Continental Movement Year 9 Unit 4.1 Flashcards
Describe Continental Drift:
All continents were once joined together to form a supercontinent called Pangea. And they have since separated and drifted apart over the course of millions of years. This is from a result of the motion of tectonic plates.
Describe all of Wegners Observations and their conclusions.
- Continents appeared to fit together like a jigsaw forming Pangea.
Conclusion: Continents were once connected. - Fossils of same prehistoric species were found in Continents very far apart from each other
Conclusion: Animals/Plants couldn’t have possibly travelled across oceans to get to other continents.
What’s the lithosphere
Composed of the Earths crust and the upper mantle.
Define Tectonic plates
Tectonic plates are rigid segments of the Earth’s lithosphere. They float on the semi-fluid asthenosphere.
Explain the two different types of crust
Oceanic crust - Below sea level (sea floor), thin (5km) and very dense due to presence of iron and magnesium
Continental Crust - Above sea level (continents) thick (30km) and less dense due to the presence of aluminium and silicon
Explain how rifting works
Convection currents
move tectonic plates
away from each other
(diverge)
- This movement creates a
rift (gap) in the Earth’s
crust. - Rifting results in the crust
thinning / cracking /
fracturing
What’s Harry Hesses theory of seafloor spreading
When rifting occurs
underwater the seafloor
spreads.
- Rifts are filled with new
molten rock which
cools, creating oceanic
ridges
How does riftin create ocean basins
When rifting occurs and
the basin is filled with
water
- Water may ‘leak’ in from
other water bodies or
slowly fill with rain
3 types of evidence for seafloor spreading Hess
- Magnetic striping
- Age of the sea floor
- Sediment thickness
Define magnetic striping
The patterns of
magnetism trapped in
rocks on either side of
ocean rifts
What is magnetic striping
Many rocks contain the magnetic
iron oxide mineral called
magnetite.
- Magnetite has a north seeking
pole and south seeking pole (like
a compass) - When molten rock (containing
magnetite) solidifies, all the
magnetite lines up with Earth’s
magnetic field.
The pattern of magnetism is
symmetrical: - the rocks at equal distances on
each side of the ridge were
formed at the same time - magnetic fields pointing in the
same direction - Geomagnetic reversals:
- the magnetic poles have been
swapping over long periods of
time (thousands to millions of
years)
Geomagnetic Reversals
Dynamo effect:
* As molten iron in Earths
core swirls around, it
generates electric currents,
creating a magnetic field.
Geomagnetic reversals:
* Flow patterns of the molten
iron change due to
complex interactions of
heat, convection currents,
and the Earth’s rotation.
Age of sea floor
Dating of rock (radioisotope
dating) on the sea floor
support:
- The further the rocks of the
sea floor were from the
ridges, the older they were - The sea floor was very
young compared with the
rocks in the continents
Sediment Thickness
Sedimentary rock is formed by
sediments sinking to the
ocean floor
- The sedimentary rock layers
on the ocean floor become
thicker as they move away
from the ridges. - This means that sediments
had been sinking for longer on
the rocks on the sea floor
further away from the ridges
Types of plate movement
- Constructive (Diverging)
boundaries (plates are
moving apart from each
other) - Destructive (Converging)
boundaries (plates are
colliding with each other) - Transforming boundaries
(plates are sliding past each
other)
Constructive boundaries
Referred to as constructive
boundaries as it forms new crust
- Also referred to as diverging
boundaries - Plates diverge (separate) from
one another leaving a rift (deep
crack). - Magma rises up into the rift and
solidifies as it cools forming a
ridge (mountain range) of new
crust
Destructive boundaries
Referred to as destructive
boundaries as it destroys crust
- Also referred to as converging
boundaries - Plates converge (collide) with
each other where one plate
often subducts under another - These collisions form mountains,
island chains and underwater
trenches depending on the crust
density
Oceanic/Continental
The denser oceanic crust is
usually subducted under the less
dense continental crust. This
subduction creates a deep
trench in the ocean floor (e.g.
Peru-Chile Trench).
- The subduction of the oceanic
crust can lead to the formation
of continental volcanic
mountain ranges on the
continental plate’s edge.
