Module 3 - Continental Drift Flashcards
(119 cards)
1
Q
What is continental drift?
A
Continental drift is the large-scale horizontal movement of continents during geological time
2
Q
What are Tillites?
A
Tillites are ancient glacial deposits preserved within a rock sequence
3
Q
What are Evaporites?
A
Evaporites are minerals formed by the evaporation of saline water, for example, halite (rock salt) and gypsum
4
Q
Who set out evidence for continental drift using continental fit, the distribution of rocks and ancient glaciations?
A
Alfred Wegener in 1915
5
Q
What led to widespread acceptance of continental drift?
A
In the 1950’s the evidence from palaeomagnetism and exploration of the sea floor
6
Q
Which huge landmass is believed to have existed 250Ma and which continents did it contain?
A
Gondwanaland •South America •Africa •Antarctica •India •Australia
7
Q
When did Gondwanaland break up?
A
In the Jurassic about 167Ma
8
Q
Why don’t we get an exact jigsaw fit of the continents?
A
- sea level is constantly changing, so a coastline is a temporary feature
- deposition and erosion has occurred since the two continents drifted apart
- where there is erosion, there is a gap
- where there is deposition, there is an overlap
9
Q
Where do the continents fit better?
A
At the edge of the continental shelf or a specific depth like 1000m or 500m
10
Q
How can you prove that two rocks on either side of the ocean were once together?
A
- distinctive characteristics - of mineral composition and physical features
- age determined by radiometric dating
11
Q
What are some examples of matching rocks?
A
Precambrian cratons, Carboniferous coals and Tillites, Permian red sandstone and evaporites and Upper Triassic flood basalts
12
Q
What are mountain chains?
A
Fold mountain chains are linear features 100’s of km long. The trend of fold mountains provide a way to match geology across continents
13
Q
What are the two main fossils providing evidence of continental drift?
A
Mesosaurus (land-based reptiles) and Glossopteris (plants) found in both Africa and South America
14
Q
What is Glaciation?
A
Sedimentary deposits of Angular, poorly sorted and scratched pebbles (clasts) in a fine grained matrix. Glacial striations are used to trace the movement of the glaciers to one common source area in Southern Africa.
15
Q
What is Palaeomagnetism?
A
Iron-rich minerals in some rocks hold a record of earths magnetic field. A large number of rocks are collected and dated and direction of Palaeomagnetism measured. The data is then plotted as an apparent polar wandering curve
16
Q
How is an apparent polar wandering curve depicted?
A
An apparent polar wandering curve is depicted by a line on a map, which joins up the apparent positions of the magnetic North Pole over time
17
Q
What is an erratic?
A
A boulder that is out of place in the surrounding rock
18
Q
When were sonars developed?
A
1940s
19
Q
When were accurate maps of the worlds ocean floor produced?
A
1950s
20
Q
Why were the maps surprising?
A
They showed areas of high mountains forming ridges on the ocean floors and also deep trenches. The topography of the ocean floor is more extreme than the land surface with trenches 12km deep compared to the highest mountains e.g. Mount Everest at 8km high.
21
Q
What did Harry Hess propose in the 1960s about sea floor spreading?
A
That basaltic magma from the Mantle rises to create new ocean floor at mid ocean ridges. On each side of the ridge, the sea floor moves away. As the continents drift apart, the oceans become wider
22
Q
What are submersibles?
A
Capable of working in high pressures at depth. They film the sea floor, collect water and rock temperatures and collect samples
23
Q
What do cores of sediments and underlying igneous rocks allow us to calculate?
A
The age of the sea floor.
24
Q
What are mid ocean ridges?
A
They are large mountain chains running down the centre of all the oceans basins. In the middle is an axial rift (a deep valley with steep-sided mountains either side.
25
Name an example of an island created from the MOR rising above sea level?
Iceland
26
Describe the MORs characteristics
Closest to the ridge, the crust is hot and expanded, so forming the ridge itself, further away the crust cooks and contracts, creating the flatter areas of the ocean basins.
27
Where do MORs show the highest heat flow?
Directly above the axial rift
28
Which type of volcanoes form at the axial rift?
Fissure volcanoes - non explosive, low viscosity, basaltic lava
29
How are pillow Lavas formed?
If the basalt cools quickly underwater, the outside rapidly solidifies while the inside remains liquid for longer
30
What is caused by the rising magma at the MOR
Shallow earthquakes. They also occur along linear transform faults.
31
What do transform faults have?
Large horizontal displacements, they are perpendicular to the ridge and can be thousands of km long.
32
What can satellite be used to measure the exact location of and what does this show?
The location of fixed receivers on the surface of the earth and it records their movements to see how fast the sea is spreading.
33
What is a magnetometer?
