The Movement of plates results in mountain building
gather, process and present information from secondary sources which compares
formation, general rock type and structure of mountain belts formed as a result of
thermal uplift and rifting with those resulting from different types of plate convergence

A good way to compare information is to structure the information in a table.
A table like the one below is an effective tool to assist you to gather, process and present
information. Well-designed tables assist you to identify useful information and will assist you to
notice trends and patterns. Try using a table to sort out the information from the notes provided
after the table.
Mountain formed by
Formation
Mountain Belt features
General Rock
Structures found in
Type
mountain belt
(including folds and
faults etc)
Thermal uplift and
rifting
Example……………
Oceanic/oceanic
convergence
Example……………
Oceanic/ continental
convergence
Example……………
Continental/
continental
convergence
Example……………
1) Use information from the class notes and handouts and the following information to fill in the table
Divergent boundaries
Mountain belts formed from the action of thermal uplift and rifting are of two main types:
1. Mid-ocean ridges form a near-continuous underwater mountain chain that extends for
60 000 kilometres right around the globe. Mid-ocean ridges rise to over 2.4
kilometres above the floor of the 5 kilometres deep ocean basins. A mid-ocean ridge
can be a wide a 2000 kilometres.
Mid-ocean ridges result from convective upwelling of mantle beneath thin oceanic
lithosphere. They are formed along structurally weak zones created where the ocean
floor is being pulled apart lengthwise along the ridge crest. New magma from deep
within the Earth rises easily through these weak zones and eventually erupts along the
crest of the ridges to create new oceanic crust. This process is called seafloor
spreading.
At the top of the oceanic crust at mid-ocean ridges are basalt lavas. The lavas often
form as pillow basalt. Beneath are numerous basaltic dykes and deeper down are
gabbros. The topography near the ridge axis is very rough and mountainous. At the
centre of each ridge there are steep-sided troughs, several kilometres wide, which are
similar to rift valleys that occur on continents. Mid-ocean ridges are offset by
transform faults that run perpendicular to the ridge axis. The faults are only active
between the spreading centres.
2. Young rift zones occur within continental landmasses and are caused by
convective upwelling of mantle beneath weak continental lithosphere. When
continental crust stretches beyond its limits, tension cracks begin to appear on the
Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to
erupt and form volcanoes. Rift zones generally have intensive basaltic igneous
activity. The rising magma, whether or not it erupts, puts more pressure on the crust to
produce additional fractures and, ultimately, the rift zone. The uplift produces
plateaus adjacent to the rift. These plateaus generally slope upwards towards the rift
valley. Escarpments in the rift valley are formed from normal faulting into the rift.
Such features are seen in Africa along the East African Rift Zone. Main rock is basalt
however some Rhyolite due to partial mixing with continental crust.
Convergent boundaries
At convergent plate margins, great slabs of oceanic lithosphere slide ponderously into Earth’s
internal abyss—the deep mantle. As they slowly disappear from the surface, spectacularly
deep trenches form graceful arcs on the seafloor. The subducted plates strive to reach
mechanical and chemical equilibrium with the mantle, and, in the process, many of Earth’s
most dramatic landscapes and structures are created. Earthquakes, volcanic arcs, deep-sea
trenches, and the continents themselves are the result of converging plates. But perhaps the
most fascinating phenomena resulting from plate collision are the great mountain ranges of
the world: the Alps, Andes, Rockies, and Himalayas.
Convergent plate margins are where continental crust is born, just as divergent plate margins
are the birthplaces of oceanic crust. This is perhaps the most important fact to remember as
you study these important plate boundaries. This new granitic crust is so buoyant that it can
never sink into the denser mantle below. Consequently, the rocks of the continents are much
older than those in the ocean basins. They preserve a record of much of Earth’s ancient
history—a record in the form of faults, folds, mountain belts, batholiths, and sediments.
The three types of convergent boundaries result in the following mountain types:
Ocean/ocean boundaries:
Mountains formed at ocean/ocean boundaries are of the volcanic island arc type. They form
on an oceanic plate that has another oceanic plate subducting under it. There are two types of
mountains that can form at ocean/ocean boundaries.
1. Those that comprise elongated mounds of ocean floor sediments that have been
tightly folded and chaotically mixed in the trench by the faulting (reverse) and
folding caused as they are scraped from the down-going oceanic plate. The southern
line of islands of the Indonesian Archipelago is a good example of this type.
Those formed of chains of explosive volcanoes. These volcanoes form from andesitic
magmas that are generated as the subducted plate partially melts when it comes in contact
with the hot asthenosphere. Steam and other volatile substances find paths upwards, creating
vents for magma to reach the surface to create the volcanoes. The northern line of islands of
the Indonesian Archipelago is a good example of this type. The Tonga Islands in the western
Pacific show the structure and topography of a simple island arc. The volcanoes are
dominated by the eruption of andesite, and the backarc region is extending to form a basin.
Ocean/continent boundaries:
As an oceanic plate is subducted beneath a continent, the sediments on the upper surface of
the lower plate will be scraped off to produce a wedge of sediment called an accretionary
wedge. Where the accretionary wedge is forced directly against the leading edge of
continental crust, the subducting plate will be forced down steeply into the asthenosphere
where the plate will be partially melted. Steam produced in the process also partially melts
the upper mantle. Andesitic magmas are produced from these processes. Mountains will be
produced in the continental plate from the compression and uplift of the low density wedge
sediments and the sediments and rocks of the continent, and from the intrusion of magma
produced from the partial melting in the subduction zone. These mountains rise to very high
altitudes and contain highly folded and faulted sedimentary rocks produced from the
compressional forces. The upper sections of sedimentary mountain ranges remain poorly
consolidated and quickly erode, producing large amounts of sediment for the rivers that drain
from them. The intrusions of magma are in the form of large granitic batholiths beneath
the volcanic belt. The mountains contain explosive andesitic volcanoes. The explosive
volcanoes produce much pyroclastic sediment that is deposited in the mountain areas. The
explosive volcanoes frequently form calderas where they develop from eruptions from large,
shallow magma chambers. The Andes Mountain chain in South America is a good example
of this ocean/continent type of mountain building.
Continent/continent boundaries:
When two continents collide, the ocean between them has been subducted under one of them.
The continents will have been flanked by accreted sediment from the ocean floor that was
scraped off from the subduction. This sediment forms into a huge wedge as it is folded,
compressed and uplifted. Rocks from old oceanic plate, called ophiolites, can also be
squeezed between the two continents and be uplifted as part of the mountain range formed.
Ophiolites (remains of oceanic crust) are very mafic and are composed of rocks like basalt
and gabbro. Eventually the two older sections of colliding continents meet. These older
sections of the continents are called cratons. Cratons are made up of crystalline igneous and
high-grade metamorphic rocks. They are old and incompressible. The rocks of the craton
splinter and fault at low angles, stacking on each other as they are compressed to form
mountains. The Himalayas are an example of a mountain range that has been formed from
compressed ocean floor sediments and fractured cratons. Low-angle thrust faults are common