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Tectonic change 1
To many geographers one of the most amazing things about the subject is
that one of the most fundamental ideas affecting the subject has only
become apparent to us over the last 45 years, i.e. the complex history of
our planet.
In that time the theory of plate tectonics has been accepted, refined
and widely applied as a remarkably successful framework for
understanding the history of the Earth.
Theme 2 – Investigating Tectonic and Hydrological Change
Key Question 2.1 What are the processes associated with plate
tectonics?
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Patterns of plates and plate boundaries.
Processes associated with constructive, destructive and
conservative plate margins.
Key Question 2.2
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Hazards associated with tectonic activity.
Demographic, economic and social impacts of
tectonic hazards.
Local and regional impacts of tectonic hazards .
Key Question 2.3
managed?
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What are the hazards associated with
tectonic events?
How are tectonic hazards perceived and
Different perceptions and awareness of tectonic
hazards by groups with conflicting interests.
Strategies to manage tectonic hazards.
The effectiveness of management strategies.
Continental Drift
The idea that continents can move around
on the Earth's surface has been suggested
since the 17th century, but serious
evidence in support of the idea of
Continental Drift was only put forward by
the German meteorologist Alfred
Wegener in 1912. His ideas were initially
greeted with scepticism, but by now the
idea is fully accepted and maps of the
world at various geological ages have
been produced. The big change since the
1950's has been the science of plate
tectonics, which shows how the
continents move, explains that our Earth
is a dynamic entity with a history of
turbulence, destruction, creation and
upheaval
Evidence for plate tectonics
Evidence in support for the theory of plate tectonics has come from a number of areas:
[1] the analysis of seismic (earthquake) waves telling us about the internal composition of
the Earth;
[2] the use of sonar and radio-sounding equipment giving us information about the deep
ocean floor, information previously denied us by its inaccessibility;
[3] the magnetic analysis of deep ocean floor rocks;
[4] the existence of rocks formed in tropical climates in polar areas, and of
glaciations in tropical areas;
The Gondwana
glaciation of c.300
mya
[5] the anomalous distributions of various plants and animals which are hard to
explain except by the process of continental drift;
[6] the comparison of the surfaces of the Earth and other planets by the
development of spacecraft.
Creation of the Solar System
The Sun forming out of a cloud of dust and gas
Planetisimals colliding and accreting
We have long known that the inside of the Earth is hot - the evidence is there in
volcanoes and hot geysers. But why is the centre of the earth hot and where does the
heat come from? The answer lies in understanding what happened when the Earth was
formed. The Earth has existed for approximately 4650 million years. It was probably
formed by small pieces of material, known as planetesimals, accumulating or accreting in
space. These initially came together to form a homogenous, undifferentiated mass. As
the cold planetesimals accumulated, the planet began to heat up and then separate into
different layers.
Impacts and radioactivity
This heating was caused by two different factors: firstly the impact of bodies striking
the planet gave out heat, and secondly the process of the Earth contracting and
squeezing itself into a smaller space also gave out heat. Since that initial period a third
source of heat has also been active, i.e. the disintegration of radioactive elements like
uranium and thorium, although scarce, is sufficient to generate high temperatures
inside the confines of the Earth.
Differentiation and outgassing
During the process of differentiation most of the Earth's iron and nickel and other
heavy metals sunk to the Earth's core, leaving the lighter elements to make up a
disproportionate amount of the upper layers of the Earth.
Outgassing
Gradually the surface of the Earth cooled until by
4000 million years ago it had cooled sufficiently to
allow an ocean to form, an ocean which
precipitated from the gases which were
outgassed from the crust by volcanoes.
The Archean Period
At this time continents did not exist. The oldest rocks so far found on the earth
date from around 3900 million years ago. This period of the Earth's history is called
the Archean period and lasted until 2500 million years ago. As the surface of the
earth cooled plates of solid rock formed, but these were dragged down into the
hot molten mantle on convection currents.
Microplates
Gradually some of these plates became cold enough
as they were pulled down into the mantle that they
stayed solid for longer, and were subjected to greater
heat. This produced a lighter material which floated
to the surface and formed a different kind of plate.
Initially these plates were in the shape of long strips,
reflecting the shapes of the fissures from which they
emerged. Being lighter they resisted being pulled
back into the mantle and stayed on the surface as
continents.
Over the years
partial melting of
basaltic crustal
material from
oceanic plates has
formed continental
crust which cannot
be subducted.
Continental crust is
lighter and through
weathering, erosion
and deposition by
rivers, the sea ,
wind and glaciers is
now composed of a
wide variety of
rocks.
