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Lecture 4

EESA06H3 Lecture 4: EESA06 Lecture Four

Environmental Science
Course Code
Nick Eyles

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Lecture Four
How oceans form: The East African Rift
- History of Ocean basins
- East African Rift
o It’s a long crack that runs North Eastern corner of Africa and it’s opening
o Right now, it’s dry land but soon the sea will break through, called embryonic
Key words, Themes and Concepts
- Types of plate margins: convergent, divergent and transform
o Transform plate boundaries: plates slide past each other
- The rock cycle:
o Igneous rocks formed at convergent margins: “acidic” (Water and silica-rich)
E.g. granite typical of continental crust
o Igneous rocks formed at divergent margins:
“basic” (dry, iron-rich and silica poor)
E.g. basalt typical of oceanic crust
o Igneous means “born out of fire”
Rocks that once was magma
- Density of magma explains the behavior of when they erupt at the Earth surface or when
it doesn’t come near the Earth surface
- Viscosity: high viscosity silica-rich magmas and low viscosity silica-poor magmas
o Viscosity is the speed of which something flows
o Magma has a low viscosity flows very easily
o A lot of magma contains gas (sulfur dioxide, carbon dioxide and water vapor)
Magma with low viscosity causes these gases to come out and spread
o High viscosity magma gases can’t be released readily, instead it gets trapped
and when it gets released, it explodes
That’s why we have dangerous volcanos that blow up
o Viscosity plays a vital role in terms of the type of volcano we have
- Melting in the mantle: the crucial importance of water content: decompression melting
and decompression thickening
o The mantle is not fluid, but hot rock that seems like plastic
It not only becomes fluid when the hot rock rises to the surface and then it
- The Wilson Cycle and the history of oceans
o 1. Embryonic
o 2. Mature
o 3. Dying
o It’s an organized principle of Earth history
- How embryonic oceans form: Triple junctions and “failed rifts”

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o Aulacogens
- Flood basalts (magma types): Aa, ropey and pahoehoe lavas
How Lithospheric plates slide across the asthenosphere: Memorize this slide
- Plate tectonics is the movement of lithospheric plates (drift of rocky sphere)
- Note: The Moho defines the base of the crust
- Note: The lithosphere encloses the crust (whether it’s continental crust or oceanic crust)
and the weak upper part of the mantle
- Plate tectonics is actually the movement near the asthenosphere, not the moho
One of the big breaks in late 60s, is mapping earthquake epicenters
- Need to know this diagram
- Spreading centres where plates are created by the addition of magma that comes from
the depth
o Pushing oceanic plate apart
- Convergent plate boundary
o Where we have oceanic crust dominated by basalt; it’s dense
- Earthquake focus
o Red, yellow, blue shocks on diagram
- Two continents colliding there is no subduction because the continents are of equal
density/age; but instead there is a process called obduction which results in thickening of
crust/ compression and associated with big, tall mountains
o There is also a suture zone that marks the boundary of two colliding continents
- Where oceanic crust converges with oceanic crust
o Rule: Older oceanic crust of the two plates is what subducts because the older one
is more dense
- Divergent plate boundaries where plates move apart such as mid-ocean ridges
o E.g. Iceland and continental rifts
o E.g. East African Rift (Spreading centers)
- Convergent plate boundaries
o Oceanic crust converges with oceanic (e.g. western Pacific) or continental crust
(e.g. Western South America (“subduction zones”)
o Continental crust converges with continental
E.g. India and Himalayas
(“obduction zones”)
This causes big mountains
- Transform plate boundaries
o Where plates simply slide past each other
o It gets close to continents
- Look at this map and he’ll ask what type of boundary is occurring there

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The Rock Cycle
- Three major groups of rock:
o Igneous
Born of fire; material come apart from deep interior of the mantle
At the earth surface it sits in an oxidizing environment dissolution to rain
water, snow and heat the rocks break down and produce particles and
soon gets washed into lakes and oceans as sediments
The process of lithification causes sedimentary rock
Loose particle gets sedimented together
Lithification is an important process because fossil fuels come
from oil deposits; oil deposits are found in the open pore spaces
between grains
o Sedimentary rock
Can be accumulated in ocean basins;
When these rocks get pushed down and heated, the sedimentary rock can
become metamorphic rock
o Metamorphic rock
Can’t be identified from what it came from originally
It can either be igneous rock or sedimentary rock
- Look at the rock cycle
Classifying igneous rocks according to plate tectonic setting
- Decompression melting/ Divergent margins: “Dry magma”
o Basalt and gabbro rock all silica poor
o It’s an important process when the hot mantle rock rises (all the pressure takes
off) and what happens is that the rock will start to melt, and this only occurs at
deep mantle rocks which are silica poor
Silica poor mantle rocks are rich in iron and magnesium
o When these rocks go to the surface, they melt due to the lowering of pressure
- Compressional margins: “wet magma”
o Andesite, diorite and granite (all silica rich) are the main type of rock
o Silica tends to dominate continental crust
“Silicon” + oxygen silica (glass lenses for example)
o It’s one of the most common minerals found in continental crusts
- When we melt oceanic crust that is dominated by silica poor rocks, but we can add water
and sediments that has been eroded by the continents
o We end up with granitic rocks and when they come out of volcanos, they’re called
Andesitic volcano
- Note: Silica poor magma flows very easily
o Silica rich magma flows very slowly and instead blows up
- There is a lot of magma that never makes it to the surface
o It’s called granitic magma; it has the property of being higher viscosity magma
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