36.3 Earth Sci 10

Rift/Rift Valley

-A region of Earth's crust along which divergence is taking place. -Landform created at a divergent boundary on continental crust such as the Great African Rift Valley

Escarpment

a steep cliff or slope between a higher and lower land surface

Three Types of Convergent Boundaries

(a) an ocean floor plate collides with another ocean floor plate. (b) an ocean floor plate collides with a less dense continental plate. (c) a continental plate collides with another continental plate.

Divergent Plate Boundaries

-Divergent Boundaries are the boundaries between two plates that are diverging or moving away from each other. -Divergent means: moving apart. -The Mid-Atlantic Ridge is an example of a Divergent Plate Boundary -Another example of a Divergent Plate Boundary is Iceland, which is splitting apart down the middle.

The Features of a Transform (Sliding) Boundary are:

Earthquake zone or fault zone

The Himalayas: Two continents collide

Among the most dramatic and visible creations of plate-tectonic forces are the lofty Himalayas, which stretch 2,900 km along the border between India and Tibet. This immense mountain range began to form between 40 and 50 million years ago, when two large landmasses, India and Eurasia, driven by plate movement, collided. Because both these continental landmasses have about the same rock density, one plate could not be subducted under the other. The pressure of the impinging plates could only be relieved by thrusting skyward, contorting the collision zone, and forming the jagged Himalayan peaks. About 225 million years ago, India was a large island still situated off the Australian coast, and a vast ocean (called Tethys Sea) separated India from the Asian continent. When Pangaea broke apart about 200 million years ago, India began to forge northward. By studying the history -- and ultimately the closing-- of the Tethys, scientists have reconstructed India's northward journey. About 80 million years ago, India was located roughly 6,400 km south of the Asian continent, moving northward at a rate of about 9 m a century. When India rammed into Asia about 40 to 50 million years ago, its northward advance slowed by about half. The collision and associated decrease in the rate of plate movement are interpreted to mark the beginning of the rapid uplift of the Himalayas. Artist's conception of the 6,000-km-plus northward journey of the "India" landmass (Indian Plate) before its collision with Asia (Eurasian Plate). Solid lines indicate present-day continents in the Indian Ocean region, but no geologic data exist to determine the exact size and shape of the tectonic plates before their present-day configurations. The dashed outlines for the "India" landmass are given for visual reference only, to show the inferred approximate locations of its interior part in the geologic past. The "India" landmass was once situated well south of the Equator, but its northern margins began to collide against the southward-moving Eurasian Plate about 40 to 50 million years ago (see text). The Himalayas and the Tibetan Plateau to the north have risen very rapidly. In just 50 million years, peaks such as Mt. Everest have risen to heights of more than 9 km. The impinging of the two landmasses has yet to end. The Himalayas continue to rise more than 1 cm a year -- a growth rate of 10 km in a million years! If that is so, why aren't the Himalayas even higher? Scientists believe that the Eurasian Plate may now be stretching out rather than thrusting up, and such stretching would result in some subsidence due to gravity. Sunset view of towering, snow-capped Mt. Everest, from the village of Lobuche (Solu-khumbu), Nepal. (Photograph by Gimmy Park Li.) Fifty kilometers north of Lhasa (the capital of Tibet), scientists found layers of pink sandstone containing grains of magnetic minerals (magnetite) that have recorded the pattern of the Earth's flip-flopping magnetic field. These sandstones also contain plant and animal fossils that were deposited when the Tethys Sea periodically flooded the region. The study of these fossils has revealed not only their geologic age but also the type of environment and climate in which they formed. For example, such studies indicate that the fossils lived under a relatively mild, wet environment about 105 million years ago, when Tibet was closer to the equator. Today, Tibet's climate is much more arid, reflecting the region's uplift and northward shift of nearly 2,000 km. Fossils found in the sandstone layers offer dramatic evidence of the climate change in the Tibetan region due to plate movement over the past 100 million years.At present, the movement of India continues to put enormous pressure on the Asian continent, and Tibet in turn presses on the landmass to the north that is hemming it in. The net effect of plate-tectonics forces acting on this geologically complicated region is to squeeze parts of Asia eastward toward the Pacific Ocean. One serious consequence of these processes is a deadly "domino" effect: tremendous stresses build up within the Earth's crust, which are relieved periodically by earthquakes along the numerous faults that scar the landscape. Some of the world's most destructive earthquakes in history are related to continuing tectonic processes that began some 50 million years ago when the Indian and Eurasian continents first met.

Convergent Boundary with Mountain Building

A major geological event; occurs when continental plates of equal density converge. Example: Indian plate converging with Eurasian plate forming Himalayas mountain range where the highest peak, Mt. Everest can be found.

Deepest Part of the Ocean The Challenger Deep in the Mariana Trench is the deepest known location in Earth's oceans.

