Download Rift Valleys (1)

Rift Valleys (1)
(a) Nature
The Rift valley is a feature of plate tectonics in which large portions of the earth's
upper mantle and crust have moved slowly (about 5 cm per year) but persistently
through geological time to form the shapes of the continents, oceans, and mountains
as we know them today. Places where the earth's crust has formed deep fissures and
the plates have begun to move apart, develop rift structures in which elongated
blocks have subsided relative to the blocks on either side. The sides of the rift valley
tend to be rugged, steep, with scarp faces.
(b) Distribution
Associated with constructive plate margins
(b) Cause
The doming up of the crust as a result of intense pressure from convection currents
within the mantle, leads to rocks splitting along their weakest points. As the plates
move apart, faults form and the fractures are widened by tension. Rift valleys occur
where two parallel faults have created a trough.
Case Study: The East African Rift Valley
Processes similar to those which formed the Atlantic may be happening beneath the
continental crust of East Africa.
Here the crust is brittle and has fractured as sections have moved apart. As the crust
has become stretched, arched and weakened, the central section has subsided to form
the Great African Rift Valley which extends 4000km from the Red Sea in the North to
Mozambique in the South. This fracturing of the crust has produced steep rift valley
sides up to 600m high with the valley width ranging from 10-50km. The rift is also
associated with volcanic activity as has been seen in the recent eruption in the Congo.
The East African Rift valley trends roughly north-south through Kenya and
Tanzania. The East African Rift is a world-famous example. It is characterized by
1) topographic deep valleys in the rift zone,
2) sheer escarpments along the faulted walls of the rift zone,
3) a chain of lakes within the rift, most of the lakes highly saline due to evaporation
in the hot temperatures characteristic of climates near the equator, and some of the
lakes are the largest in Africa,
4) voluminous amounts of volcanic rocks that have flowed from faults along the
sides of the rift, and
5) volcanic cones where magma flow was most intense.
Due to the East African Rift, the total volume of phonolite (alkaline extrusive rocks)
lava in Kenya exceeds that present elsewhere in the world by several orders of
magnitude.
Ocean Ridges (2)
(a) Nature
Oceanic constructive margins are characterised by a central submarine spreading
ridge where new oceanic crust is injected into the central rift. Oceanic constructive
margins represent the next evolutionary stage on from continental rift systems such
as the East African Rift Valley and Red Sea area.
(b) Distribution
These are associated with constructive margins and are found in the three main
oceans – the mid-Atlantic, eastern Pacific and central Indian. Iceland is the largest
island on the mid Atlantic ridge.
(c) Cause
Where two plates are diverging (convection currents in the mantle), molten magma
from the mantle rises and reaches the surface as basalt, creating new crust. The
newly added rock then horizontally pushes previously created ocean crust away
from the rift in a conveyor belt fashion. Because of this process, we find that the age
of oceanic crust increases as we move away from the rift zone. As the plates move
apart, giant fractures appear through which the magma is able to reach the surface as
lava, building a ridge on the ocean floor. This ridge is full of long parallel cracks
called fissures which vary in width from 1cm to 1km. Magma from the mantle fills
these fissures as they form and solidifies. Transform faults (large cracks) are often
created at right angles to the plate boundary as the plate moves. Underwater, lava
soon cools in pillow-like lumps. Where eruptions occur above the water’s surface,
fountains of lava can often be seen.
Case Study: Mid Atlantic Ridge
The rate of seafloor spreading and the formation of new oceanic crust varies over
time, giving rise to different widths of age bands and magnetic stripes. At times, an
excess of magma is injected into the spreading line, and this overflows onto the
seafloor to build up a large submarine ridge with a series of smaller, sub-parallel
ridges. An example of this is the Mid Atlantic Ridge. The ridge forms a very
prominent feature near the spreading line, where it develops on the crust domed up
by the rising convection currents in the underlying mantle. As the newly-formed
crust is carried away from the spreading line, the doming is reduced as the
underlying mantle is cooler. In places, the magma outpouring and doming can be so
extensive that volcanic islands can be built up, such as Iceland, which is currently
located directly on the Mid Atlantic Ridge. As these volcanic islands are carried away
from the spreading line by plate movement they subside, become volcanically extinct
and sometimes become submerged to form guyots. Numerous examples of these
extinct volcanic islands and guyots can be seen in the Atlantic basin - e.g., the Azores,
Madeira, Cape Verde and the Canary Islands. These islands were formed along the
spreading line and were carried away by plate motion so that the age of the islands
increases with distance from the spreading zone.
The opening of the Atlantic Ocean was initiated about 180 million years ago in the
Jurassic period during the fragmentation of the super-continent known as Pangaea,
and the separation of the American plates from the African and Eurasian plates. The
early phase of this plate separation was similar to the situation in the East African
Rift Valley today. The gradual enlargement of the ocean basin since Jurassic times
can be traced in the pattern of ages of ocean crust preserved in the ocean basins. New
crust is continually being created at the spreading ridge, and gradually moves away
from the ridge as spreading continues as if on a conveyor belt. This creates a pattern
of symmetrical age bands on either side of the spreading ridge, as shown in the
diagram below.
Case Study: Iceland
Iceland is the largest island sitting atop the mid-Atlantic ridge. Through the centre of
the country are major fault lines, rifts and fissures caused by tension as the two plates
pull apart. This is the zone where most tectonic activity occurs. Whilst most of the
lava has spilt out through fissures, there are some shield volcanoes similar to those
on Hawaii. Basaltic plateaus, up to 5km thick, are a relict of older tectonic activity
and are found outside the current active zone on both sides of the island.
