Transcript plate tectonics 2009..

Plate Tectonics
This Dynamic Earth
http://pubs.usgs.gov/gip/dynamic/dynamic.html
and the distribution of major landform features
I. The discovery of Plate Tectonics –
Continental Drift
 17-18th centuries: outlines of the continents
similarity of the shape of the coast lines of the
Atlantic Ocean
 19th century: geology of southern continents:
Africa, S. America, Australia, India
“super-continent – Gonwanaland”
I. The discovery of Plate Tectonics –
Continental Drift
 1915
Alfred Wegener (German scientist)
All the continents were jointed together
“Pangaea”
The single ocean “Panthalassa Ocean”
- For reasons yet unclear:
200 m.y.a. Cracks began to divide Pangaea
into 2 supercontinents,
north: Laurasia south: Gondwanaland
They then began to be broken apart by
additional cracks, finally the continents today
I. The discovery of Plate Tectonics –
Continental Drift
I. The discovery of Plate Tectonics –
Continental Drift
 Many geographers and geologists did not
support Wegener’s theory at that point
 as many points not adequately explained
 The idea of continental drift took on new
meaning in the 1960s and 1970s
 when geophysical evidence from
palaeomagnetic and ocean floor studies
supports the theory.
 The idea was combined in plate tectonics, the
leading theory now.
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 1. Ancient Ice Sheet
 2. Fossils
 3. Sea-floor Spreading
 4. Palaeomagnetism
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 1. Ancient Ice Sheet (discovered in late 19th C)
- Evidence suggesting that an ice age existed about 200
million years age (~ same age) in 3 continents in the
northern hemisphere – South America, Australia and
Africa.
Rock outcrops with a smooth appearance and a series
of parallel grooves, probably made by a huge mass of ice
of about thousands metres thick moving very slowly over
solid rock.
Glacial deposits can also be identified, looking quite
different from fluvial deposits by rivers and streams.
What does the direction of the glacial grooves tell us?
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 2. Fossils
- Presence of identical fossil plants on different continents
separated by hundreds of kilometres of ocean.
- Similar rock layers also occur on different continents
Could it be the strongest kind of evidence to support
the theory?
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 3. Sea-floor Spreading
- The discovery of a mid-oceanic ridge
system some 40,000 km long extending
through the major ocean basins
- Observations: rate of sea-floor spreading
~ 2 – 18 cm / yr
- New oceanic crust is continuously being formed as the
molten materials rises to fill the cracks, solidifies and
forces it way out in opposite direction from the ridge
- Could it be the strongest kind of evidence to support
the theory?
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 3. Sea-floor Spreading
Evidence of sea-floor spreading?
a. Fossils showing the age of rock to increase from the
mid-oceanic ridges outwards
b. Increasing thickness of marine sediment away from the
mid-oceanic ridges, absence of sediment at the ridge
crest
c. Arrangement of alternate reversal polarity pattern found
at either side of the mid-oceanic ridges – almost
symmetrical in rock bands
d. Alignment of volcanoes of different ages from hot spots
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 3. Sea-floor Spreading
- The concept is defended successfully only if:
a. the earth is expanding (vs: + 2% in past 200 m.y.)
b. other surface material is somewhere returning to the
earth’s interior (vs: major bending only evident within
some mt. belts and ocean)
- How can we account for all of the newly formed crust?
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 3. Sea-floor Spreading
- How can we account for all of the newly formed crust?
- Some crustal materials is being “consumed” by
downward currents at the deep oceanic trenches
- At the oceanic trenches, the older crust sinks towards
the mantle, resulting in earthquakes and volcanic activity.
- What was the mechanism of spreading?
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 3. Sea-floor Spreading
- What was the mechanism of spreading?
- It seems probably that there is a convective movement
of material within the mantle.
- A series of convective cells, driven by internal energy
produced by decay of radioactive elements appears to
exist within the asthenosphere.
- So, the asthenosphere behaves in a plastic or semi-fluid
manner over a long period of time.
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 4. Palaeomagnetism
- The several reversal of the earth’s magnetic field in the
North and South Poles
- When molten rocks spread away from the central ridges,
rocks with metallic materials would adopt a magnetic
orientation reflecting that of the earth’s magnetic fields.
I. The discovery of Plate Tectonics –
Continental Drift
 Evidence
 4. Palaeomagnetism
- The transect across the ocean floor suggests that:
a. palaeomagnetism of rocks varies as if the ocean floor
had been formed at different times.
b. The pattern on either side of the mid-oceanic ridge:
almost identical!
II. The Theory of Plate Tectonics
 Introduction
 Tectonics means the study of tectonic activity.
 The study of deformation (e.g. breaking & bending) of
earth materials and the structures that result from
deformation.
 It is the study of geological activity on a global scale.
II. The Theory of Plate Tectonics
 History - 4 major scientific developments
(1) demonstration of the ruggedness and youth of the
ocean floor
(2) confirmation of repeated reversals of the Earth
magnetic field in the geologic past
(3) emergence of the seafloor-spreading hypothesis and
associated recycling of oceanic crust
(4) precise documentation pointing to the concentration of
the world's earthquake and volcanic activity along
oceanic trenches and submarine mountain ranges
Further reading: http://pubs.usgs.gov/gip/dynamic/developing.html
II. The Theory of Plate Tectonics
 The development
 Plate tectonoics is a scientific concept developed in the
1960s to explain the pattern of the earth’s structural
components and the mechanisms by which they were
formed.

The lithosphere is broken into segments called “plates”.
They float upon the soft and plastic asthenosphere
below.
II. The Theory of Plate Tectonics
 Basic assumptions
 1. Large areas of the earth’s outer portion act as a rigid
caps on a sphere. These rigid plates undergo no
significant internal deformation.
2. Each plate is in relative motion with respect to the
other plates on the surface deformation only at their
boundaries with each other.
[except for surface geologic processes, most
geologic activity is concentrated along the plate
boundaries.]
II. The Theory of Plate Tectonics
 Plate movement
 Due to convection currents generated by heat from
the centre of the earth
 Plates movement: moving towards, away from or
sideways along adjacent plates
 It is at plate boundaries that most of the world’s
major landforms occur, and where earthquakes,
volcanic and mountain zones are located.
II. The Theory of Plate Tectonics
 Plate movement – the rules
 1. Continental crust does not sink due to its
relatively low density
 2. Continental plates may consist of both continental
and ocean crust, e.g. Eurasian Plate
 3. Plates do not overlap
 4. No ‘gap’ may occur on the earth’s surface.
 5. New oceanic crust being formed at one place,
elsewhere older oceanic crust must be destroyed
 6. Plate movement is slow but is usually continuous.
 7 Most significant landforms are found at plate boundaries.