Oceanic/Oceanic
The denser (or faster) oceanic
crust is subducted beneath the
other. This creates a deep trench
in the ocean floor. (e.g. the
Mariana Trench is between the
Pacific Plate and the Mariana
Plate).
- The subduction of one oceanic
crust beneath another often leads
to the formation of volcanic island
arcs. These are chains of volcanic
islands in the ocean
Island arcs/chains
Destructive boundaries
● The denser / faster moving plate subducts under the other and is
melted forming magma
● The magma rises through to the surface creating a chain of
volcanic islands along the boundary
Hot spots
● If tectonic plates move over a hot spot in the crust, a volcano will
be formed. The active volcano is carried away from the hotspot
and becomes inactive. A new part of the plate is now above the hot
spot and gradually forms a new volcano.
Continental/Continental
Continental crusts have similar
densities, so they usually do not
subduct beneath each other.
Instead, they crumple and
compress.
- The collision of continental crust
can lead to the formation of high
mountain ranges, often with
extensive folding and faulting
What’s the first way that volcanoes can be formed
- Tectonic plate boundaries:
Constructive
Magma rises up into the rift between two diverging plates and solidifies as it
cools forming a ridge (mountain range) of new (often thin) crust
Magma rises through the lithosphere, creating a volcano
Destructive
When an oceanic plate converges with a continental plate (or another
oceanic plate) it will subduct
Subduction generates massive amounts of heat and causing the crust to
weaken.
Magma rises through the lithosphere, creating a volcano
Whats the second way that volcanoes can be formed
- Hot spots:
Isolated weak / thin areas in the lithosphere where magma is closer to the
surface and the significantly hotter.
Magma rises through the lithosphere, creating a volcano
3 main parts of a volcano
Magma Chamber: A reservoir of molten rock beneath the Earth’s
surface.
Main Vent: The opening through which magma, ash, and gases are
released.
Crater: The bowl-shaped depression at the top of the volcano,
usually formed after a volcanic eruption.
What are the 2 types of eruptions
Effusive eruptions
The relatively gentle flow of lava down the sides of the volcano.
Lava Flow: Magma (molten rock) from the aesthenosphere which flows
out of the vent
Average speed 1km/hr – 10km/hr
700 °C - 1,200°C
Explosive eruptions
Gas-rich magma is violently expelled, leading to ash clouds and
pyroclastic flows. Often followed by effusive eruption of lava.
Ash clouds: Ash (tiny rock and mineral fragments), Volcanic glass
(obsidian), Sulfur dioxide & Water vapor (steam).
Pyroclastic flow: a mixture of hot gas, ash, and volcanic rocks that can
flow at very high speeds
Average speed 100km/hr.
200°C - 700°C
What is a volcano
A volcano is a natural
geological formation that
occurs when there is a rupture
in weak / thin areas of the
lithosphere releasing hot
material to the surface.
What is an earthquake
An earthquake is the natural
geological event caused by the
sudden release of energy
(seismic waves), typically along
a fault line in the Earth’s crust
due to tectonic plate movement
What are the two causes of earthquakes
Destructive
When two plates converge, one will subduct
Stress builds at the boundaries due to the friction between the plates until it
is suddenly released along a fault line in the form of an earthquake.
Transform
Plates slide past each other horizontally.
Stress builds at the boundaries due to the friction between the plates until it
is suddenly released along a fault line in the form of an earthquake.
Focus, epicenter and fault
Focus: The point in the Earth’s crust where the earthquake
originated from.
Epicentre: The point on Earth’s surface directly above the
focus.
Fault: Fracture in the Earth’s crust (where the focus is generally
located)
Two main types of seismic waves and explain
Body waves: Waves which originate from the focus
P waves: Primary longitudinal waves (First and fast)
S waves: Secondary transverse waves (Second and slower)
Surface waves: Waves which originate from the epicenter (Third
and the slowest but most destructive)
Love waves: Horizontal waves
Rayleigh waves: ‘Rolling’ waves
What is a seismograph and its use
Seismic activity is measured using a seismometer and produces a
seismograph. Can be used as an early detection strategy.
Contains a base anchored to the ground and a suspended mass
(pendulum) with a spring attached to it.
When the ground shakes due to seismic waves, the seismometer’s
base moves while the suspended mass remains relatively stationary
due to inertia.
Measures energy of an earthquake in MMS units (Moment magnitude
scale)