Basaltic plateau
The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers
scientists a natural laboratory for studying on land the processes also occurring along
the submerged parts of a spreading ridge. Iceland is splitting along the spreading
centre between the North American and Eurasian Plates, as North America moves
westward relative to Eurasia
Krafla fissure lava fountain
The consequences of plate movement are easy to see around Krafla Volcano, in the
NE part of Iceland. Here, existing ground cracks have widened and new ones appear
every few months. From 1975 to 1984, numerous episodes of rifting (surface cracking)
took place along the Krafla fissure zone. Some of these rifting events were
accompanied by volcanic activity; the ground would gradually rise 1-2 m before
abruptly dropping, signalling an impending eruption. Between 1975 and 1984, the
displacements caused by rifting totalled about 7 m.
Fold Mountains (3)
(a) Nature
These are mountains which form the highest points on the earth’s surface. Within
each mountain range, there are several long parallel ranges, separated by high
plateaus and deep valleys. Young fold mountain ranges are very rugged (glacial
erosion) and spectacular. Old fold mountains are not so spectacular.
(b) Distribution
Associated with destructive plate margins.
Young fold mountains – (formed mainly within the last 100 million years), occur
where subduction zones border continents. They run in lines along the margins of
the plates which created them. The 2 young fold mountain systems are:
(i)
around the Pacific ocean, reaching its greatest height in the Andes
(ii)
East-west band across Europe and Asia, including the Alps and
Himalayas.
Old fold mountains- formed along old, previously active margins. Eg: Caledonian
mountains, Scotland, the mountains of Norway and the Appalachian mountains
(USA) were all one mountain range before Europe and America drifted apart. 400
years of weathering and erosion has resulted in today’s low mountains.
(c) Cause
Cycle of mountain building: (1)Erosion- rocks of the land are weathered and eroded
by rivers, waves, glaciers and wind. (2)Deposition- sedimentary and volcanic rocks
are deposited on ocean floor where they accumulate. (3) Deformation- as the crust of
the ocean floor slides under the continent in the subduction trench, these materials
are deformed. (4)Uplift-pressure of colliding plates causes crustal material to be
folded and uplifted into mountain ranges. (5) Volcanicity- some of the crust melted
by pressure and heat in the subduction zone is intruded as giant batholiths of granite
into the core of the mountain range; the rest reaches the surface and builds up
volcanic cones.
When continental plates collide head on with other continental plates, the result is
upheaval! With both having similar density, neither plate will be forced downwards
as happens at the destructive margin. Movement is slow, in the order of just a few
centimetres a year, so there is no massive and sudden collision shock. Instead, the
gradual forward movement of each plate creates extreme pressures within the
leading edges. Over a period of time (long in our human terms, but relatively short
in the time scale of the planet), the rock strata deform and become folded (plastic
deformation), rising to create mountain ranges.
OROGENESIS – What is it?
This is the name given to the process of fold mountain building. It involves at least
two movements: horizontal folding of sediments and vertical uplift. In some, a third
movement is the extrusion of volcanic materials to build up cones with peaks higher
than the general level of the mountains.
Case Study: The Alps
The Alps were formed about 35 million years ago by the northward movement of the
African plate colliding with the Eurasian plate. The pressure in the collision zone has
thrust the continental margins upwards, creating the Alpine mountain range. A line
of high peaks from SE France to Austria reflects the most intense period of
deformation. Around the edges of the range, folding is less intense, peaks are lower
(eg: Jura mountains in France). As the uplift continues, the Mediterranean Sea is
becoming shallower and will eventually disappear altogether (Lake Titicaca, in the
Andes, is the last remnant of a sea, uplifted by the formation of the Andes
mountains). Careful analysis of rocks at the highest levels of the Alps shows the
presence of marine sediments from the ancient Tethys Sea.
Creation of Alps
Swiss Alps: 3000m peak
Island Arcs (4)
(a) Nature
Island arcs are chains of volcanic islands
(b) Distribution
These are associated with destructive plate margins, where two oceanic plates
converge. The main band of island arcs stretch across the northern, and down the
western, side of the Pacific ocean from Alaska to New Zealand, including Japan, the
Philippines and Indonesia. This is the most active tectonic zone in the world.
(c) Cause
Where two oceanic plates converge, some sediments scraped off by collision do
contribute to the creation of land, but it is mainly the lava eruptions which are
responsible for the creation of landforms. The faster moving plate is deflected
downwards under the other plate where its oceanic crust is consumed in ocean
trenches, which take it back to the mantle. Most of this crust is melted or reabsorbed
before reaching a depth of 300km. Because it is less dense than that of the mantle, it
rises towards the ocean floor where most of it erupts as lava.
The volcanic arc can be considered an intermediate stage where eventually the arc
will be swept into a continent, where its deformed material will contribute to a new
fold mountain range, perhaps in 100-200 million years time.
Case Study: Kamchatka
Along the Pacific coast of eastern Russia, subduction zones have created the violent
volcanic region of Kamchatka with substantial eruptions occurring at regular
intervals - and with equally violent and regular earthquakes. The Aleutian Islands
form an island arc leading westwards from Alaska (home to some of the world's
most powerful recorded earthquakes) to Kamchatka. South, the Kurile Islands form
an offshore island arc (the world's deepest ocean trenches are found off the Kurile
and Mariana Islands) leading to Japan, an intensively active volcanic and seismic
region.