THE THEORY OF PLATE TECTONICS Mr. Halfen Jan. 2016

What is a theory? theory - a scientific concept which explains a large range of observations and experiments and can be used to make predictions This is vastly different from the “everyday” use of the word where it is used to mean a guess or hypothesis.

Wegener’s Evidence Wegener’s evidence for continental drift: the general fit of the continents to one another the continuation of mountain ranges across continents Large bodies of granite are found in adjacent locations in So. America and Africa Large deposits of coal are found in adjacent locations in No. America and Europe

Wegener’s Evidence Wegener’s evidence for continental drift: the distribution of fossil ranges across continents the distribution of glaciation across the continents Fossil plants are found in the Arctic Ocean

Review - Continental Drift Wegener’s Theory of Continental Drift stated that the continents were once joined in a large landmass called Pangea. His theory explained numerous geologic and geographic observations It was rejected because Wegener could not supply a reasonable means to move continents across the Earth.

Volcanoes Volcanoes erupt melted material from inside the Earth. Volcanoes are not distributed randomly; they are found near ocean ridges, deep ocean trenches and “hot spots” where they make chains of volcanic islands.

Hot Spots Hot Spots are locations of volcanoes that are long-lived and voluminous. Associated with many hot spots are chains of inactive volcanoes. The most famous chain is the Hawaiian Islands/Emperor Seamount chain.

Major Plate and Volcanoes

Earthquakes Earthquakes release energy from built-up stresses in the Earth. Earthquakes do not occur randomly across the surface of the Earth. They are found in the same places as volcanoes and along faults.

Deep Earthquakes In deep earthquakes, the earthquakes follow a pattern of going deeper into the Earth away from the deep ocean trench.

Earthquake Epicenters

Ring of Fire The Pacific “Ring of Fire” is the region running from New Zealand to Japan to Alaska to Chile where earthquakes and volcanoes are common.

Sea-floor Spreading The sea floor seems to be moving apart at mid-ocean ridges. Observation from sea floor explorations: Volcanic ridges are found on ocean floor The youngest rocks are found near the ridges and the oldest are furthest away. Ocean floor basalts record Earth’s magnetic field; the alternating polarity of the magnetic field show parallel and symmetric patterns on either side of the ridge. Deep ocean trenches are found near ocean/continent boundaries that have volcanoes.

Data Overload Given the vast number of observations, geologists began to re-visit Wegener’s theory. All the data seemed to fit his idea, but there still was no mechanism to move large sections of the crust.

Structure of the Mantle Highly sophisticated analysis of earthquake data revealed the existence of a plastic-like layer about 200 km down into the mantle. The mid-ocean ridges seemed higher than would be expected by their thickness. There had to be something occurring beneath them to raise their level.

Convection Geologists realized that the mid-ocean ridges were being elevated by upwelling magma. It seemed reasonable that the deep-ocean trenches were where the crust was being pulled down. It was tentatively concluded that convection cells were present in the mantle and the plates were moving with the convection cells.

Plate Tectonics With a mechanism for moving large bodies of crustal plates and the large volume of observational evidence, plate tectonics became the underlying theory in geology in the late 1960’s.

Theory of Plate Tectonics The lithosphere is broken into 7 major plates and numerous minor plates. Plates are composed of the uppermost mantle and oceanic and/or continental crust (aka, the lithosphere). These plates interact in predictable ways and produce many geologic phenomena. All of the observations cited in the previous slides, as well as the formation of mountain chains, are explained by this theory.

Plate Boundaries

Plate Boundary A plate boundary is where the two or more of the Earth’s plate come in contact

Divergent Boundaries Movement of Plates: away from each other Examples: Mid-Atlantic Ridge, East Pacific Rise, East African Rift Zone Features: graben, voluminous basaltic volcanism, shallow earthquakes

Convergent Boundaries – continent-continent Movement: towards each other Examples: Appalachians, Alps, Himalayas Features: long mountain chains, thrust faults, andesitic volcanoes, shallow & deep earthquakes

Convergent Boundaries – ocean-continent Movement: towards each other Examples: Andes, Appenines Features: mountain chains, thrust faults, andesitic volcanoes, shallow & deep earthquakes

Convergent Boundaries – ocean-ocean Movement: towards each other Examples: Japan, Phillipines, Aleutian Islands Features: thrust faults, basaltic and andesitic volcanoes, shallow & deep earthquakes

Strike-Slip Boundaries Movement: side-by-side in opposite directions Examples: San Andreas Fault, Alpine Fault in southern New Zealand Features: hills, shallow earthquakes, extensive faulting