Tectonic Forces: Plate Tectonics=volcanism, earthquakes, and folding and
faulting
Continental and Oceanic plates are found on the earth’s crust
They are moved by convection currents (when the material expands it rises and
when it cools it drops = a circle) in the asthenosphere
Divergence occurs when the convection current rises and splits the oceanic plate
apart (see diagram above= ridge)
Main location=Mid-Atlantic Ridge or Rift=the youngest rock is found here and you
also get transform faults and the creation of shield volcanoes=Iceland
The arrow point to the ridge in this remotely sensed image.
Basaltic lava (highly viscosity=less fluid, not as eruptive) is a result of sea-floor
spreading
Subduction is created when an oceanic plate dips into a continental plate=composite
volcanoes are found along these plate boundaries=Andes Mountains (Chile),
Cascades (North America)
Oceanic plates can also collide=denser plate subducts, creating volcanic island
arcs=Phillipines
Continental plates also collide forming huge fold mountains=Himalayas are a result
of collision between the Indian plate and the Eurasian Plate
Plates can also move past each other creating a tear (aka. strike-slip/transform)
fault. Most famous transform fault: San Andreas Fault.
The three types of plate interactions are pictured below:
On the provincial exam:
K
V
E
You are given an image like the one above-most often you will be asked what type of
interaction is occurring along distinct plate boundaries
Z=subduction zone (Japan=Pacific plate subducts into North American
Y=Convergence (continent to continent=Indian plate into Eurasian=Himalayas)
W=Tear/Transform fault (San Andreas=North American plate moving past Pacific)
X=Rift/Divergence (East African Rift Valley=Arabian and African plate boundaries
V=Divergence (Mid-Atlantic Ridge = Iceland)
K=Subduction (Juan de Fuca subducts into North American plate (our
location)=Cascade Mountain Range
E=Subduction (Nazca into South American Plate=Chile=Andes Mountain Range)
The Rock Cycle:
Changes in Rock Formation
As Igneous Rocks (granite, basalt) undergo weathering and erosion they will form
sediments. Those sediments are deposited and lithification (compaction) to form
sedimentary rocks (sandstone, shale, limestone). Intense heating and pressure will
cause changes in the crystalline structure (no melting) metamorphic rocks (gneiss,
marble)
Look at this real world example below and try to trace the changes:
Volcanism:
1) Cinder Cone
- basaltic magma ejected under high pressure from a narrow neck cools to
form cinder and ash which falls back down to build the sides of the cone
- cone can build rapidly with steep sides i.e. 30 - 40 degree angle
- if the narrow neck of the cone becomes plugged the volcano can explode violently
2) Shield Cone
eg Kilauea in Hawaii
- often formed from basalt lava which has a low viscosity (i.e. basaltic) allowing the
lava to move for some distance forming a broad gently sloped volcano i.e. 10 -20
degree angle
- less explosive tendency because the magma flows freely allowing the gases to
escape
3) Composite Cone (or sometimes called Stratovolcanoes)
eg Mt. St. Helens, Mt. Baker, Mt. Rainier, Mt. Etna in Sicily
- consists of alternating layers of lava and ash
- lava layers permit the cone to attain considerable width while the cinder/ash
layers encourage vertical growth
- as a result, some of the world's highest volcanic mountains are in this category
- lava is a mixture of the andesite type (i.e. silica-rich) and the basalt type
(i.e. magnesium-rich)
- can be very explosive spewing much pyroclastic material (i.e. cinders, ash, and
rock fragments) high into the
atmosphere and / or rapidly down the side of the volcano as a pyroclastic flow.
Such a flow sears everything in its path and may also contain a nuee ardente - a
very hot cloud of gases (i.e. hydrogen, carbon monoxide, sulphur, and chlorine).
- mud flows, known as lahars, are frequently triggered as heat from the volcano
melts snow and ice
Mt. St. Helens:
Earthquakes:
Built up-stress between two plates is released
Deformed plates snap back into position=Elastic-Rebound theory
Focus: where the earthquake occurs.
Epicentre: spot on earth's surface directly above the focus.
Magnitude
Magnitude is the total amount of energy released by an earthquake, and
consequently, the amount of ground motion it causes. Magnitude is measured using
the Richter Scale. The scale grows by a power of 10 for each 1.0 increase in
magnitude. Hence, a 5.0 earthquake is 1000 times (10 x 10 x 10) greater than a 2.0
earthquake.
Intrusive Forms
Due to magma that solidifies in cracks and cavities that it has made as it forces
the surrounding rock apart.
wall or dike: magma that cools in a vertical crack
sill: magma that flows between strata of sedimentary rock forming a thin but
extensive sheet
laccolith: dome of igneous rock formed from a large enough volume of magma to
raise up the overlying strata
batholith: a huge storage basin of magma several kilometres thick.
Folding and Faulting:
Folding is the compression of rock strata (horizontal or compressed)
Gets compressed into folds
PARTS OF A FOLD:
ANTICLINE: the layers of
rock that bend up.
ANTICLINE
SYNCLINE: The layers of
rock that bend down
SYNCLINE
Faulting is the process when the crust fractures and the blocks are either
vertically or horizontally displaced. This displacement is caused by tension (apart)
or compression (together).
1) NORMAL FAULT:
• When a fracture is caused by
tension, separation occurs and one
side of the fault drops.
2) REVERSE FAULT:
• If compression produces the
fracture, one block may override the
other. (also called a thrust fault)
3) STRIKE-SLIP, TEAR, OR
TRANSFORM FAULT
• When the forces cause the rocks to be
displaced horizontally along the fault line
and there is no vertical displacement. The
best example is the San Andreas Fault.
4) RIFT VALLEY OR
GRABEN
• When two parallel fractures occur and the
block between them drops. The African
Rift Valley is the best example.
5) HORST
• When two reverse faults occur causing a
block of rock in the middle to be pushed
up. through a process of tension or
compression
Caused by divergence and tension
Local/Global landforms resulting from tectonic processes:
Western Canada=Coast Mountains=volcanic mountain range=created by the Juan de
Fuca plate subducting beneath North American Plate
No active volcanoes in British Columbia=presence of hot springs and geysers
South=Cascades (active volcanoes)=Mt. St. Helens erupted in 1980
San Andreas Fault=active earthquake zone=large population living in that area
(Southern California)
Globally-Mt. Vesuvius (Italy), erupted because of the African Plate subducting
beneath Eurasian.
Mt. Pinatubo (Philippines) = biggest volcanic eruption
Himalayas (highest Mountain ranges) = created by Indian plate converging into
Eurasian plate.
Effects of Tectonic Processes on cultural, economic, political and social activities:
Most of the major landforms are created by tectonics = volcanoes, mountain ranges
How are they beneficial to humans?
Himalayas=attracts tourists=income for the government of Nepal=jobs for the
locals.
However, tourists bring garbage and western influences the local culture.
Water melting from the mountain feeds rivers such as the Gangesion
Mountains are a barrier for transportation.
Culturally, the people worship the mountain goddess.
Many people have lost their lives trying to climb Everest and K2 (Himalayas)
Melting can cause flooding.