It is towed behind a ship to measure changes in magnetism. It can detect the strength and direction of the magnetic field
34
What is a gravimeter?
It can be towed behind a ship to measure the changes in gravity.
35
What happens when Earths magnetic field undergoes complete reversal?
North becomes south and south becomes north. It can occur up to four times in a million years. The sea floor records these reversals.
36
What does magnetic reversals involve?
New magma forms at the MOR. The iron particles line up parallel to the Earths magnetic field. As the rocks cool down, they remain permanently magnetised. The result is a striped magnetic anomaly pattern on the sea floor. They form a symmetrical pattern parallel to the MOR.
37
What is the important thing to note about magnetic reversals?
The pattern is symmetrical on each side of the MOR. The only possible interpretation is that the crust on both sides is moving apart.
38
How can you date the ocean floor?
Thin oozes and other fine grained sediments can be dated from pelagic microfossils that sank to the sea floor after death. You can then measure the distance from the known ages rock to the MOR to calculate the rate of spreading.
39
Why does the thickness of the sediments increase away from the ridge?
Because the ocean floors age is greater away from the ridge and so have had more time for sediments to be deposited. The ocean floor is up to 200Ma and the sediment can be at a thickness of 2000m thick.
40
What is the structure of the ocean crust?
* basaltic pillow lava is the material from volcanoes that cooled rapidly underwater forming fine crystals
* dolerite dykes are the vertical feeder pipes from the magma chambers supplying the volcanoes. The crystals are medium sized
* Gabbro cools slowly, forming coarse crystals in the magma chamber
* peridotite is part of the upper Mantle where the magma originates by partial melting
41
How can you calculate rate of spreading?
Annual rate of spreading (cm/yr) = distance moved in cm / the number of years taken
42
What are the major features under the oceans?
* continental shelf
* continental slope
* abyssal plain
* mid-ocean ridge
* median Rift Valley
* oceanic trench
* seamounts
* ocean basins
43
What is the continental shelf?
Although it's below sea level at the present time it is still part of the continent
44
What is the continental slope?
The steeper (about 4 degrees) slope of the continental margin between the edge of the continental shelf at about 200m and where it merges into the abyssal plain at 1500-3500m deep
45
What is the abyssal plain?
A deep ocean basin 3-5km deep. It's flat with the magic rocks of the oceanic crust below it. It is covered by thin beds of fine-grained, slowly deposited, pelagic sediment derived from wind blown dust. The area is aseismic.
46
What does aseismic mean?
That there are no earthquakes there
47
What is the Mid ocean ridge?
An elongated submarine ridge in the middle of the ocean rising 2-3km above the abyssal plain and up to 1000km wide. Built up of basalt extruded at the divergent plate boundary where two oceanic plates move apart by sea floor spreading. An axial Rift Valley splits the summit of the ridge. Has frequent shallow focus earthquakes due to rising magma and movement along transform faults
48
What is a Deep-Sea trench?
It's very deep (up to 11km) narrow (up to 150km), occurs along the edges of the ocean alongside gold mountain belts and island arc systems.
49
What is a seamount?
A submarine basalt volcano rising about 1000m above the ocean floor without reaching the sea level. They occur singly/in groups/in chains.
•some seamounts are topped by a coral atoll
50
What is a seamount with an eroded top called?
A guyot
51
What does mafic mean?
It describes an igneous rock with 45-52% silica such as basalt.
52
What are pelagic organisms?
They live in/ and are sediment deposited from the main body of sea water
53
What is the term planktonic associated with?
Organisms that float in sea water
54
What are clastics?
Sediments made up from fragments of pre-existing rock.
55
What percentage does the continental land surface and the continental shelf make up?
40%
56
What is the continental shelf in terms of the land?
The area of the sea floor around the edge of the continental land mass which gentle slopes to a depth of 150-200m. It's covered with beds of clastic sands, clays and carbonate limestone deposits.
57
What are fold mountains?
Fold mountain ranges/the eroded roots of former mountain chains form belts along the margins of the continents. They are made of very thick sequences of sedimentary rocks that have been strongly folded and faulted by compressive forces.
58
What are the two main mountain ranges?
The Rockies and the Andes. Several 1000km long. Their width is generally 300-800 km. Earthquakes are quite likely
59
What is the continental shield/craton?
They are stable blocks of rocks that form the ancient cores of the continents. They are made of highly deformed crystalline Precambrian metamorphic and igneous rocks.
60
What is the Major Rift Valley and Rift system?
A Rift Valley is a linear strip of crust that has slipped down along normal faults, which dip towards the Valley.
61
What are Earthquakes?
They are caused by a vibration in the rocks of the Earths crust and upper Mantle, caused by a sudden dislocation of the rocks along a fault.
62
Where are earthquakes mainly confined to?
