Development of plate tectonics theory illustrates some important principles of how science operates and the general development of scientific theories.
Plate tectonics history/development: 1. One common observation about scientific ideas illustrated by plate tectonics is that many good ideas are not really new. Instead they have been discovered and rediscovered several times and some are quite old. Ideas of continental drift had been around for several centuries before plate tectonics provided an encompassing theory of the Earth. 2. A good scientific theory often explains and connects many observations which were formerly thought to be unrelated. Plate tectonics provides an overall explanation of such phenomena as seafloor topography and sediment thickness; distribution of earthquakes and volcanoes; continental drift; and spatial and temporal episodes of mountain building. 3. Technology often leads to new observations which cry out for a new understanding and eventually lead to development of a new theory. The technologies of oceanography, paleomagnetism, and seismology were particularly important to development of plate tectonics.

Think, pair, share: Define scientific theory; how test?

Theory: Systematically organized knowledge applicable in a relatively wide variety of circumstances, especially a system of assumptions, accepted principles, and rules of procedure devised to analyze, predict, or otherwise explain the nature or behavior of a specified set of phenomena.

CONTINENTAL DRIFT

Fit between continents Fig. 12-5 Early champions of continental drift: Alfred Wegener: German Meteorologist published book on continental drift in 1912. He argued for existence of a former megacontinent Pangea which had broken apart to produce current configuration of continents. Killed on expedition to Greenland ~1920. Alexander du Toit: South African geologist developed many of Wegener’s ideas into formidable theory published in Our Wandering Continents in 1937. A man on a mission with detailed knowledge of geology of southern continents. A classic argument for continental drift is the fit of the continents surrounding the present Atlantic Ocean. But what would be the mechanism for continental drift? How would you plow the continents through the ocean basins which have hard crust?
Rock Evidence Fig. 12-6 Late Paleozoic and Early Mesozoic geological histories of southern continents show remarkably similar sequences of rock types.
Glacial Evidence Fig. 12-8 Episode of Late Paleozoic (Permian) glaciation evidenced in southern continents. Glacial deposits of Late Paleozoic age and glacial striations (scratches) on older rocks. Can work out distribution of glaciers AND directions of motion. Motions of these glaciers did not look like any modern glaciers which move from land to sea. Instead, with continents in present position, these glaciers appeared to move from the oceans ONTO land?? But when the southern continents are reconstructed into the Gondwana supercontinent, the glaciers can be seen to have moved out from a south pole located in southern Africa.

Fossil Evidence Fig. 12-9 Glossopteris: fern-like plant whose seeds cannot be carried by winds and would not survive transport in sea water. Also present climatic zones of areas yielding Glossopteris flora are highly varied. Mesosaurus: Late Paleozoic fresh-water reptile which cannot be expected to survive ocean swimming. Cynognathus and Lystrosaurus: Both land reptiles of Triassic Period not physically capable of serious swimming. So how did opponents of continental drift refute these arguments? They simply said that continental drift was mechanistically impossible. No mechanism meant that it cannot have happened. Coney’s first rule: If it happened, it’s probably possible.
APW Paths Fig. 12-10 Paleomagnetism provided records of the past positions of magnetic poles with respect to continents. If magnetic poles are aligned with geographic poles, these polar wandering paths require: 1. Continents have drifted over Earth’s surface during their history; they were at very different paleolatitudes than where they are now. 2. Continents must have moved with respect to each other; their present relative geography is not their paleogeography.
Magnetic anomalies Fig. 12-11 Oceanic crust is like the bar code on packages in super market; it has a magnetic record of the polarity of the Earth’s magnetic field at the time it was formed and cooled at spreading ocean ridge. We can remote sense the polarity of magnetism in oceanic crust by measuring small variations in Earth’s magnetic field at the sea surface. Match the pattern of magnetism in oceanic crust with the known magnetic polarity time scale. This is the speedometer of plate tectonics.
Think, pair, share, calculate: In-class extra-credit opportunity on calculating rate of sea floor spreading. Up to 3 points extra credit.
Age of world’s oceans Fig. 12-12 Mapping magnetic anomalies produces maps of ages of oceanic crust in ocean basins. 1. Young oceanic crust near ridges; progressively older with distance from ridge. 2. Spreading rates differ between ridges. East Pacific ridge faster than mid-Atlantic. 3. Oldest oceanic crust ~150 million years old. Oldest continental crust ~4 billion years. 150 million years Ö 4000 million years Å 0.038 < 4% of Earth history! Ocean basins are very young! Because of generation of oceanic lithosphere at ridges and subduction at trenches.
Supercontinent cycle Fig. 12-1.1 Cycle of making and breaking supercontinents. Pangea was supercontinent of Late Paleozoic and Early Mesozoic; broke up in Jurassic Period (~200 million years ago). Rhodinia was supercontinent of ~700 million years ago.
History of divergent plate boundary Fig. 12-15a,b,c,d Upwarp - Crustal extension and rifting - Begin sea floor spreading - Open an ocean This happened to eastern North America in Late Triassic time as North America began to break away from Africa to open the Atlantic Ocean. An early stage is happening to the Red Sea now. A very early stage is happening in the East African Rift now.
Ocean-Continent convergent zone Fig. 12-18 Elements of an ocean - continent subduction zone: Oceanic plate subducts down into mantle. Ocean bottom trench mark the location where the oceanic plate begins to subduct. Melting of portion of oceanic plate can generate magma which rises through the over-riding continental plate to produce a volcanic arc. Sediments scraped off the descending oceanic plate can pile up to yield high-pressure & low-temperature metamorphic rocks (blueschist). Can get compressional deformation of over-riding plate.
Continent-Continent convergent zone Fig. 12-19 Elements of an continent - continent subduction zone: Neither plate can subduct into mantle because of low density continental crust. A magmatic arc on the former over-riding plate will shut off when continents collide. May have trapped pieces of oceanic crust in the collision zone. Major compressional deformation of both plates in the collision zone. High plateaus may develop from thickening of continental crust.
Ridge-Ridge transform Fig. 12-20a Transform faults have plates moving parallel to fault boundary between plates. The transform fault "transforms" one boundary into another. A ridge into a ridge; a trench into a ridge, etc. Faulting occurs only between the features offset, so transform faults have beginnings and ends.
Trench-Trench transform Fig. 12-20b Draw it on the board, show how it works.
Ridge-Trench transform Fig. 12-20c Draw it on the board, show how it works.

Rates of plate motion Fig. 12-22 Mid-Atlantic spreading rate slower than East Pacific Rise. Large and frequent EQs and active volcanic arcs where convergence rate is large (Peru-Chile trench; Aleutians in Alaska, western Pacific subduction zones).

Hawaiian Islands Fig. 12-23 The hot spot game. Darwin noticed the aging of the Hawaiian Islands. Radiometric dating has proven that the age of the volcanic rocks does increase systematically from SE to NW. This increasing age pattern continues for the seamounts with tops under water to the NW.

Slab-pull & Ridge-push Fig. 12-25 What makes the plates go? Topography of the ridge gives the plate a push away from the ridge. "Ridge Push" High density of the cold subducting oceanic plate gives the plate a pull. "Slab Pull" Pacific plate moves fast because the ridge push on the SE side adds nicely to the slab pull on the NW side. So why does Earth have plates which are created at ridges and destroyed at trenches?