- Theory that earth’s lithosphere is broken into “PIECES” or “PLATES” that are in motion over underlying plastic ASTHENOSPHERE

Why study PT?
- Has led to many tragic events
e.g.,  Two tragic events in 1985 related to PT

1. September 19, 1985: Earthquake killed more than 9,000 people in Mexico City.
2. Two months later, just 3,200 km to the South, a VOLCANIC ERUPTION: NEUADO DEL RUIZ killed more than 23,000 people

- These two events occurred in the “Ring of Fire”.
- A zone of intense seismic and volcanic activity around the PACIFIC OCEAN BASIN.

- CONTINENTAL DRIFT:
- Began in 15th century by Leonardo da Vinci (Italian).
- Supported by many others till 1910 when Frank B. Taylor (American Geologist) joined.
- However, Alfred Wegener (1912) – German Meteorologist is accredited with hypothesis of "Continental Drift”.
- He proposed:  All landmasses were once a SINGLE SUPERCONTINENT – “PANGEA”
- Present continents have broken up from PANGAEA & moved apart since then.
- He used a good amount of data:
a. GEOLOGICAL  b. PALEONTOLOGICAL c. CLIMATOLOGICAL

1. GONDWANA:  Southern Landmass
- Australia, Africa, Antartica, South America (+ India from the north)

2. LAURASIA: Northern Landmass
- North America, Greenland, Europe, Asia (except India)

Evidence for Continental Drift
 

1. Fit of shorelines of Continents:
- A close resemblance observed between coastlines of continents on opposite sides of the Atlantic Ocean (South America & Africa)

2. Similarity of Rock Sequences & Mountain Ranges:
- If continents were together, rocks and mountain ranges of same age in adjoining locations, on opposite side continents should closely match
(This was the case)
   a. Rock sequences of Pennsylvania and Jurassic age are similar for all 5 GONDWANAN continents: S. America, Africa (India), Australia, Antartica
   b. The folded Appalachian Mountains of N. America trend through Eastern Us & Canada, & terminate abruptly at Newfoundland coastline
- These mountains are found again in Greenland, Ireland, Great Britain & Norway
 

3. Glacial Evidence:
- Similar evidence of glaciation found in all continents of the GONDWANA (except Antartica)
- Evidence will not be similar if continents were not together.

4. Fossil Evidence:
- Fossils are preserved remains of plants & animals
   a. Fossils of GLOSSOPTERIS (floral plants) are found in equivalent Pennsylvanian & Permian Age coal deposits of all 5 continents of the GONDWANA
   b. Fossils of:
     i. MESOSAURUS, a freshwater reptile found in Permian age rocks in Brazil & S. Africa (NO WHERE ELSE  in the world)
     ii. CYNOGNATHUS in S. America & Africa only
     iii. LYSTROSAURUS in Africa, India & Antartica

5. Paleomagnetism & Polar Wondering:
- In 1950s geologists studied the earth’s magnetic field – They discovered
  a. ocean basins are geologically young
  b. the continents have indeed moved in the past
 

- In 1950s, paleomagnetic studies were conducted to map ocean basins. It was discovered that:

a. MIDATLANTIC RIDGE was part of World-wide oceanic ridge system, more than 65,000 km long(as above)

b. Oceanic Ridges are characterized by the following:

i. high heat flow
ii. basaltic volcanism (upwelling of basalt)
iii. Seismicity

c. A series of negative and positive magnetic anomalies were recorded to show even spread of the ocean bottoms

- Conducted by Harry H. Hess of Princeton University while serving in the World War II.
- Hess discovered and studied flat-topped submarine volcanic island (Guyots)
- Based on these evidences, HESS proposed the hypothesis of “SEA FLOOR SPREADING” in 1962.  He stated:
- Hot magma upwells from the  mantle at Mid-Oceanic Ridges (MOR)
- Create new oceanic crust at MOR
- Old crust is then pushed farther away from MOR on both sides of spreading
- Cold (Old) crust is SUBDUCTED back into mantle at deep-sea trenches where it is heated and recycled.

