Topic 7 Flashcards
What is partial melting?
The process by which most igneous rocks melt. Since individual minerals have different melting points, most igneous rocks melts over a range of temperatures of a few hundred degrees. If the liquid is squeezed out after some melting has occurred, a melt with a higher silica content results.
Upper mantle = ultramafic igneous rock called peridotite
Partial melting of peridotite makes mafic magma
What is the lithosphere?
It is the earth’s outer shell that includes the crust and the upper most solid part of mantle. It is cool and rigid and ~100km thick It bends or breaks in response to deformation. Made primarily of C and O
What is the asthenosphere?
Lies beneath the lithosphere. Is part of the mantle. Softer, weaker and hotter layer then the lithosphere. Rock behaves plastically (deforms slowly by flowing). Allows movement of the lithosphere which moves independent of the asthenosphere. ~100-350km in depth
What are the different layers of the earth’s crust?
Continental crust -,higher then the oceanic crust (30-70kn in depth), silica rich and low density
Oceanic crust - 5-10km in depth, rich in Fe and Mg, basalt high density
The mantle is richer in Fe and Mg and higher density
Explain how the modern theory of plate tectonics arose
- Came from Alfred Wegener’s hypothesis of Continental Drift (1915)
- Which came from Edward Seuss idea of the supercontinent (1831-1914)
TOPIC QUESTION What is the evidence to support modern plate tectonics theory
- Fits of continents
- similarity between coastlines
- tried to fit shorelines but instead need to look at the seaward edge of the continental shelf - Fossils
- fossils of mesosaurus found only in black shale from the Permian (~250ma) in eastern south America and Southern Africa
- if it could swim far enough to cross the ocean the fossil would be widespread and it is not and it only lived in freshwater - Rock types and structure match
- rock types and mountain belts found in separate continents match in age, rock type and fossil - Ancient Climates
- paleoclimatic support for the idea of mobile continents
- possible coral reefs in Alberta
- glacial sediments in Southern Africa, South America, India and Australia
- tropical ice sheets rejected because fossils of the same age from other locations indicate warm climates
Interpretation of largest pre-existing data set can help test theory
- ocean drilling programs
- mantle hot spots
- paleomagnetism
- earthquake depths
- age of crust
Who proposed the theory of contentinal drift?
Alfred Wegener
Who proposed the theory of supercontinent
Edward Seuss
TOPIC QUESTION What are the issues with Wegener’s theory?
- compelling evidence but idea was not accepted until 1960
- Wegener was unable to find a mechanism by which continents could move
- gravitational forces of the moon and sun refuted because forces that string would stop the earth’s rotation
- icebreaker continents were refuted because no evidence suggesting the ocean floor was weak enough
TOPIC QUESTION Describe the three types of plate boundaries?
- Divergent boundaries - two plates move apart and creates new lithosphere (constructive margins)
- Convergent boundaries - plates move together and lithosphere is destroyed (destructive margins)
- Transform fault boundaries - lithosphere is neither created or destroyed
TOPIC QUESTION Describe divergent plate margins
- ocean ridges spread between 2-15cm per year average 5cm
- elevated
- new oceanic crust is hotter and larger volume
- ocean lithosphere away from the ridge cools and contracts increasing its density
- accumulates sediment
- site of submarine volcanoes and earthquake activity
- ridge ~2-3km above ocean floor pillow basalt in ocean floor
- continental rifting
- divergent plate boundaries can develop within continents - cause landmass splits like break up of Pangaea
TOPIC QUESTION Describe the three types of convergent plate convergences?
- Oceanic - continental
- example - western margin of the USA which is the subduction of the Juan de Fuca plate
- associated with volcanic arc because water is squeezed from subducted plates, which lowers the melting temperature of surrounding rock - magma is less dense and rises - Oceanic-oceanic
- When an oceanic plate meets another one cosmic plate the thicker more dense plate subducts
- example Aleutian Islands, Japan, The Antilles, most of the west pacific
- typically, simple structures made of numerous volcanic cones undressing by oceanic crust (<20km thick) - Continental - continental
- also called collision boundaries
- examples Himalayas, alps, Appalschians, Urals
TOPIC QUESTION Describe and explain the characteristics of transform plate boundaries
- plates slide horizontally past one another with destroying or creating new lithosphere (conservative)
- most occur in ocean basins (offsets along mud-Atlantic range) a few occur in continental crust (e.g. San Andreas fault)
- transform fault running length of California’s coast located at the boundary between the Pacific and North American played
TOPIC QUESTION
Diagram and explain the plate tectonic cycle
No answer will need to look up diagram and explanation
- Mantle convection
- Ridge push
- slab pull
