Geology 101 - Gale Martin - Class Notes
History Behind Plate Tectonics Theory
To understand the theory of plate tectonics, it's best to know the history and development of the idea. The theory was developed through many years of scientific study and 'arguments' (scientific discussions).
Several geologists, from many different continents, had commented on the similarity of rocks, fossils and structural geology through geologic time. In the early 1900's, Alfred Wegener published a book comparing and summarizing the evidence into one hypothesis called Continental Drift.
Several pieces of evidence support the concept of continental drift. One obvious line of evidence is the external outline of the continents. Over the centuries, many explorers and scientists had commented on the similarity of the coastlines (especially South America and Africa). Wegener placed all the continents together into one large continent, which he called Pangaea. He noted now the scientific evidence of rock and fossils supported a single landmass. Mountain ranges and their structural features matched between South Africa/Argentina and eastern North America/ Greenland/Great Britain and Norway. Late Paleozoic/Early Mesozoic rock types, typically developed in distinctive climatic zones (glacial deposits, coal beds and desert sands), seem randomly situated with the present configurations for the continents. When Pangaea in 'reunited', distinctive climate zones with a single equatorial region is evident. Plant and animal fossils for species of very specific regions (land based or climatic restrictions) also form distinctive patterns within Pangaea. With the breakup of the continent, the fossil patterns diverge and adapt to new climatic zones on separate continents.
Though the evidence collectively pointed to the existence of a single continent, the hypothesis was greatly opposed. Wegener had envisioned the continents breaking apart and pushing along the ocean floor, scraping up mountain ranges along the leading edge of motion. The mechanism for how and why the continents moved caused the greatest opposition. Support for the idea would have to wait for evidence from the ocean itself.
Sea Floor Spreading
During World War II, evidence from oceanographic studies reveled more information than military strategies. It became evident that the ocean floor was not a flat featureless region: there were trenches, long mountain ranges and individual sea mounts scattered throughout the ocean basins. In 1962, Harry Hess published the idea of sea floor spreading. He postulated that the features on the ocean floor were created by upwelling magma released as the crust separated along mid-oceanic ridges. As the new crust is developed along the ridge, old crust is subducted at deep ocean trenches. (Evidence to supported this would later come from seismology in the form of Benioff Zones.) Thus the ocean crust was constantly being consumed and regenerated.
Paleomagnetics, a field developed in the 50's, supported Hess' idea. Mafic lavas, as they cool, preserve the orientation of the Earth's magnetic field. Vine and Matthews noted that there have been reversals of the Earth's magnetic field throughout geologic time. A distinctive pattern of magnetic stripes is evident along the ocean floor. This pattern is centered along ocean ridge systems and evenly reflected on both sides of the ocean basin. The pattern must be created as the crust cracks and splits, pulling apart at the mid-ocean ridge.
In 1968 the Deep Sea Drilling Project (DSDP) began exploring the ocean floor using the ship Glomar Challenger . DSDP supplied evidence that the ocean floor is basaltic in composition (i.e., a volcanic origin). The youngest basalt occurs along the ocean ridge; it becomes progressively older as the distance from the ridge increases. The oldest basalt found, located along the continental edge, was approximately 250 million years old. Overlying sediment confirms the age trend for the basalt; sediment is thicker further away from the ridge system (older the basalt has accumulated more 'dust'). It became evident that sea floor spreading was, in fact, happening.
The process of sea floor spreading supplied an appropriate mechanism for continental movement. The continents did not physically 'push' their way across an ocean floor but, instead, 'hitched a ride' along with the ocean crust as it spread apart. In the late 1960's this idea was coined: Plate Tectonics.
Plate Boundary Configurations
The lithosphere is broken into many pieces referred to as plates. Geology in the interior of the plates is relatively inactive. The edges of the plates, where they interact with one another, is where the major geologic activity occurs. The shifting and sliding of plates causes earthquakes, volcanic activity and various types of faults and mountain building events.
The mechanism for motion is still under study. It is believed that the heat in the mantle causes convection in the plastic asthenosphere. Hot material slowly rises and pushes against the rigid lithosphere, cracking it. The plates are pushed or dragged away as the hot material spreads out when it reaches the lithosphere. When the material cools, it sinks, potentially dragging the plate downward into the mantle. In this fashion, ocean floor is created and destroyed, while continents are geologically altered as they pass over various 'convection cells'.
The styles of tectonics are commonly grouped according to the type of stress found. Where plates are pulled apart they are referred to as divergent in nature. Collisions are produced along converging zones and transform motion in produced in regions of shear. (Refer to your text for drawings of each. This course will remain very basic in nature.)
