Chapter 4: The way the earth Works; Plate tectonics
4.2 What do we mean by plate tectonics
The lithosphere, the crust and the upper most part of the mantle, is relatively hard. It does not flow.
The asthenosphere, the part of the mantle that is able to flow, lies under the lithosphere. The mantle
rock is the same, the difference between the lithospheric and asthenospheric mantle is the temperature.
The upper part is cooler than 1280 degrees so it doesn’t flow, and the lowest part is warmer than 1280
degrees so it does flow.
Because continental lithosphere is thicker and has a less
dense crust, its surface sits higher than does the surface of
the seafloor.
The lithosphere exists of 20 plates, and the contacts
between them are plate boundaries.
Some plate boundaries coincide1 with
continental margin, the
boundary between continent
and an ocean, these are called active margins. If they do not, the margin
is called passive margin.
Continental shelf is the shallow seafloor with a very thin sediment pile
because it is at a passive margin.
Identify plate boundaries
The location of plate boundaries is found by looking at a map of earthquakes. An earthquake is a
vibration due to the suddenly shears2 along a fault. The focus of the earthquake is the spot where the
fault slips, and the epicenter marks the point on the surface of the Earth directly above the focus.
Three types of plate boundaries exist: 1* Divergent boundary – two plates moving apart from each
other 2*Convergent boundary- two plates moving toward each other 3*Transform boundary- two
plates slide sideways past each other.
Archimedes’ principle of buoyancy
Buoyancy is the upward force acting on an object immersed or floating in a fluid. A
object less dense than water, sinks only until the mass of the water displaced equals
the total mass of the object. So if the object is denser than water is will
always sink.
4.3 Divergent-plate boundaries and seafloor spreading
At a divergent boundary, two oceanic plates move apart by the process of
seafloor spreading. New seafloor forms at the divergent boundary. This takes place at Mid-ocean
ridges. During seafloor spreading hot asthenosphere rises and melts into magma. The ultramafic rock
1 Coincide=samenvallen
2 Shear=slijden
, does not melt equally. During the melt, the composition of the rock changes into a mafic rock (more
silica) than the ultramafic source rock. Because of the density of magma, it rises. Some of the magma
accumulates in a magma chamber. Some of the material cools along the sides of the magma chamber
and solidifies to become a coarse-3grained mafic igneous rock called gabbro. Some rises still higher
into vertical cracks in which it solidifies to form sheets of basalt, these sheets are called basalt dikes.
Magma that makes it all the way out is called lava and when it cools it forms a layer of pillow basalt.
The tension (stretching force) causes the new crust to break, that’s why you see faults. Movement on
these fault generates earthquakes.
Lithospheric mantle is colder
than 1280 degrees, so it does
not exist at the ridge axis,
because it’s to hot because of
the presence of the rising hot
asthenosphere. As oceanic
lithosphere continues to move
away from the ridge axis, it
continues to cool so the
lithospheric mantle grows
progressively thicker. As
lithosphere thickens it gets
cooler and denser so it sinks
down in the asthenosphere, so
older ocean floor is deeper than younger ocean floor. You can find the oldest seafloor to the passive
continental margins on either side of the ocean and the youngest at mid-ocean ridges. ** why at
passive?
Chimneys4 spewing hot, mineralized water are called black smokers. Sea water percolates 5 down into
the oceanic crust, warms up because of the heat of magma, dissolves minerals and carries them along
as it spews back into the sea. The mineral grains precipitating in the cold seawater causes the black
color.
4.4 Convergent-Plate boundaries and subduction
At convergent-plate boundaries, two plates, at least one of which is oceanic, move toward each other.
The oceanic plate bends (krommen) and sinks down into the asthenosphere beneath the continental
plate. This is called subduction. The overall density of the oceanic plate once it has aged at least 10
million years exceeds that of the underlying asthenosphere. Therefore when it bends and starts to slip
into the mantle it will continue to sink. ** Is it denser because it cools?
The viscosity 6of the asthenosphere, is large thus it can flow only very slow. That’s why it takes so
long. And because of the asthenosphere’s high viscosity, lithosphere can only sink when it enters the
3 Coarse=grof
4 Chimney= rotskloof
5 Percolate=filter
6 Viscosity= stroperigheid
4.2 What do we mean by plate tectonics
The lithosphere, the crust and the upper most part of the mantle, is relatively hard. It does not flow.