Active seismic zones around the world where stresses are acting on the crust and upper Mantle
63
What is depth of focus related to?
It is related to the plate boundaries
64
What are earthquakes like at Mid-Ocean areas?
Shallow focus earthquakes. High heat flow and eruption of basaltic pillows at the MOR indicates rising magma. When magma moves, it vibrates to produce harmonic waves which are detected as small, shallow-focus earthquakes.
65
What type of earthquakes occur at axial rift systems?
Shallow focus earthquakes. Extension of the crust at the Rift Valley along the centre of the MOR causes formation of normal faults. Movement along the faults, usually subsidence causes earthquakes.
66
Why do Earthquakes not originate at depths greater than 720km?
Because deeper rocks are not brittle enough to fracture
67
Why do shallow focus earthquakes occur at Transform faults?
Because the oceanic crust spreads away from the MOR in sections between transform faults. The horizontal movement along the transform faults cause earthquakes
68
What is a subduction zone?
Where an oceanic plate is subducted beneath a continental plate, the depth of origin of earthquakes ranges from shallow to deep
69
What are earthquakes like under deep ocean trenches and fold mountains?
Shallow earthquakes occur here and along the oceanic sides of fold mountains.
70
What do trenches mark?
The line at the surface where two plates meet
71
What happens when you get further inland towards fold mountains?
The focus of the earthquake gets progressively deeper along a zone that slopes away from the trench at about 45 degrees. It marks the top of the boundary between the two plates where one moves beneath the other.
72
What is the Benioff zone?
The area where most of the earthquakes occur in subduction zones. They also occur under island arc systems
73
What are collision zones?
Where two continents collide, such as India and Asia, there will be shallow and medium earthquakes along deep faults. Compression is taking place as the Indian plate is still being pushed north from the Indian Ocean MOR. Shallow earthquakes are common
74
What are the ideas behind plate tectonics?
The surface are of the earth can't get any bigger, while the sea floor is created at the MOR it is consumed at subduction zones. In 1965 Wilson suggested that the surface of the earth is divided into a small number of sections. They are thicker than just the crust but are lithospheric plates
75
What is the lithosphere plate?
A layer or shell of the crust and upper Mantle, in which the rocks are rigid and brittle
76
How do the lithospheric plates move?
They are carried by the moving underlying asthenosphere. The base of the lithosphere is taken as 1300 degrees c isotherm and varies from 5km at a MOR to 300km below fold mountains on a continent
77
What is a continental plate?
It carries a continent but will also have a oceanic crust
78
What is an oceanic plate?
It has no continental material
79
What are the 7 major plates?
Pacific, South American, North American, African, Eurasian, Indian-Australian and the Antarctic plates
80
What is the plate tectonic theory?
It gives a coherent explanation for many of the major global geological events such as earthquakes, volcanoes, faults, folds and fold mountains
The uppermost layer of the Earth is divided into a number of sections, which are constantly in motion relative to each other carried by moving material beneath
The rigid lithospheric plates are carried by the plastic asthenosphere (rheid layer)
81
How can you identify plate margins?
New crust is formed at the MOR and destroyed along subduction zones. Where plates meet there will be displacement and therefore earthquakes, which suggests a way of marking out The boundaries between them.
82
What other methods are there for mapping out the plate margins?
* earthquake zones
* changes in heat flow
* chains of volcanoes
* trenches and rift valleys
* changes in gravity
* fold mountain belts
83
What is the thickness of the lithosphere, density, age, average composition and examples of oceanic plates of the oceanic crust?
Thickness - 5km under the MOR to 200km away from the ridge
Density - 2.9 g/cm^3
Age - 0-200Ma
Average composition - peridotite of the upper Mantle topped with basaltic oceanic crust with a thin layer of sediments on top
Examples - Pacific, Cocos and Nazca plates
84
What is the thickness of the lithosphere, density, age, average composition and examples of continental plates of the continental crust?
Thickness - 300km under major mountain ranges to 75km under the continental platform
Density - 2.7 g/cm^3
Age - 0-3960 Ma
Average composition - peridotite upper Mantle topped by granitic crust that is deformed and contains a mix of igneous, metamorphic and sedimentary rocks
Examples - Arabian Plate is almost entirely continental crust most other plates carry continental crust
85
What are divergent plates?
Plates are moving apart.
86
What is the alternative name for divergent plate margins?
A constructive plate margin
87
How are pillow Lavas formed?
At volcanic vents, the lava erupts as globules, which cool rapidly in the cold sea water to form fine grained basalt in rounded masses of pillow Lavas
88
How do dolerites and Gabrro form?
In the feeder pipes of volcanoes, magma which doesn't reach the surface cool as medium grained dolerite, forming sheeted dykes. Below the dykes, magma cools slowly to solidify in layers of coarse-grained gabbro
89
What and where are Divergent plate boundaries?
