The modern picture of plate tectonics:
or Continents don’t “plow through” the oceanic crust, as Wegener thought. Instead, they are carried by the oceanic crust as if on a conveyor belt. The driving force is thought to be convection in the asthenosphere.
Both continental and oceanic crust form tectonic plates. Plate margins are often associated with earthquakes (shown as red dots) and with volcanoes.
1 GPS (Global Positioning System) and other space-based technologies are now used to Mid-Oceanic Ridges measure directly how fast tectonic plates are moving. • New oceanic crust is constantly being produced at the mid-oceanic ridges. • Along the ridges, molten rock wells up, Plates today are moving at solidifies, and forms new crust. speeds of between 2 cm/ • Another name for this process is seafloor year and 15 cm/year. This is in line with estimates from spreading. the geologic record of how • Older crust is pushed away to either side. fast they’ve moved over the past millions of years.
Fossils on the sea floor, No oceanic crust is older the amount of sediment than about 210 million cover, and direct dating years, whereas very techniques all show that many continental rocks the crust nearest to the are that old and older. mid-oceanic ridges is the youngest, and that This is just what you age increases away would expect if oceanic from the ridges. crust is constantly being created and destroyed.
(Check out the National Geophysical Red=youngest; blue=oldest Data Center for more maps like this)
2 This is one of the few places in the world where an actively spreading rift crosses a land area. . .
Map of Iceland showing how the Mid-Atlantic Ridge crosses the island. Red triangles are active volcanoes.
Allmanagjá, Thingvellir, Iceland
Subduction Zones • If new crust is being formed, and the Earth isn’t getting bigger, then crust must also be destroyed. . . • Oceanic crust is destroyed at subduction zones, where one plate sinks under another. • Subduction zones are marked by trenches and by island arcs. Cross-sectional diagram of a subduction zone. As the plate sinks into the asthenosphere, not only is a • Examples: Aleutians, Philippines, Japan, trench formed, but melting rock rises and forms a line of Indonesia. (NOT Hawaii!!) volcanoes parallel to the trench. That’s an island arc.
3 Example of an island arc: The Aleutian Islands, Alaska
Seismic waves travel faster in cold rock than in hot rock. This actually makes it possible to “see” the colder plate (shown in blue) diving into the hotter asthenosphere Pacific Plate (shown in green and red). This is a cross-section of the movement Tonga subduction zone in the southern Pacific Ocean.
Arc volcanoes may join to continents, rather ✖ Mt. Garibaldi The subduction of the small Juan de than form islands. . . ✖ Mt. Baker ✖ Glacier Peak Fuca Plate below the North American plate ✖ Mt. Rainier has produced a ✖ Mt. St. Helens volcanic arc on the ✖ Mt. Hood continent. The red ✖ Mt. Jefferson X’s mark active ✖ Three Sisters subduction-related volcanoes. ✖ Mazama (Crater Lake)
✖ Mt. Shasta
4 Transform Faults There’s only a few places in the world • At the plate boundaries we’ve mentioned, where transform plates are moving either towards each other faults cut across land, or away from each other and this is an aerial • Transform faults occur when plates move photo of what is sideways with respect to each other probably the best- • Transform faults connect two spreading known. . . the San Andreas fault, seen in zones, two subduction zones, or a spreading this aerial photo. zone with a subduction zone.
“Hot Spots” The San Andreas • Some points in the asthenosphere are hotter fault connects spreading zones in than others. the Gulf of California • As a continent moves over a hot spot, a with zones in the chain of volcanoes is formed. . . north Pacific. Coastal • . . . but this time the chain is parallel to the California and Baja direction the continent is moving, not California are slowly perpendicular like a true island arc. sliding northwest • At any given time, the newest volcanoes are relative to the rest of North America. right over the hot spot, with older ones progressively farther away.
5 The Hawaiian chain is much longer than the islands in the Diagram of Hawaii, a classic “hot spot” State of Hawaii. Its “dogleg” results from the Pacific Plate volcanic chain changing direction as it moved over the hot spot.
Hawaii is also home to a great diversity of Recall that the youngest Hawaiian Islands are over a mid- Drosophila (fruit fly) species—here are three out of oceanic “hot spot”, and the older ones are farther away. about 700 endemic species. . .
6 The same story may be told Prognathogryllus Notice how a cladogram of Hawaiian Drosophila mauka about Hawaiian crickets. . . Hawaii puna Equivocal spadix species mirrors the order of appearance of islands. . . kohala weli Island Formation waikemoi Maui kipahulu Migration kukui hana Time stridulans makai alternatus alapa opua Oahu kahea awili oahuensis elongatus epimeces hypomacron kahili parakahili alatus flavidus Kauai hea robustus pihea victoriae
. . . and Hawaiian spiders...... and Hawaiian silverswords!
Tetragnatha 5 Tarweed (California USA) OG T. pilosa 1 T. kauaiensis Dubautia paleata Kauai T. perreirai 6 4 Dubautia waialealae Kauai 4 T. tantalus T. kamakou 2 4 Dubautia laxa indigenous T. kamakou 3 4 3 3 Wilkesia gymnoxiphium Kauai 2 2 1 T. quasimodo T. waikamoi Wilkesia hobdyi Kauai T. quasimodo 3 Argyroxiphium caliginis Maui T. quasimodo 1 6 3 1 Argyroxiphium grayanum Maui 4 T. polychromata T. quasimodo http://www.entomology.cornell.edu/Faculty_Staff/Blackledge/tetragnatha.JPG Argyroxiphium virescens Maui 3 T. restricta 4 Argyroxiphium sanwicense M&H 6 T. macracantha T. restricta 1 2 Argyroxiphium kauense Hawaii 3 T. brevignatha N 4 1 T. brevignatha
3 T. stelarobusta 4 T. pallescens http://www.calflora.net/bloomingplants/images/redrocktarweed4.jpg
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