The asthenosphere, the part of the mantle that is able to flow, lies under the lithosphere. The mantle
rock is the same, the difference between the lithospheric and asthenospheric mantle is the temperature.
The upper part is cooler than 1280 degrees so it doesn’t flow, and the lowest part is warmer than 1280
degrees so it does flow.
Because continental lithosphere is thicker and has a less
dense crust, its surface sits higher than does the surface of
the seafloor.
The lithosphere exists of 20 plates, and the contacts
between them are plate boundaries.
Some plate boundaries coincide1 with
continental margin, the
boundary between continent
and an ocean, these are called active margins. If they do not, the margin
is called passive margin.
Continental shelf is the shallow seafloor with a very thin sediment pile
because it is at a passive margin.
Identify plate boundaries
The location of plate boundaries is found by looking at a map of earthquakes. An earthquake is a
vibration due to the suddenly shears2 along a fault. The focus of the earthquake is the spot where the
fault slips, and the epicenter marks the point on the surface of the Earth directly above the focus.
Three types of plate boundaries exist: 1* Divergent boundary – two plates moving apart from each
other 2*Convergent boundary- two plates moving toward each other 3*Transform boundary- two
plates slide sideways past each other.
Archimedes’ principle of buoyancy
Buoyancy is the upward force acting on an object immersed or floating in a fluid. A
object less dense than water, sinks only until the mass of the water displaced equals
the total mass of the object. So if the object is denser than water is will
always sink.
4.3 Divergent-plate boundaries and seafloor spreading
At a divergent boundary, two oceanic plates move apart by the process of
seafloor spreading. New seafloor forms at the divergent boundary. This takes place at Mid-ocean
ridges. During seafloor spreading hot asthenosphere rises and melts into magma. The ultramafic rock
1 Coincide=samenvallen
2 Shear=slijden
, does not melt equally. During the melt, the composition of the rock changes into a mafic rock (more
silica) than the ultramafic source rock. Because of the density of magma, it rises. Some of the magma
accumulates in a magma chamber. Some of the material cools along the sides of the magma chamber
and solidifies to become a coarse-3grained mafic igneous rock called gabbro. Some rises still higher
into vertical cracks in which it solidifies to form sheets of basalt, these sheets are called basalt dikes.
Magma that makes it all the way out is called lava and when it cools it forms a layer of pillow basalt.
The tension (stretching force) causes the new crust to break, that’s why you see faults. Movement on
these fault generates earthquakes.
Lithospheric mantle is colder
than 1280 degrees, so it does
not exist at the ridge axis,
because it’s to hot because of
the presence of the rising hot
asthenosphere. As oceanic
lithosphere continues to move
away from the ridge axis, it
continues to cool so the
lithospheric mantle grows
progressively thicker. As
lithosphere thickens it gets
cooler and denser so it sinks
down in the asthenosphere, so
older ocean floor is deeper than younger ocean floor. You can find the oldest seafloor to the passive
continental margins on either side of the ocean and the youngest at mid-ocean ridges. ** why at
passive?
Chimneys4 spewing hot, mineralized water are called black smokers. Sea water percolates 5 down into
the oceanic crust, warms up because of the heat of magma, dissolves minerals and carries them along
as it spews back into the sea. The mineral grains precipitating in the cold seawater causes the black
color.
4.4 Convergent-Plate boundaries and subduction
At convergent-plate boundaries, two plates, at least one of which is oceanic, move toward each other.
The oceanic plate bends (krommen) and sinks down into the asthenosphere beneath the continental
plate. This is called subduction. The overall density of the oceanic plate once it has aged at least 10
million years exceeds that of the underlying asthenosphere. Therefore when it bends and starts to slip
into the mantle it will continue to sink. ** Is it denser because it cools?
The viscosity 6of the asthenosphere, is large thus it can flow only very slow. That’s why it takes so
long. And because of the asthenosphere’s high viscosity, lithosphere can only sink when it enters the
3 Coarse=grof
4 Chimney= rotskloof
5 Percolate=filter
6 Viscosity= stroperigheid