Stromatolites
and blue-green
algae
Stromatolites at Shark Bay, Western Australia
Life started very early on in the Earth's history, although did not progress beyond
simple single celled organisms for most of its history. It is likely that it thrived in
pools of water near hydrothermal springs. The oldest `fossils' found date from
3550 million years ago in rocks from Western Australia, and are algal structures
known as stromatolites. Around 2500 million years ago a radical change took
place. Blue-green algae developed the ability to photosynthesise and so free
oxygen became much more common in the atmosphere. Life became more
diverse, and the free oxygen became broken down high in the atmosphere to form
a layer of ozone, which prevented the harmful ultra-violet rays from reaching the
surface, and so stimulated the development of more types of life.
Cratons
The Proterozoic period (2500 - 550 million years ago) saw
the establishment of the Earth's continents. Land areas
were transformed from narrow strips of crust to continent
size chunks. These ancient hearts of continents are known
as cratons. The low lying eroded remains of these ancient
continents can be seen today in the ancient shields of
Canada, Siberia, Australia, central Africa, the Amazon and
Antarctica
Supercontinents
By the end of the Proterozoic
period life was plentiful in the
sea, although not on land, and
the atmosphere was rich in
oxygen. It is possible that at
one or several times during the
Proterozoic era there were
supercontinents comprising all
of the Earth's land surface, as
continental drift brought all of
the smaller continents
together.
Evidence of a glacial
period 750 mya is to be
found on the Skerries,
north of Anglesey
Snowball Earth
At least twice during this long period in Earth's history the positions of the
continents and their effect upon ocean currents caused the Earth to enter an ice
age (at about 2300 and 700 million years ago). There is evidence for this second ice
age in rocks from the Skerries just to the north of Anglesey. Gradually the
supercontinents break up and their parts drift round the Earth until they meet up
again and form another supercontinent. This pattern is called the Wilson
Supercycle and is thought to occur over periods of c400 million years. This process
has been important for the last 2500 million years.
Hypsometry
So much for the early history of the Earth, but
how does continental drift actually occur,
according to the theory of plate tectonics?
Interest in continental drift began again during
the 1950's when evidence from a number of
areas began to filter through to geologists.
Thorough surveys of the sea floor enabled graphs
to be made plotting the relative distribution of
surface areas of different heights (a hypsometric
graph). For the Earth it shows a bimodal
distribution (with two peaks).
If the data is plotted as a cumulative frequency curve (a hypsometric curve) it
suggests that the break between oceans and continents does not occur at sea
level but at a depth of c200m. A map of major global morphological features
shows large areas of fairly flat terrain, both on the continents and oceans, and
long thin areas of mountains.
Structure of
the Earth
Seismic evidence has given us a
picture of the interior of the
Earth. There are three main
zones, the crust , mantle and
core. The boundary between the
core and mantle is very sharp
and located at a depth of
2900km. The crust-mantle
boundary is marked by the
Mohorovicic discontinuity
(Moho), with an average depth
of 35km beneath continents but
only 10km beneath oceans.
Lithosphere and asthenosphere
In the upper part of the mantle is located a semi-rigid layer known as
the lithosphere which behaves as if it were part of the crust. The
thickness of the lithosphere shows considerable variability and its
lower boundary is gradational and shows no sharp change.
Underneath the lithosphere is the asthenosphere, where the plastic
like properties of the rocks permits them to flow slowly over periods
of time. It corresponds to a zone of low velocity seismic waves.
Remanent
magnetism
It was found earlier in this century by Brunhes in France and
Matuyama in Japan that rocks displayed remanent magnetism,
i.e. they took on the polarity of the magnetic field of the Earth
at the time they cooled and hardened. But the polarity of the
Earth has changed over time and at certain periods seems to
have reversed, i.e. north becoming south etc. Analysis of rocks
on either side of the Mid Atlantic Ridge seemed to show a
pattern of parallel lines aligned on either side of the ridge
itself.
Sea-floor
spreading
In 1962 H.H.Hess of Princeton University in the USA
suggested that the mid ocean ridges were where new
oceanic crust is generated by the upwelling of hot mantle
material. The crust, he suggested, spreads laterally away
from these ridges until it reaches an island arc or
mountain belt where it descends into the mantle along
the adjacent oceanic trench. This `conveyor belt' view of
the ocean crust was termed sea-floor spreading.
Plate
tectonics
The plate tectonics theory proposes that the Earth comprises seven major and at least
a dozen minor lithospheric plates composed of the crust and the upper more rigid part
of the mantle. These plates are constantly in motion with one another and the motion
of one plate influences the motion of the others. Rates of movement vary up to
100mm/year (100km/million years) averaging 70mm/year. Much of but not all of the
Earth's seismic activity occurs along the boundaries between these plates. The narrow
zone marking the relative movement between two plates, usually clearly demarcated
by seismic activity is termed a plate boundary, while the peripheral region adjacent to
this boundary is called a plate margin.