Divergent Plate Boundary Zones
Divergent Plate Boundaries occur where upwelling mantle physically rips the crust apart. This can begin within a continent (ex.: East Africa, Pangaea) where tensional forces extend and thin the crust. Long linear valleys, known as rift zones, are created as pieces of crust drop along normal faults. Any crack that extends into the asthenosphere acts as a conduit for the hot rising fluids beneath. Thin veneers of mafic rich lavas cover the rift valley floor. As the plates continue to diverge, the crust drops low enough that the ocean eventually floods the region producing a long linear sea (ex.: Red Sea, Gulf of Aden). Given sufficient time, the rift zone will enlarge and form an ocean (ex.: Atlantic Ocean). Along the spreading center, the newly formed basaltic ocean crust is hot and buoyant, resulting in a raised mid-ocean ridge. As the crust pulls away from the ridge, it cools and sinks forming a deep ocean basin. Divergent Plate boundaries are the regions where ocean crust is made.
Geology within rift zones consists of block faulted mountains. Fissure eruptions of basalt are common (ex.: Iceland). When the eruptions occur underwater (ex.: mid-ocean ridges), hydrothermal alteration of the sea floor produces rich mineral deposits. Earthquakes are commonly shallow and volcanic in nature.
Transform Plate Boundary Zones
When the lithosphere cracks along divergent ridges, the break is not smooth and straight. Offsets occur between segments of the ridge system. These areas are known as Transform Plate Boundary Zones. Here the plates slide past one another in a shearing motion. Geology along transform zones is usually restricted to earthquake activity. As the two spreading ridges pull apart, shallow earthquakes occur along the stressed offset zone. Transform Boundary Zones in continental crust (ex.: the San Andreas Fault Zone) produce larger earthquakes due to the length of the fracture and the complexity of the crust it involves.
Convergent Plate Boundary Zones
In regions where the cold convecting material sinks into the mantle, plates collide and may be dragged into the Earth's interior. The types of geology that occur along these Convergent Plate Boundary Zones will depend on the types of crusts involved in the collision. Three combinations can occur: ocean-ocean, ocean-continent and continent-continent.
Ocean-Ocean Convergent Zones
As two plates whose leading edges are oceanic collide, one of the plates gives and is pushed beneath the other plate. This process of subduction is evident by the trace of earthquakes that occurs, known as the Benioff Zone. Earthquake activity is shallow along the deep oceanic trench formed at the site of collision. Foci depth increases at an angle into the interior of the Earth. It is assumed that the trace of foci shows the descending slab of rock being pushed into the asthenosphere. As the slab descends into the subsurface, it is pushed into regions where it begins to partially melt. (Remember: different minerals melt/crystallize at different temperatures. Review Bowen's Reaction Series.) The magma produced is more intermediate/felsic in composition and as it rises may also be altered by assimilation. A line of andesitic/rhyolitic volcanoes, known as an island arc, will be produced on the overlying lithosphere above the deep seated earthquakes (ex.: Japan, Aleutian Islands). Sediments deposited on the ocean crusts will be folded and thrust onto the colliding plate forming complex folded mountains.
Ocean-Continent Convergent Zones
During an ocean-continent collision, the plate with the oceanic leading edge will be subducted. This occurs because ocean crust is denser than continental crust. Ocean crust therefore sinks, while continental crust remains 'floating'. Once produced the only way to 'destroy' continental crust is through erosion. The geology along an ocean-continental collision is similar to an ocean-ocean collision to some degree. With the subduction of the oceanic slab, Benioff Zone earthquakes are produced. The earthquakes occur as a slanted zone that becomes progressively deeper toward the interior of the continent. A rhyolitic/andesitic volcanic arc is produced above the melting slab (ex.: Andes, Cascades). Often the magma is more felsic in composition due to the thickness of the lithosphere it must travel through to reach the surface. Felsic magmas tend to be thicker in character and, therefore, may get 'stuck' beneath the surface. Large granitic batholiths are common along ocean-continent collision zones. The edge of the continent goes through more structural changes: mountains with major thrust faults and complex folds are common (ex.: Rocky Mountains).
Continent-Continent Convergent Zones
Collisions between to plates with continental leading edges produce no subduction. Both plates are buoyant and refuse to be subducted. Earthquakes are shallower in character (no Benioff Zones) and confined to the lithosphere in depth. The collision produces a large complex of folded, faulted and thrusted rock with little, if any, volcanism (ex.: Himalayan and Appalachian Mountains).
Plate convergence in often an 'evolutionary' process. With the advent of ocean-ocean subduction, a small island arc is produced on the surface. This land formation is felsic in composition and cannot be subducted. If it becomes involved with a collision, it will act as a small 'micro-continent'. This means that is will either 'suture' itself to another continent (via continent collision) or act as the nucleus to another continent. Continents grow by accretion, the 'suturing' of small pieces through several collisions (ex.: interior of North America). As the continental grows larger it becomes known as a craton. It's interior regions become geologically 'inactive' and only the edges are altered by collisions. When two large continents collide (ex.: Asia with India) the collision results in the end of convergence at that boundary. The convergence will shift to a region along the coast where oceanic crust will 'give' and subduct. This process continues until the next collision occurs or the continent shifts off the convection cell deep in the mantle interior.
Throughout Earth's history the surface of the plate has been altered and changed by the movement, collisions and shifting of the lithospheric plates. The current shape of the continents is only a brief configuration for the present time. The Earth's surface is destined to be changing and evolving as the Earth's internal forces shape the land surface.