CONTINENTAL DRIFT AND PLATE TECTONICS

DRIFT AND PLATES• Continental drift concept

• Introduction• did continents originate at their present

locations or did they move to where they are today?

• drift concept supplied information to aid in the formulation of rock plate or tectonic plate concept

• a solid lithosphere floating on top of the asthenosphere gives an idea of how plates can float

• Accretion of continental material took place

DRIFT AND PLATES• convection cells in Earth’s interior is the

force which split the lithosphere and are the driving force in continental separation

DRIFT AND PLATES Accretion of Continental Materials during Drift---Western USA

DRIFT AND PLATES• who was the first to observe this map fit? • Alfred Wegener was the first to publish a

summary of ideas about continental drift in 1912.

• some ideas in the publication were: 1) the protocontinent or supercontinent was named Pangaea; 2) southern section was named Gondwana or Gondwanaland and northern section, Laurasia; 3) the protocontinent broke in pieces about 150-200 million years ago

DRIFT AND PLATES• Some important data confirming continental

drift• map fit--already mentioned• mountain chains formed on continents with

longitudinal axes perpendicular to movement of the continents--examples: Himalayan, Rocky Mountain and Andes Mountain chains

• same fossils (in rocks),same rocks , mountain ranges, and glacial features located on different continents in areas representing prejoined positions prior to continental drift

DRIFT AND PLATES• Initial ideas of continental drift

• jig saw puzzle or map fit of continents

Matching of Fossils from Common Locations on Separated Continents

Matching of Mountain Chains from Common Locations on Separated Continents

Matching of Glacial Deposits from Common Locations on Separated Continents

DRIFT AND PLATES• presence and shape of global ocean ridge

• a ridge is a raised region on ocean basin which is believed to represent prejoined or splitting area of separated continents--the shape of the ridge contours shape of coast lines of separated continents--- this is most evident in the mid Atlantic ocean

Global Ocean Ridge

DRIFT AND PLATES• seafloor spreading

• age of rocks located on equivalent positions on both sides of the ridge are the same--rocks progressively increase in age away from the ridge

• magnetic intensities in the rocks are found to be the same on equivalent sides of the ridge--on both sides of ridge magnetic intensity alternates between normal(high intensity) and abnormal(low intensity)

DRIFT AND PLATES

Magnetic Intensities and Sea Floor Spreading

DRIFT AND PLATES• Lithospheric plates and plate tectonics• where are the spreading continents going?• the answer was important in initiating the rock

plate concept• rock sections or plates which diverge from

spreading zones move towards or converge on subduction zones

• Rock plates• ocean basin and continental sections float in the

asthenosphere, some plates diverge, some converge and some move parallel (to each other)

DRIFT AND PLATES• Classification of rock plate boundaries

• volcanism and seismic activity are abundantly associated with plate boundaries

• boundaries are classified as divergent, convergent, or transformed boundaries

• divergent boundary--includes all ridges and rifts--tensional forces caused by convection cells drive the plates apart

DIVERGENT PLATE BOUNDARY

DRIFT AND PLATES• kinds of divergent plate boundaries--1)ocean

basin to ocean basin boundary;

example is the mid ocean ridge

DRIFT AND PLATES• 2) continental to continental boundary

DRIFT AND PLATES• convergent boundaries--includes all

subduction boundaries--compressional forces caused by convection cells drive plates towards each other--volcanic arcs are associated with convergent boundaries

Convergent Plate boundary

3 types of convergent plate boundaries

1) Ocean basin to continental boundary

(Example is the Cascade Mountain chain)

2) Ocean basin to ocean basin boundary

(Examples are Aleutian Islands and Japan)

Aleutian Islands and Japan

3) Continental to continental plate boundary

(Example is India and Asian continent boundary)

DRIFT AND PLATES• transformed boundary--plates move

parallel to boundary

Example are those off the Coast of Western U.S.A.

DRIFT AND PLATES• Major Plates on Earth and Movement Rates

Movement rates are about as fast as your fingernails grow