PLATE TECTONICS THEORY
 

- Based on simple model that lithosphere (crust + upper mantle) is broken into “pieces” or “plates”
- Plates vary in size and thickness
Upto 250 km thick (in continental crust) and 100 km (in oceanic crust)
- Plates move over HOTTER, WEAKER & PLASTIC Asthenosphere
- As they move over asthenosphere, they
a. Separate
b. Collide & are subducted back into the mantle at oceanic trenches
- Plate tectonics unifies most of the earlier theories of CONTINENTAL DRIFT and SEA-FLOOR SPREADING
- It explains most processes relating to earthquakes, seismicity, volcanism and mountain building, etc.

An important and most acceptable theory

PLATE BOUNDARIES
- Plates move relative to one another, & have 3 different types of boundaries

a. Divergent Boundary
b. Convergent Boundary
c. Transform Boundary

    Divergent Boundary:

- Oceans where plates are spreading & new oceanic lithosphere is forming
- Occurs mainly at ocean ridges e.g., Mid Atlantic Ridge
- Rugged topography with high relief
- Shallow focus earthquakes
- High heat flow
- Basaltic flow
- May also occur under continents at early stage of continental break ups

    Convergent Boundary:

- Where plates come together or collide
- Old oceanic crust is destroyed
- Subduction zone
- Area of many earthquakes (3 types)
- Oceanic – Oceanic
- Oceanic – Continental
- Continental – Continental

Oceanic – Oceanic Con. Boundary
- One plate subducts beneath the other
- As one plate subducts, it descends in asthenosphere, gets heated & partially melted to generate magma of ANDESITIC COMPOSITION
- Less dense than surrounding rx, thus it rises P (????) through rx to form VOLCANISM & Volcanic Island Arcs.  Japan, Philippines Island Arcs.
- Scotia & Caribbean  Islands
 

Oceanic – Continental
- Oceanic & continental plates meet
- Denser oceanic plate is heavier and there descends under continental plate.
- Volcanic arcs are formed
e.g., Pacific Coast of South America

Continental – Continental
- 2 Continental plates meet
- One plate eventually goes down below the other.  But confusion because of struggle for plate that goes down
- Lots of dangerous activities
e.g., collision between INDIAN & ASIAN plates ->form the HIMALAYAS

TRANSFORM BOUNDARIES
- Occur where plates slide past each other.
- At boundary, lithosphere is neither created or destroyed.
- Intense shatter of rock
- Numerous SHALLOW – focus earthquakes
e.g., the San Andreas Fault in California
è Separates the Pacific plate & North American Plate
- Home of many earthquakes affecting California today

HOT SPOTS

Plate motion is studied from magnetic reversals, satellite & lasers
 

- Distance is relative between plates
- Don’t measure absolute
- To measure absolute distance moved by plate – use HOT SPOTS
 

- Are places where plates are originally created
e.g., HOT SPOT are found in HAWAIIAN ISLAND CHAIN

DRIVING MECHANISM OF PLATE TECTONICS

- Geologist believe that PT is kept alive by
1. Convection cells
- in Asthenospher
- Asthenosphere + remain part of mantle
2. Most recently other driving forces proposed by geologist
 

Many metallic mineral deposits, e.g. Cu, Au, Pb, Tin, Zinc are all related to igneous rx & magmas.

- Magma is generated by partial melting of rx from mantle.  These precipitate & concentrate METALLIC SULFIDE ORES.

- Major (many) ore deposits in the world are found in CONVERGENT plate boundaries
- In the Andes, Rockies, Coast Ranges of S & N America, Japan, Philippines, Russia & Pakistan.
- Majority of world’s GOLD is in CONVERGENT plate boundaries
 

>>>>In Sulfides in South Africa, Canada, California, Alaska, Venezuela, Brazil, S. India, Russia, Western Australia -> JOHANESBURG
>>>>Cu (Copper) in N & S America

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