Ngā Whāinga Ako 🔗
- To understand the theory of plate tectonics and Wegener’s theory of continental drift.
- To describe the evidence to support Wegener’s theory.
Write the date, title and whāinga ako in your book.
What is plate tectonics? 🔗
Imagine that there is a pot of soup with pieces of bread floating on top. In this scenario the soup is Earth’s mantle and the bread is Earth’s crust.
Pātai: What might we observe as the soup boils?
Whakatika: As the soup boils the hot water rises from the bottom to the top and the bread will move around as those circular currents (convection currents) occur. The heated water will cool down and then sick back towards the bottom where it can be heated again, continuing the cycle.
Earth’s crust is the bread floating on the soup (the mantle). The mantle is being heated by the hot core of the Earth!
Pātai: If Earth’s crust is moving, why are we not normally aware of it? What situations are we aware of it in? Whakatika: This is because the movement is so slow that we cannot notice it. They move at approximately the same speed that our fingernails grow!
Pātai: What is the effect of the crusts moving? Whakatika: Three different types of plate boundary exist!
Divergent Boundary 🔗
- Where two plates move apart. New crust is formed from cooled mantle in the gap that they leave.
This is an example of a rift valley - these plates have been moving apart for millions of years.
Convergent Boundary 🔗
- Where two plates move towards each other. One plate sinks (subducts) into the mantle, and the lighter plate rises up forming a mountain range.
- The plate that subducts into the mantle is melted (destroyed).
These are the Southern Alps of New Zealand. They are an example of uplift at a complex plate boundary!
Transform (Conservative) Boundary 🔗
- Where two plates move past each other. No crust is created or destroyed in this type of boundary.
This is an example of conservative plate movement in Canterbury after the 2010/2011 earthquakes!
Question: How do the plate boundaries relate to areas of volcanic activity and earthquakes?
Who Discovered All This? 🔗
Alfred Wegener was a German, born in 1880 and died in 1930. He was a key figure in developing modern ideas around Earth’s structure and movement.
Pangaea 🔗
In 1912, Wegener proposed the idea the idea that the continents were once in a different location and once were all together in a super-continent called Pangaea.
He suggested that Pangaea started to break up around 200 million years ago and the pieces drifted apart to form the modern day continents.
Task/Ngohe: Add the breakup of Pangaea to your timeline of the history of Earth.?
Wegener was dismissed at the time by geologists for his theory of continental drift because he could not provide an explanation for how the continents were able to move. They had not yet learned about the mantle and the convection currents that drive it.
Evidence of Continental Drift 🔗
- Sea floor spreading
- The continent shapes seem to fit together
- Similar fossils have been found in South America and West Africa
- Dating and types of rocks are similar
- Climate similarities in ancient times
Ngā Whāinga Ako 🔗
- Use plate tectonics to explain why and where earthquakes and volcanic eruptions happen.
Mahi Tuatahi 🔗
Explore this website: http://dinosaurpictures.org/ancient-earth#240
- How did Earth look when life first arose?
- How about when the dinosaurs were around?
- And the first primates?
- Did NZ exist? Did it look as it does now?
Recap: What is the theory of plate tectonics? 🔗
- The explanation for how the continents move came from observations of seafloor spreading and other effects. In 1967, these ideas were linked in the theory of plate tectonics.
- According to this theory, the Earth’s crust is like a jigsaw puzzle made up of giant sections called tectonic plates.
- These plates ‘float’ on top of the mantle and so can move around the Earth’s surface.
- There are 10 major tectonic plates and several minor plates.
What is the Ring of Fire? 🔗
- It is about $40,000km$ of ocean trenches and volcanic belts where many earthquakes and volcanoes occur.
- Is has over 75% of the world’s active and dormant volcanoes.
- 90% of the world’s earthquakes occur on it.
- There are 10 major plates and several minor plates on Earth - many of them are involved in the Ring of Fire.
- Look at this URL to see a live earthquake map: https://seismo.berkeley.edu/seismo.real.time.map.html
NZ Tectonics 🔗
What plates does New Zealand straddle?
Where plates move apart, new mountains form. Where plates meet, one can go under the other or they push together and form new mountains.
We call the areas where plate boundaries fault lines.
- E.g. Alpine fault
- E.g. Hutt Valley, Wellington
- Lots of smaller faults (Christchurch Greendale fault)
- NZ sits on the boundary of the Pacific and Australian plate.
- On the East of the NI there is a deep trench as the Pacific Plate goes under the Australian Plate.
- In the SI the two plates slide past each other, along the Alpine Fault.
- Below the SI they think that the Australian plate goes under the Pacific plate.
According to information published by the Geology Department at New Zealand’s, University of Otago, the Alpine transform boundary is unique because the Pacific plate is thrusting over the top of the Australian plate. This behaviour is typically only found at convergent boundaries or subduction zones, and not at transform boundaries. As a result, New Zealand’s Southern Alps are increasing in height by approximately seven millimetres per year.
The trench visible north and south of New Zealand shows that the Pacific plate is diving below the Australian plate, creating lots of volcanoes in the north island.
Visit this website here to see all the earthquakes that occurred in New Zealand in the last 365 days. See how they align quite well with the fault lines at the plate boundaries?