Measuring the Greatest Ocean Depth The Challenger Deep in the Mariana Trench is the deepest known point in Earth's oceans. In 2010 the United States Center for Coastal & Ocean Mapping measured the depth of the Challenger Deep at 10,994 meters (36,070 feet) below sea level with an estimated vertical accuracy of ± 40 meters. If Mount Everest, the highest mountain on Earth, were placed at this location it would be covered by over one mile of water. The first depth measurements in the Mariana Trench were made by the British survey ship HMS Challenger, which was used by the Royal Navy in 1875 to conduct research in the trench. The greatest depth that they recorded at that time was 8,184 meters (26,850 feet). In 1951, another Royal Navy vessel, also named the "HMS Challenger," returned to the area for additional measurements. They discovered an even deeper location with a depth of 10,900 meters (35,760 feet) determined by echo sounding. The Challenger Deep was named after the Royal Navy vessel that made these measurements. In 2009, sonar mapping done by researchers aboard the RV Kilo Moana, operated by the University of Hawaii, determined the depth to be 10,971 meters (35,994 feet) with a potential error of ± 22 meters. The most recent measurement, done in 2010, is the 10,994 meter ( ± 40 meter accuracy) depth reported at the top of this article, measured by the United States Center for Coastal & Ocean Mapping. Exploring the Challenger Deep The Challenger Deep was first explored by humans when Jacques Piccard and Don Walsh descended in the Trieste bathyscaphe in 1960. They reached a depth of 10,916 meters (35,814 feet). In 2009 researchers from Woods Hole Oceanographic Institution completed the deepest dive by an unmanned robotic vehicle in the Challenger Deep. Their Nereus robotic vehicle reached a depth of 10,902 meters. Why is the ocean so deep here? The Mariana Trench is located at a convergent plate boundary. Here two converging plates of oceanic lithosphere collide with one another. At this collision point, one of the plates descends into the mantle. At the line of contact between the two plates, the downward flexure forms a trough known as an ocean trench. An example of an ocean trench is shown in the diagram. Ocean trenches form some of the deepest locations in Earth's oceans. Mariana Trench earthquake: Map showing the location of the Challenger Deep, the epicenter of an April, 2016 earthquake, and the relative movement directions of the Pacific and Philippine Plates. USGS map with annotations by Geology.com. Underwater volcanic vent: As the Pacific Plate is pushed into the mantle and heated, water in the sediment is volatilized, and gases are liberated as the basalt of the plate melts. These gases migrate to the surface to form a number of volcanic vents on the ocean floor. This photo shows gases escaping and bubbles moving towards the surface, expanding as they ascend. NOAA image. Earthquakes in the Mariana Trench The Mariana Trench occurs along a plate boundary between the Philippine Plate and the Pacific Plate. The Pacific Plate is on the eastern and southern side of this boundary, and the Philippine Plate is on the western and northern side of this boundary. Both of these plates are moving in a northwesterly direction, but the Pacific Plate is moving faster than the Philippine Plate. The motion of these plates produces a convergent plate boundary because the greater speed of the Pacific Plate is causing it to collide into the Philippine Plate. This collision produces a subduction zone at the Mariana Trench as the Pacific Plate descends into the mantle and under the Philippine Plate. This collision occurs at variable speeds along the curving boundary of the plates, but the average relative motion is in the range of tens of millimeters per year. Recurrent earthquakes occur along this plate boundary because the Pacific Plate's descent into the mantle is not smooth and uniform. Instead, the plates are usually stuck with pressure accumulating, but with sudden slips as the plates move a few millimeters to a few meters at a time. When the plates slip, vibrations are produced, and those vibrations travel through Earth's crust as earthquake waves. As the Pacific Plate descends into the mantle, it is heated by friction and the geothermal gradient. At a depth of approximately 100 miles, the rocks have been heated to a point where some minerals begin to melt. This melting produces magma that rises towards the surface because of its lower density. As the magma reaches the surface, volcanic eruptions are produced. These eruptions have formed the Mariana Island Arc.

Ocean Trench

Deep and narrow depressions in the seafloor where the subducted plate moves into the asthenosphere.

Felsic

Feldspar, quartz, magnesium, igneous Describes magma or igneous rock that is rich in feldspars and silica and that is generally light in color.

Mountain Ranges

Upward depressions caused by convergence of two continental plates.

Volcanic Island Arcs

Formed when an oceanic crust converge with another oceanic crust. Japan, Indonesia, Philippines, Caribbean, Aleutian Islands

Mafic

Magnesium iron describes magma or igneous rock that is rich in magnesium and iron and that is generally dark in color

Mid-ocean Ridge

A divergent plate boundary on oceanic crust.

Volcano

An upward depression on Earth's crust with a crater and a magma reservoir.