Types of plate boundary
There are three types of plate boundary:
* at a divergent boundary new crust is formed and attached to the upper part of adjoining
lithospheric plates while new upper mantle is accreted to the
lower part. Plate movement is laterally away from the spreading
ridge.
These can also be called constructive boundaries.
* at a convergent boundary two plates are in motion towards each other, with one plate slipping
down below the other along a subduction zone. The surface area of
a plate is reduced at a convergent boundary but increased along a
spreading ridge.
These can also be called destructive boundaries.
•
along a transform boundary two plates simply move past each other without any major
element of divergence or convergence.
These are also called transform faults or conservative
boundaries.
The most famous example is the San Andreas Fault in California.
At some locations three plates meet. These areas are called triple junctions.
But how do the convection currents cause sea-floor spreading? Here researchers differ in
opinion, although some theories have more adherents than others. One idea is that lateral
flow in the asthenosphere, arising from convection currents in the mantle, drag the
overlying lithosphere (A-C on next slide). Convection currents rise and diverge below midoceanic ridges and descend along subduction zones. Convection occurs when too much
heat is present at depth to be conveyed upwards solely by thermal conduction.
Temperatures increase and material begins to rise as it is less dense than surrounding
material. Cooler denser material moves laterally to take its place. Most geologists support
the idea that convection occurs in the mantle. However whether this is solely or even
primarily responsible for plate movement is another matter.
The depth of
convection
currents
The depth at which convection currents start is
a major bone of contention, with some
researchers arguing that it is confined to the
asthenosphere (A), others suggesting 700km as
a likely depth for convection's lower limit (B).
More recently some have suggested whole
mantle convection (C), as large scale convection
cells such as this fit in with the size of the plates
themselves.
Ridge-push, Gravity sliding and Slab pull
Other processes have also been postulated to
account for plate movement. The injection of lava at
ocean ridges would push plates apart, it has been
argued (D). Others have said that the thickening of
the plates as they move away from the hot mid
ocean ridge area and add material at their base
through accretion would make a plate slide under
the force of gravity downwards from a mid-ocean
ridge towards a subduction zone (E). Cool, thick, old
oceanic lithosphere is gravitationally unstable as it is
generally denser than the asthenosphere over
which it lies. Therefore a cold, dense lithospheric
slab descending up to 700km into the mantle at a
subduction zone will tend to pull the remainder of
the plate with it (F). As rock warms and cools so
slowly the centre of the slab will be up to 1000oC
cooler than the surrounding mantle at a depth of
several hundred km retaining its high density
characteristics.
Slab
pull
The argument over the relative significance of these processes continues today, but the
indications are that it is the last process, lithospheric slab pull that is the predominant
driving force. Those plates attached to long subduction boundaries tend to move more
quickly than those without extensive subducting boundaries
The Pacific Ocean
The Atlantic Ocean
Landforms of divergent boundaries
Divergent or constructive boundaries occur in two main places –
in the middle of the sea at a MID-OCEAN RIDGE
and underneath a continent at a RIFT VALLEY
Mid-ocean ridges have many submarine volcanoes where pillow lavas are
erupted, a central rift valley, hydrothermal vents and black smokers.
The Mid-Atlantic Ridge
Pillow
lavas
Lava is squeezed out of a lava
tube like toothpaste. It solidifies
immediately forming pillow-like
shapes. Local examples dating
from 450 mya can be found at
Ynys Llanddwyn.
Black smokers
Water from the seafloor had seeped into the
rock, become heated and dissolved minerals
from the rock. When meeting deep ocean
water at 2°C the minerals precipitate into a
chimney.
In 1977 a deep sea submarine of the East
Pacific Rise discovered chimneys of
superheated steam at 300°C escaping from
hydrothermal vents, along with whole
ecosystems of animal and plant life
associated with the minerals in the water.
Shield and fissure volcanoes
Shield volcanoes are found where the
mid-ocean ridge breaks the surface of
the sea, e.g. Iceland. They are built up
from layer upon layer of fluid basaltic
lava.
Fissure volcanoes, e.g. Krafla in
Iceland are very flat and occur
where the crust opens up in a
line. Lava is basaltic and fluid.
These volcanoes are effusive, i.e.
material flows rather than
explodes from them.
Mid-continental rift
valleys
In East Africa is located the largest rift
valley system on land. The continent is
opening apart and the two sides leave a
central area lower than the sides.
Horst and graben
The Great Karas Mountains
from Namibia, an example of
a horst
The uplifted block on either side is called a horst,
and the central rift area is called a graben.
