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SMITH - EARTH SPACE – 2nd PERIOD - OFF-SITE LEARNING PACKET DAY 3
Chapter 9 The Earthlike Planets Lesson Three – Plate Tectonics
Lesson Objectives
• define the bellwork vocabulary (subduction zone, folded mountain chain, lithosphere) with 100% accuracy
• state the mechanism of volcanism at subduction zones with 100% accuracy • parse the term plate tectonics with 100% accuracy
Associated Text:
Chapter 9 Lesson 3
Plate Tectonics Our fourth stage in planet building was slow surface evolution. Earth process through this stage is greatly enhanced by the creation and movement of crustal sections. Earth's surface is active in that it is constantly renewed as large sections of crust drift about. Geologists refer to this activity as plate tectonics. Plate refers to the sections of crust, and tectonics comes from the Greek word for "builder." Plate tectonics is the builder of Earth's surface. It pushes up mountains, destroys old crust, and creates new crust.
The plates move because the mantle is highly plastic just below the crust. Convection currents rise through the mantle and help break the crust into segments, which then slide on the plastic mantle at speeds of a few centimeters per year. The recognition of the importance of plate tectonics is one of the great scientific revolutions of the 20th century. Two features of the ocean floor give us dramatic evidence of plate tectonics. In many places, the ocean floor is marked by a long underwater mountain range called a midocean rise, which is split by a midocean rift where the ocean crust is pulling apart and
lava flows are welling up to form new ocean crust. (You may have heard this called sea floor spreading.) In other places, sections of ocean crust are pushing below continents into subduction zones where they build mountains and trigger volcanism.
World map showing the relative movement of the crustal plates. See if you can identify rift zones and subduction zones.
The Alps in Italy, majestic reminders of the huge forces that shape the Earth’s surface through the mechanism of plate tectonics.
Diagram showing the major components of plate tectonics.
The evidence is clear that midocean rises are young. The rocks near a midocean rift are geologically young basalt, rock formed by solidified lava, and the farther basalt is from the rift the older it is. Also, the ocean floor nearest a rift has accumulated less sediment than the ocean floor further away. Further evidence lies in the magnetic properties of the ocean floor. As molten rock wells up along a midocean rift and hardens, it traps a record of the Earth's magnetic field. As the ocean crust spreads and Earth's magnetic field periodically reverses, the newly created basalt records the changing direction of Earth's magnetic field in a series of bands that run parallel to the rift and mirror each other on opposite sides of the rift. This was first detected along the midocean rift that runs from the South Atlantic to north of Iceland.
Like a conveyor belt, ocean crust is created in midocean rifts, spreads outward, and disappears down subduction zones to be remelted by the heat of Earth's mantle. Where ocean crust slides downward under a continent, it crumples the crust at the edge of the continent to build mountains, and the melting of the ocean crust releases low-density magma that rises to the surface to create volcanoes. The coastal volcanoes (including Mount Saint Helens) extending from northern California into Alaska are caused by a tectonic plate plunging below the continent.
The seafloor does not always slip below a continent. The floor of the Atlantic Ocean is locked to North and South America and is pushing the continents westward. Tracing this continental drift backward in time, Earth scientists find that North and South America were in contact with Europe and Africa only 200 million years ago. At that point, the land masses on Earth were joined in one great continent, Pangaea. About 200 million years ago. Pangaea broke into two land masses, Laurasia in the north and Gondwanaland in the south. These land masses further fragmented and drifted apart to form the continents we know. Continental drift continues today, driven by moving plates. Continents can split apart in a long, straight, deep depression called a rift valley. Africa has split from Arabia, opening a rift valley now filled by the Red Sea. In contrast, continents can collide. The collision of Africa with the east coast of North America, roughly 250 million years ago, folded the crust into the Appalachian Mountains, just as
Subduction zone. The water and carbon rich oceanic plates plunge under the continental crust, getting superheated and producing a very volatile magma.
