THE THEORY OF PLATE TECTONICS The seafloor is constantly being made at ocean ridges and destroyed at subduction zones. As the seafloor is moved, the continents are carried along with it.

The theory of plate tectonics suggests that the outermost layer of the Earth is divided into a number of rigid pieces that move relative to each other. These pieces are called tectonic plates. Tectonic is another word that comes from Greek. The Greek word means relating to building or construction or architecture. The Earth’s surface is built out of these tectonic plates. And the building of new rocks, mountains and valleys occurs near the edges, or boundaries, of these plates.

There are 3 basic things that can happen where the edge of one plate meets another. 1. the plates can push against each other - convergent plate boundary 2. the plates can move apart - divergent plate boundary 3. the plates can slip past each other side to side - transform plate boundaries. Wherever plates grind against each other, you can expect earthquakes!!!

DIVERGENT BOUNDARIES The pressure from convection currents below the surface of a weak plate can break the plate. The 2 sides move away from each other. Molten rock cools creating new seafloor. This process creates mid-ocean and spreading ridges.

CONVERGENT BOUNDARIES There are 3 types of convergent boundaries: 1.Continental Plate vs Oceanic Plate The dense and heavy oceanic plate slides underneath the lighter and more buoyant continental plate. This creates a subduction zone. A narrow oceanic trench is formed beside a volcanic mountain range.

CONVERGENT BOUNDARIES (cont’d) 2. Oceanic Plate vs Oceanic Plate Remember that oceanic plates are born at midocean ridges where molten rock rises from the mantle, cools and solidifies. Little by little, as new molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created. The farther the plate gets from the ridge that created it, the colder and heavier it gets. When two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink. A subduction zone is formed.

CONVERGENT BOUNDARIES (cont’d) 3. Continental Plate vs Continental Plate When two huge masses of continental plates meet head-on, neither one can sink because both plates are too buoyant. At these boundaries solid rock is crumpled and faulted. Huge slivers of rock, many kilometres wide are thrust on top of one another, forming a towering mountain range.

CONVERGENT BOUNDARIES (cont’d) 3. Continental Plate vs Continental Plate An example is the Himalayas. This immense mountain range began to form between 40 and 50 million years ago, when two large landmasses, India and Eurasia, driven by plate movement, collided. Because both these continental landmasses have about the same rock density, one plate could not be subducted under the other. The pressure of the impinging plates could only be relieved by thrusting skyward, and forming the jagged Himalayan peaks.

TRANSFORM BOUNDARIES At transform plate boundaries plates grind past each other side by side. They are usually recognised by deep valleys and broken old stream beds. Earthquakes occur regularly in these areas.

Folding When continental plates collide, the rock of the Earth’s crust is subjected to extreme pressure both horizontally and vertically. Under these conditions rock acts like plasticine and begins to buckle and fold without breaking (scientists call this plastic behaviour). It can be folded to build mountain ranges or hills. The folded rock can form an arch (called an anticline) or a trough (a syncline) or may even fold over another fold (an overfold).

Identifying different boundaries The map below shows the three types of plate boundaries. Arrows indicate the directions that the tectonic plates are moving at the boundaries. Key

1. Look on the map for plates that are moving apart. Use a coloured pencil to trace along all these plate boundaries. Also colour the matching symbol in the key with the same colour. Beside the symbol, write ‘ocean ridges at divergent boundaries’. 2. Look for plates that are moving together. Trace along these plate boundaries using a second colour. Colour the matching symbol in the key with the same colour. Write the words, ‘subduction zones at convergent boundaries’. 3. Now identify the plates that are sliding past each other. Trace these plate boundaries using a third colour. Trace along these plate boundaries using a third colour. Colour the matching symbol in the key with the same colour. Write, ‘faults at lateral boundaries’. ACTIVITY USING “TECTONIC PLATES” MAP

Why do tectonic plates move? One explanation was suggested by Scottish geologist, Arthur Holmes, in He hypothesised that convection currents in the mantle pull the crust across the surface of the Earth. The hot material in the mantle rises upwards and the cold material sinks downwards.

However, scientists think that tectonic plates themselves are involved in the process too. They are not just pushed by convection currents. At ocean ridges, the plates are hot and magma pushes up between the plates forcing them apart. At trenches, an oceanic plate is cold and dense. Gravity pulls the plate edge down into the mantle, dragging the rest of the plate after it. This explanation of plate movement takes account of: ridges trenches convection currents gravity.

PLATES TECTONICS IN THE FUTURE Most plates on the Earth’s surface are moving at about 2 to 3 cm per year. This does not seem very fast. However, when you consider the long period of time involved, the continents have moved quite large distances. In the last 10 years, Australia has moved about 25 cm. Since the arrival of the First Fleet in 1788, Australia has moved five metres further to the north. Since the arrival of the first Aborigines, Australia has moved ten kilometres closer to Indonesia. Australia has moved km northwards since it first separated from Antarctica about 50 million years ago. The fastest-moving plates are the Indian-Australian, Pacific and Nazca Plates. These plates have long convergent boundaries (subduction zones) and long divergent boundaries (ocean ridges).

Plotting Ocean Trenches 1.Plot the ocean trenches on the world map using the coordinates in the following table. Draw a straight line between each plot point.

Decide which hypothesis is supported by each piece of evidence listed.