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Chapter 2

Detailed Textbook Notes Chapter 2

Earth Science
Course Code
Terry Botrrill

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Pre-Lecture 2 Notes: Crustal Tectonics A Mobile Earth
2.7 What do we mean by Plate Tectonics?
The Concept of a Lithosphere Plate
Lithosphere: Relatively rigid, nonflowable, outer 100- to 150-km-thick layer of
the Earth; constituting crust & top part of mantle
Asthenosphere: Layer of mantle that lies between 100-150 km & 350 km deep;
asthenosphere is relatively soft & can flow when acted on by force
Asthenosphere can undergo convection but lithosphere cant
Continental Lithosphere & oceanic lithosphere differ in their thickness; On
average, continental lithosphere has thickness of 150 km, whereas oceanic
lithosphere has thickness of about 100 km
Geoscientists distinguish 12 major plates & several microplates
Distinguish 2 continental margins:
oActive Continental Margin: Continental margin that coincides with a
plate boundary
oPassive Continental margin: Continental margin that is not a plate
The Basic Principles of Plate Tectonics
Earths lithosphere divided into plates that move relative to each other & relative
to underlying asthenosphere
Plate movement occurs @ rates of about 1-15 cm/year
As plate moves, internal area remains largely rigid & intact, but rock along plate
boundaries undergoes deformation
As plates move, so do continents that form part of the plates, resulting in
continental drift
Identifying Plate Boundaries
3 types of plate boundaries:
oDivergent Boundary: A boundary at which 2 lithosphere plates move
apart from each other; they are marked by mid-ocean ridges
oConvergent Boundary: A boundary at which 2 plates move toward each
other so that 1 plate subducts beneath the other; only oceanic lithosphere
can subduct
oTransform Boundary: A boundary at which 1 lithosphere plate slips along
the side of another plate
2.8 Divergent Plate Boundaries & Sea-Floor Spreading
As plates move apart, new oceanic lithosphere forms along divergent boundary
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Process takes place @ submarine mountain range called mid-ocean ridge that rises 2 km
above adjacent abyssal plains of ocean
Formation of new sea floor takes place only along the axis (centreline) of the ridge
How does Oceanic Crust Form @ a Mid-Ocean Ridge?
As sea-floor spreading takes place, hot asthenosphere rises beneath ridge
As asthenosphere rises, it begins to melt, producing magma
Magma has lower density than solid rock, so it rises
Eventually accumulates in crust below ridge axis, filling region called magma
As magma cools, it turns into mush of crystals
Some magma solidifies completely along the side of chamber to make gabbro; the
rest rises still higher to fill vertical cracks, where it solidified & forms wall-like
sheets, or dikes, of basalt; Some magma ruses all the way to surface of sea floor
@ ridge of axis & spills out of small submarine volcanoes
As soon as it forms, new oceanic crust moves away from ridge axis
Magma from the mantle rises to Earths surface @ the ridge, solidifies to form
oceanic crust, then moves laterally away from the ridge
How does the Lithospheric Mantle Form @ a Mid-Ocean Ridge?
As newly formed oceanic crust moves away from ridge axis, crust & uppermost
mantle directly beneath it gradually cool as they lose heat to the ocean above
As soon as mantle rock cools below 1,280 degrees C, it becomes part of
2.9 Convergent Plate Boundaries & Subduction
Oceanic plate bends & sinks down into asthenosphere beneath the other plate
Convergent boundaries also known as subduction zones
Subduction occurs for 1 reason: Oceanic lithosphere, once it has aged @ least 10 million
years, is denser than asthenosphere & thus can sink through asthenosphere
As lithosphere sinks, asthenosphere flows out of its way; however asthenosphere resists
flow, so oceanic lithosphere can sink only very slowly (15 cm/year)
Continental lithosphere cant be subducted because it is too buoyant
Geologic Features of a Convergent Boundary
Chain of volcanoes known as volcanic arc develops behind accretionary prism
If volcanic arc forms where oceanic plate subducts beneath continental
lithosphere, resulting chain of volcanoes grows on the continent & forms
continental volcanic arc
If volcanic arc forms where one oceanic plate subducts beneath another oceanic
plate, the resulting volcanoes form chain of islands known as volcanic island arc
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