Below are two views of the East African Rift Valley
and the lakes which fill part of the graben.
Half graben
If one side moves more than the other an asymetrical half-graben can result.
Landforms of convergent
boundaries
Convergent boundaries occur in three places,
between one ocean plate and another (an island arc)
between an ocean plate and a continental plate
between two continental plates (a collision zone)
In the first two subduction can take place because oceanic rock is heavy,
so a slab of cold oceanic crust is thrust down into the mantle. Earthquakes
occur at increasing depths (the Wadati-Benioff Zone). An ocean trench is
formed. Some material partially melts and rises as a diapir to crate
explosive andesitic volcanoes.
Island arc
These are numerous in
the Pacific Ocean,
Indonesia and around
the Caribbean. Here an
ocean trench is to be
found, e.g. the deepest
of all the Marianas
Trench. Magma rises to
the surface and a
volcanic arc occurs, e.g.
Mt Pelee in the
Caribbean. Diapirs
cause some spreading
in the marginal basin.
The WadatiBenioff Zone
On destructive margins, where an
ocean plate is subducted beneath
another oceanic or a continental plate,
a line of earthquakes can be detected,
getting progressively deeper away
from the ocean trench. The two
seismologists who first discovered this
feature were Egyptians Wadati and
Benioff.
Soft material on
the ocean floor,
oozes and
muds, are
scraped off by
subduction to
form melange.
Some of
Anglesey is
formed of
melange rocks
from a long
distant
subduction
episode.
Andesitic mountain
range
When oceanic crust is
subducted under
continental crust an
ocean trench, e.g.
Peruvian Trench is
formed. Diapirs rise to
form many andesitic
volcanoes containing
an explosive mix of
oceanic material
(basalt) and
continental material.
The rock is named
after the Andes
mountain range. The
compression forms
folds and fault in the
rock, hence a fold
mountain range.
Stratovolcanoes
As magma from partial melting of the oceanic
crust forces its way through to the surface
through continental crust a wide variety of
material can reach the open air. This can vary
from steam and gases (in fumaroles and
solfarata), to ash and dust, lava and the blasted
material of pyroclastic flows.
Volcanoes in andesitic mountain chains and
island arcs are often strato-volcanoes. These
are composed of alternate layers of lava, ash
and pyroclastic material (clouds of molten rock
droplets and steam). The central vent often
falls in to form a crater. Secondary vents may
occur on the slopes. These volcanoes are
much more explosive than shield volcanoes
and often represent a significant hazard to life
and property.
Pyroclastic flows
Many volcanoes in the Ring of Fire, around
the Pacific, such as this one, Semeru, in
Indonesia, are prone to pyroclastic eruptions
Intrusive rocks
Part of a granite batholith in Namibia
Not all of the magma which rises
in diapirs reaches the surface.
Magma which cools
underground is much more likely
to have large crystal sizes than
volcanic lavas. If the magma is
basaltic the rock is called
gabbro. If the rock is of
continental crust it is called
granite. Granite often forms
large features underground
called batholiths. If magma can
rise through cracks in the rock it
can form dikes, sills and
laccoliths.
Collision zones
Some areas of rock may be
broken off and forced over other
layers. This is called a nappe.
When two continents meet both plates
are light and thick and cannot be
subducted. A collision zone results. Fold
mountains are created by the pressure
and although earthquakes may occur,
volcanoes are much less common.
India and the Himalayas
The Indo-Australian Plate moved northwards
and collided with the Eurasian Plate, creating
the Himalayas over the last 50 million years.
Transform margins
Transform margins, or conservative plate boundaries
may occur under the ocean or on land. They are
especially common at right angles to mid-ocean
ridges, where they are known as fracture zones or
transform faults. They split the ridge up into smaller
blocks so that it can bend. No volcanoes are found on
these boundaries but a number of landforms
associated with seismic activity are found. An
excellent example is the San Andreas fault in
California.
Landforms of transform margins
The landforms of transform margins are those involved with
earthquake fault lines. Hills and valleys are offset, drainage channels
are diverted and escarpments and depressions may form. Lakes may
form in the depressions.
3-4 metres of rightlateral slip have offset
a ridge and created a
'shutter' ridge blocking
flow in the dry creek
channel. 1999 Hector
Mine earthquake
California
Denali fault in Alaska crossing stream after
2002 earthquake. Stream offset was 4.8m.
An escarpment along a fault line.
A fault pond or sag pond on the
San Andreas Fault in California.
Turbidity currents
In oceans earthquakes may cause large amounts of mud to flow
as a turbidity current. They have great erosive power and may
break submarine cables. Many of the largest of the world’s rivers
have created submarine canyons which produce turbidity
currents fairly regularly.