They are locations where plates moving away from one another. Rising convection currents in the Mantle push up and move along the bottom of the lithosphere. This cause the plate to move in the direction of the flow. When the plates start to move apart, pressure is reduced, allowing the peridotite Mantle material to partially melt.
90
What can magma accumulating below the ridge become?
It finds its way up through feeder dolerite dykes until it reaches the surface of the sea floor and becomes a fissure lava flow
91
Why is the Mid Atlantic ridge a good example of a divergent plate boundary?
For most of its 7000km length it is a submarine feature. However, Iceland is above sea level, probably because there is a hot spot under Iceland as well as the ridge
92
What is the evidence for divergent plate boundaries?
* Dating of basalts, sediments and magnetic anomalies on the ocean floor
* The mountain ranges along the MOR
* The heat flow is higher than average across the ridge due to hot magma rising beneath it
* Shallow focus Earth quakes are caused by moving magma
* Fissure volcanoes can form basalt lavas
93
What are black smokers?
They are hot springs occurring in narrow zones of the flanks of the MORs. High speed jets of very hot solutions (350 degrees) are emitted from a series of small vents. These hot waters are rich in sulfur, with copper, iron, zinc and gold dissolved from the rocks below giving the water its characteristic black colour. On cooling, and helped by the action of bacteria living in this warm, mineral rich environment, minerals are precipitated around the central in the form of a chimney, a few meters tall. Iron, copper and zinc sulfides are made from the elements listed above, and have the potential to form ore deposits, especially if the oceanic crust is carried onto land by plate movement to form ophiolites.
94
What are the three types of convergent plate margins?
* Oceanic-Continental
* Oceanic-Oceanic
* Continental-Continental
95
Which is the plate which always descends?
The oceanic plate because it is made of dense basalt compared to the lighter granitic continental plate
96
Heat flow anomalies of a convergent plate boundary?
The heat flow at O-O and O-C show the same pattern. A negative anomaly at the trench where the cold lithosphere is subducting and a positive anomaly at the island arc or volcanic belt where magma is being intruded
97
Volcanic activity at convergent plate boundaries?
Partial melting of the crust at subducting zones produces basaltic magma. This is less dense and so rises to form volcanoes in an island arc or chain
98
Batholiths at convergent plate boundaries?
Where continental crust is melted the material will separate where the viscosity of magmas are different can form giant granite batholiths under fold mountains
99
What are trenches?
The form above the point that the oceanic plate is being subducted. They are long, narrow, linear features that form the deepest parts of the Earths Surface arranged parallel to the edge of the plate
100
What's the other name for divergent margins?
Constructive because they add new material
101
What's the other name for convergent margins?
Destructive because they consume the oceanic crust along the subduction zones
102
What is the Benioff zone?
As the oceanic plate descends the sloping plane is marked by increasingly deep-focus earthquakes.
103
Name an example of a convergent continental-continental plate margin?
The Himalayas
104
What type of lava comes from volcanoes at oceanic-oceanic margins?
Intermediate and mafic lava e.g. Andesite and basalt
105
What type of volcanoes can you get at oceanic-continental margins?
Explosive rhyolitic volcanoes or strato volcanoes due to the high temperature which can cause the continental crust to melt
106
What happens at constructive plate margins?
Where some plates literally just slide past each other e.g. The San Andres fault.
107
What are convection cells?
They are caused by convection currents due to the variations in heat. Slow moving convection currents within the asthenosphere or the whole Mantle move the overlying lithospheric plates. Hot denser material rises upwards then flows sideways and starts to cool down
108
What are the other possible mechanisms of plate movement?
Ridge push and slab pull
109
What is ridge push?
It occurs at MORs magma rising at the MOR pushes the plates apart.
110
What is slab pull?
It occurs at subduction zones. Gravity pulls the oceanic lithosphere down into the Mantle
111
What is the complete opening and closing of an ocean known as?
A Wilson Cycle
112
How is the Earth constantly being recycled?
Opening and closing of the oceans recycles the materials
113
What is a hot spot?
A volcano within a plate, which is the surface expression of a Mantle plume.
114
What is a Mantle plume?
A stationary area of high heat flow in the Mantle, which rises from great depths and produces magma that feeds hot spots volcanoes
115
What is a diapir?
A body of lower density buoyant material rising upwards in the same way as hot wax does in a lava lamp
116
What is seismic tomography?
A computer imaging technique that is based on seismic wave velocities.
117
Why do you get island chains?
Because the plate is moving whilst the hot spot (Mantle plume) remains stationary
118
Where are the oldest islands?
The furthest away from the hot spot
119
How can you calculate the rate of sea floor spreading using hot spots?
Speed = distance / time
| Measured in cm per year
