harry williams, earth science1 lecture 11. plate tectonics; diastrophism continental drift the...

Harry Williams, Earth Science 1

LECTURE 11. PLATE TECTONICS; DIASTROPHISM

Continental DriftThe theory of continental drift formulated by Alfred Wegener in the early 20th century is the forerunner to the modern theory of plate tectonics. It was based on the observation that many continents "fit together like a jigsaw puzzle" (e.g. S. America and Africa) and,if joined back together, many similarities exist in rock types, geologic structures and plant and animal fossils.


North and South America, Europe and Africa all fit together like pieces of a jigsaw puzzle. Mountain ranges, rock types and fossils also match up.


PANGAEA

Wegener proposed that 200 million years ago all

continents were joined together in a supercontinent he

termed PANGAEA (“All land”, consisting of N. America

and Eurasia to the north - LAURASIA; and the southern

continents to the south - GONDWANALAND). Pangaea

broke apart and the continents drifted, like icebergs in

the sea, "bulldozing" their way through the denser

oceanic crust below.


Pangaea

The continents today


There were two major criticisms of Wegener's

theory - firstly, no explanation was given for the driving

force behind continental drift, and secondly, the oceanic

crust is far too dense and rigid to allow the continents to

"push" their way through it. The theory was largely

dismissed. The explanation for the drifting of the

continents had to wait until the 1940's and 1950's when

exploration of the ocean floors (offshoot of submarine

detection) resulted in the new theory of plate tectonics.


PLATE TECTONICS Recap: the upper 70 km of MANTLE and CRUST = the rigid LITHOSPHERE; beneath this is the partially molten ASTHENOSPHERE.


The lithosphere is not continuous, but is fractured into a number of LITHOSPHERIC PLATES (about 7 major plates). The plates are moving, as shown by the arrows, at typical velocities of 1 -10 cm/year (fingernail growth). It is the movement of these plates and the interaction between them that is known as PLATE TECTONICS.


SEA FLOOR SPREADING BY CONVECTIONThe exact driving force of plate motion still isn't certain, but the most likely explanation is that the plates are driven by convection currents in the asthenosphere. Plates spread outwards from mid-oceanic ridges where new ocean floor is created, and return into the earth at subduction zones were plates sink and melt.

Mid-oceanic ridge Subduction zoneSubduction zone

Convection

Sea-floor spreading


Much important geologic activity, including earthquakes

and volcanoes, is found at

PLATE BOUNDARIES

3 types:

Divergent – plates move away from each other.

Convergent – plates move towards each other.

Transform fault – plates slide past each other.


Divergent. Mid-oceanic ridges; shallow earth quakes, volcanic eruptions -> new sea floor consisting of BASALT (magma from asthenosphere forms basalt). Plate movement is initiated at mid-oceanic ridges; continents are carried along on top of plates; supporting evidence from "hot spots".


A hot spot is a stationary plume of rising magma from the Mantle, as this periodically erupts through the overlying plate, a chain of volcanoes is formed in the direction of plate motion.

E.g. Hawaiian islands


Convergent. Subduction zones; earthquakes occur down the subduction zone; volcanism from melting of subducted plate e.g. Cascades, Mt. St. Helens. Possible collisions include: ocean plate - ocean plate > island arcs e.g. Japan, Aleutian Islands. Ocean plate - continental plate collision > volcanic arc e.g. Cascade Range; Andes. Continent-continent collision > mountains e.g. Himalayas.


Island Arc. E.g. Japan. Earthquakes take place down the Wadati-Benioff zone (part of subduction zone where earthquakes occur).


Volcanic Arc: Cascade Range.

Juan de Fuca plate


Continent-continent collision; suture zone.


Transform fault - plates slide past each other; large earthquakes are common e.g. San Andreas fault.


1989 Loma Prieta earthquake.


Distribution of magnitude 5+ earthquakes, 1980-1990. The pattern closely matches plate boundaries, particularly subduction zones and transform faults.


Distribution of major volcanoes. The pattern closely matches plate boundaries, particularly subduction zones. Note the Pacific “Ring of fire”.


DiastrophismCentral to the theory of plate tectonics is the idea that the crust is subject to both vertical and horizontal movements. These movements can deform the crust by tilting it, breaking it and bending it.

Tilted Beds: angle of tilt is the dip - ranges from 0 (horizontal) to 90o (vertical). Orientation of the outcrop is the strike e.g. north-south, east-west etc.


Steeply dipping rock strata.


Folded Beds: Rocks at depth under great pressure tend to bend rather than break when subjected to stress, producing folds. Main types = syncline, anticline, monocline, overturned fold. Folds are often exposed at surface by erosion of overlying rocks, typically ridges and valleys are formed e.g. Appalachians.


The ridges and valleys of the Appalachians are exposed folds.


Faults: Brittle rock (e.g. near surface) tends to break rather than bend when subjected to stress -> fault.


(Same as a transform fault)


Sometimes, the vertical movement of rock in a fault can result in a cliff-like feature on the surface of the Earth - a FAULT SCARP.


Blocks of crust can be uplifted and depressed along faults forming GRABENS and HORSTS. e.g. Death Valley is a graben surrounded by horsts on either side.


Death Valley, California. The flat valley floor occupies a graben and is filled with sediment. The horst rises abruptly on the far side of the valley.

harry williams, earth science1 lecture 11. plate tectonics; diastrophism continental drift the...

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