Pangea
Diagram showing a midocean rift. The farther you travel on either side away from the rift, the older the rocks get. This indicates that new crust is being formed. Also notice the symmetric periodic reversals of Earth’s magnetic field.
the collision of India with southern Asia is now building the Himalayan Mountains. These folded mountain chains are typical of plate collisions. Sections of crust can slip past each other, as in the case of the Pacific plate carrying part of southern California northward along the San Andreas Fault and
generating frequent earthquakes. Plotting the location of earthquakes and volcanism on a world map reveals the edges of the moving plates. The Pacific plate is bounded by a ring of Earthquake zones known as the "ring of fire." Shallow earthquakes and undersea volcanism also occur along the midocean rises, where new crust forms and the seafloor spreads. Because Earth's crust is active, all geological features gradually change. The oldest existing portions of the crust, including parts of Canada and parts of South Africa and Australia, are only about
3.9 billion years old. The constant churning of Earth's surface has wiped away all record of older crust.
Image URLs http://www.zastavki.com/pictures/originals/2014/World___Italy_Alps_in_summer_at_the_ski_resort_of_Val_Gardena__Italy_062903_.jpg https://whybecausescience.files.wordpress.com/2013/06/worlds-plate-tectonics.jpg https://i.ytimg.com/vi/sdCrn39Lv5I/maxresdefault.jpg http://www.esa.int/var/esa/storage/images/esa_multimedia/images/2012/09/the_seafloor_acts_as_a_geological_tape_recorder/11856656-2-eng-GB/The_seafloor_acts_as_a_geological_tape_recorder.jpghttp://baef480c1b9a59094802-bb7fd020772cbf1cd099f3b22c712b0b.r79.cf2.rackcdn.com/9C60A476-E275-4880-95AE-C3DB277273B3.jpg http://eatrio.net/wp-content/uploads/2013/01/10.-pangea_politik.jpg https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Pacific_Ring_of_Fire.svg/1280px-Pacific_Ring_of_Fire.svg.png
The ring of fire.
Guided Reading Questions: (10 pts.) Use the above text and class notes to answer the following questions 1. What does the word “plate” mean in the term plate tectonics?
2. What does the word “tectonics” mean in the term plate tectonics?
3. Why do Earth’s crustal plates move?
4. What is the rate of plate movement?
5. What is a midocean rise?
6. What is a midocean rift?
7. What is a subduction zone?
8. In a subduction zone, which type of crust goes under the other type of crust?
9. From notes: What two gasses are released back into our atmosphere from the volcanoes that result
from subduction zones?
10. How long ago were North America, Europe, and Africa connected to each other?
11. What was that supercontinent called?
12. What is a folded mountain chain?
Lesson Notes Plate Tectonics
Parsing the two words
plate means sections of crust
tectonics means builder Earth’s Surface is Active
Earth’s crust is constantly being renewed and destroyed
plates move because the plastic mantle flows in convection currents
movement rate of few cm/yr
changing crust two main areas
mid ocean rifts
new oceanic crust formed here
subduction zones
crust is recycled into mantle
builds mountains
produces volcanoes
works like conveyor belt Importance of Subduction Zones
oceanic crust has lots of water and CO2 containing sediments like limestone
when subducted, it gets heated and becomes magma
this releases gasses and makes magma less dense
this thin magma causes volcanoes
these volcanoes release water vapor and importantly CO2 from the melted limestone
without this mechanism, Earth would not have enough CO2 to support plant life
Continental Movement
sometimes oceanic crust does not slip under continental crust
causes the continental plates to be pushed (a.k.a. continental drift)
200 million years ago all continents connected
called Pangaea Other Features of Plate Tectonics
rift valleys
continents moving apart
new crust being made
examples
Iceland
red sea
folded mountains chains
collision of continental plates
examples
Appalachian mountains
Himalayan mountains
transform faults
crustal plates slip past one another
example
San Andreas fault
Vocabulary: subduction zone – area where Earth’s crust is being pushed underneath another section of crust. Typically, it is the oceanic crust that subducts. folded mountain chain – a chain of mountain that is the result of two colliding plates with similar density. No igneous rocks found in abundance. lithosphere - the rigid outer part of the Earth, consisting of the crust and upper mantle