Volcanic Arc

Formed when Oceanic and continental crust collides Characteristics: large volcanoes; subduction zone; trench; oceanic crust subducts under continental crust

Ridge

Landform created at a divergent boundary on the ocean floor such as the Mid-Atlantic ridge

Active Volcanoes

Luzon- Pinatubo, Taal Mayon Visayas- Kanlaon Mindanao- Ragang

Transform Plate Boundaries

-The third type of Plate Boundary is the Transform (Sliding) Boundary. -These plates slide past each other. -An example of a Transform-Sliding Plate Boundary is the San Andreas Fault Zone in California. -there are also transform boundaries along mid-ocean ridges

Subduction

-A process wherein denser oceanic crust is pushed back to the mantle whenever an oceanic plate converges with another plate. -The process in which a denser plate is pushed downward beneath a less dense plate when plates converge; occurs at continental to oceanic boundaries and oceanic to oceanic boundaries.

3) Continental - Continental Convergent Boundary

-Continent-Continent collisions involve two continental plates. -Compressional forces cause two continental plates to move together. -Once the oceanic crust between the two continents has been entirely subducted and the continental shelves have nearly joined, the process of subduction ceases. -Lithosphere is destroyed as one oceanic slab descends beneath another. -Because of the low density of continental crust neither plate will subduct, both continental shelves get uplifted and deformed, forming folded mountains. -Such a collision occurred when India collided with Asia forming the Himalayan mountains and also during the formation of the Appalachian mountains -Global examples of continental-continental collisions include the Himalayan and Appalachian mountains Example: -India collided with Asia about 40 million years ago Mt. Everest, 29,000 feet -the alps formed through the collision of the African plate and Eurasian plate

Convergent Plate Boundaries

-Convergent Boundaries are the boundaries between two plates that are converging, or moving towards each other.

2) Oceanic - Continental Convergent Boundary

-Ocean-Continent collisions involve an oceanic plate and a continental plate. -Compressional forces cause an ocean plate and a continent plate to move together, due to a greater density (basaltic vs. granitic), the oceanic plate will most likely subduct beneath the continent forming a trench and subduction zone. -In a map view, a volcanic arc is created on the continent as a result of the collision. -Lithosphere is destroyed as one oceanic slab descends beneath another. -The composition of the molten material will most likely be granitic (felsic) since the upwelling magma created by the melting oceanic crust has to burn through a granitic (felsic) continent. -At depths of about 100 km the oceanic plate and parts of the mantle partially melt producing viscous magmas. This molten rock rises slowly where it cools and solidifies at depths producing plutons. However, some magma may reach the surface and erupt through composite volcanoes as violent volcanic eruptions. -Examples include the Rocky mountains in North America and the Andes mountains in South America.

1) Oceanic - Oceanic Convergent Boundary

-Ocean-ocean collisions involve two oceanic plates. -Compressional forces cause the oceanic plates to move together, causing one slab of lithosphere to be consumed into the mantle initiating volcanic activity which creates volcanoes to form on the ocean floor. -The volcanoes produced at this collision boundary are island arc volcanoes; they form a curved arc on the surface of the non-subducting plate. -Features called ocean trenches are formed at these boundaries. -The composition of the molten material is largely (intermediate) since the melting basaltic (mafic) oceanic crust becomes mixed with melting sediments that were located on top of the subducting plate as a result of weathering and erosion from the nearest continent (felsic). -Lithosphere is destroyed as one oceanic slab descends beneath another. -Examples include the Japan island arc and the Japan trench.

Features of a Divergent Boundary: Rift Valley

Associated with the Jordan Rift Valley to the south is the Red Sea Rift. Millions of years ago, the Arabian Peninsula was connected to Africa. Seafloor spreading caused the Arabian and African plates to rift apart. The Indian Ocean flooded the rift valley between the continents, creating the Red Sea.

36.3 b Additional Information

· Convergent plate boundaries occur where plates move together, resulting in the subduction of oceanic lithosphere into the mantle along a deep oceanic trench. · Convergence between an oceanic and continental block results in subduction of the oceanic slab and the formation of a continental volcanic arc such as the Andes of South America. · Oceanic-oceanic convergence results in an arc-shaped chain of volcanic islands called a volcanic island arc. · When two plates carrying continental crust converge, both plates are too buoyant to be subducted. ·The result is a "collision" resulting in the formation of a mountain belt such as the Himalayas.

36.3 a Additional Information

· Divergent plate boundaries occur where plates move apart, resulting in upwelling of material from the mantle to create new seafloor. · Most divergent boundaries occur along the axis of the oceanic ridge system and are associated with seafloor spreading, which occurs at rates between about 2 and 15 centimeters per year. · New divergent boundaries may form within a continent (for example, the East African rift valleys), where they may fragment a landmass and develop a new ocean basin. · Transform fault boundaries occur where plates grind past each other without the production or destruction of lithosphere. · Most transform faults join two segments of an oceanic ridge. · Others connect spreading centers to subduction zones and thus facilitate the transport of oceanic crust created at a ridge crest to its site of destruction, at a deep-ocean trench. Still others, like the San Andreas Fault, cut through continental crust.