1. Plate Margins
Think back …. can you remember what the 3 types of plate margins were?
Write them down
A bit more challenging ….. Can you give a location of an example of each …..

2. Plate Margins Divergent. Convergent. Conservative.
Each type of plate margin has different rocks and different types of hazard
Firstly a few golden rules ….

3. Golden Rule 1 – how do we melt things?
Why do plates/rocks melt:
1: addition of heat
2: decompression melting
3: wet rocks lower melting temperature
Decompression melt
Wet rocks have lower melting point
Add heat!

4. Why do plates/rocks melt:
Golden Rule 1
Why do plates/rocks melt:
Because the temperature has increased.
Because the pressure is decreased.
Because it is saturated in water.
If one or more of these situations exists then the plate/rock will melt.

5. Golden Rule 2 - chemistry
Depends on what you start off with
Chocolate chip cookie …… what melts first?
Chocolate chips
When rocks melt, the first minerals to melt are the lower temperature ones
PARTIAL MELTING

6. Golden Rule 2 What kind of magma is produced when a plate/rock melts?
igneous rocks have a range of compositions.
Silica rich GRANITE
Silica poor BASALT
We can classify igneous rocks by how much quartz (silica) they contain
Quartz has a low melting point
So melts first

7. Golden Rule 2
we classify igneous rocks by how much quartz they contain.
Felsic: > 66% silica rich GRANITE
Intermediate: 52 – 66 %
Basic: 44 – 52% silica poor BASALT
Ultrabasic: < 44%
key

8. Silicic

9. Golden Rule 2 Quartz is one of the first minerals to melt, so
When the initial magma melts
the one it produces
will be richer in silica
and so will move towards the silicic end of the scale

10. Golden Rule 2
If you melt an ultrabasic rock the magma will be more basic
If you melt a basic rock the magma will be more intermediate
If you melt an intermediate rock the magma will be more silicic

11. Golden Rule 3 – the product
As plate material is melted, the product becomes richer in silica
The more silica there is in a magma the more explosive the volcanic eruptions will be.
Ed Byrne clip from Volcano Live Summer 2012
Ed Byrne clip from volcanoes live ….

12. Silicic

13. Golden Rule 3
Why?
Because silica makes the magmas more viscous and sticky.
Gases want to come out of solution when a magma reaches the surface but they cannot escape easily.
They therefore expand putting pressure on the magma chamber.
Eventually the pressure is so great that the top/plug is blown off or the side slips and a violent eruption occurs.