ridge just north of the California coast, where the Juan de Fuca plate is diverging from the Pacific plate. This different speed of divergence on different sections of the Pacific plate cause transform faults, dispersing the tension through earthquakes. d)​ There is one other form of activity which builds new materials, and this is called​ HOT SPOT ACTIVITY​. Hot Spot activity occurs where there is a weakness in the plate, not along the plate boundary. The Hawaiian islands, the Canary islands and Old Faithful at Yellowstone are all examples of hot spot activity. They show the movement of plates as we follow the age and composition of each new island. If you look on page 347 9th ed, or page 339, 10th ed text, you will see that the Hawaiian islands are also linked to the Emperor seamounts (islands which do not break the sea level) to the north. This shows us the direction of the Pacific plate as it has moved through time. The hot spot is static and the plate moves above this creating new island or seamount along this line. Note that the age of these features becomes younger as we move to the southeast, showing the plate moved north at first, then northwest, once the Pacific plate collided with the North American plate. We know these features come from the same plume because they have the same rock composition (coming from the same magma chamber). This is ​effusive volcanism​ creating basaltic lava flows (as you will see in the atlas maps). As you synthesize chapters 12 and 13, please note the similarities between types of movement and tectonic activities discussed. For example, please note page 367, 9th ed or page 352, 10th ed text, illustrating types of stress, which cause earthquake activity. These types of stress are simply related to types of plate movement; tension stress is divergence, compression stress is convergence, and shear stress is transform movement. Try to use maps from the text and atlas to locate where such activity occurs, like I have done above. Please see how lecture 15 connects to this one. Mapping:​ Be sure that you feel comfortable identifying the maps on the Plate Tectonics map in the atlas, pages 8-9 (21st ed) and pages 22-23 (22nd ed). Note that volcanoes and earthquakes occur along the boundaries of plates (where convergence, divergence and transform movement occur). Hot spots are found in the middle of plates. Examine hot spots in the Pacific Ocean (flip back to the political map to identify which islands these create). Do the same for the Atlantic, and Indian Oceans. Use the bathometric maps of the ocean floor to locate trenches, ridges, rifts and rises that validate plate activity. These are found on pages 26-29 (22nd ed), and 10-13 (21 st ed).