To a casual observer, the crust of the earth
is permanent and fixed, but a great deal of
evidence, both direct and indirect, shows that
the earth is in constant motion. This is because
of the forces acting on its surface and within it.
Evidence of crustal movement comes in many
forms. The dynamic internal forces, collectively
known as diastrophism, generally tend to elevate
the earth’s surface. They are in constant battle
against external forces that tend to wear away
the land surface
1. Theory of Isostacy
From Greek word meaning, “equal standing”. It
states that as rock from higher region is removed by erosion and
deposited on a lower region, the higher region slowly rises while
the lower region becomes heavier and slowly sinks.
2. Contraction Theory
It states that the Earth is gradually shrinking. As
the shrinkage occurs, the stronger and the heavier blocks of the
crust sink while the weaker strata are crowded and squeezed
3. Convection Theory
A theory which would account for the pushing
and folding of rocks through convection currents. This process is
true when it occurs under continental masses. Heated portion
rises to the surface while cooler portion sinks to the ocean floor.
4.Continental Drift Theory
Is a theory which accounts for diastrophic movement
and for the folding and faulting along the edges of the
continents. Alfred Wegener, professor of meteorology in Austria
suggested that Africa and South America fit was the result of
5. Expansion Theory
The theory which states that the earth is
gradually expanding. Expansion of the Earth would change the
The process of forming a mountain not only uplifts the
surface of the Earth, it causes rocks to undergo Deformation.
Deformation: The process by which rocks are deformed
(squashed, stretched, sheared, etc…) in response to squeezing,
stretching, shearing etc.
Deformation produces a variety of geologic structures
In an undeformed sequence,
strata occurs in horizontal
layers, just like it was
• No metamorphic rocks, no
foliation, no large faults,
maybe some joints.
• Grains are round, just like
when they were deposited,
clay minerals are horizontally-
aligned from compaction
In a deformed sequence (e.g.
mountain belt) rocks are folded,
and possibly metamorphosed.
Faults with large offsets may
be present, juxtaposing different
rocks side by side.
Rocks may be highly folded
and squashed grains may create
In general you can say that a rock has been deformed if
translated (moved) from its original position.
changed in orientation (folding, rotation and/or tilting).
changed in shape (distortion)
Strain: A change in size and/or shape due to the application of
Descriptive terms: shortening/contraction, stretching/extension,
Strain = change in length / original length
Stress: A force exerted over some area that causes rocks to
Descriptive terms: compression, tension, shear.
Stress = force / area
Pressure: When stress is the
same in all directions. Causes
volumetric change, no shape
change. E.g. water or air
Compression: A stress that
Tension: A stress that
Shear: A stress that
A fold can be defined as a bend in rock that is the response to
compression forces. It can be likened to the waves in the ocean.
Each has a crust or uphold and a trough or down fold. Folds are
most visible in rocks that contained layering. For plastic
deformation of rock to occur a number of conditions must be met,
1. The rock material must have the ability to deform under
pressure and heat.
2. The higher the temperature of the rock the more plastic it
3. Pressure must not exceed the internal strength of the rock. If it
does, fracturing occurs.
4. Deformation must be applied slowly.
Hinge Line: An imaginary line that shows the location of
maximum curvature on a fold.
Axial Plane: The plane through the hinge lines on all of the
Limb: The sides of a fold that show less curvature.
Open Fold: A fold that is broad,
i.e. the angle between the limbs is
Tight Fold: A fold that is
narrow, i.e. the angle between
limbs is small.
Plunging Fold: The hinge line
Non-plunging Fold: The hinge
line is horizontal
Doubly Plunging Folds
form domes and basins.
Dome: An upside-down bowl-
Basin: A right-side-up bowl-
The simplest type of fold
which involves a slight bend in
otherwise parallel layers of rock.
Is a convex up fold in rock
that resembles an arch like
structure with the rock beds (or
limbs) dipping away from the
center of the structure.
A type of fold where the
rock layers are warped
downward. Both anticlines and
synclines are the result of
More complex fold types
can develop in situations where
lateral pressures become greater.
The greater pressure results in
anticline and synclines that are
inclined and assymetrical. (Figure
4. Recumbent fold
A fold that develops if the center of the fold moves
from being once vertical to a horizontal position. Recumbent
folds are usually found in the core of mountain ranges and
indicate the compression and/or shear forces were stronger in
one direction. Extreme stress and pressure can sometimes
cause the rocks to shear along the plane of weakness creating
a fault. We call the combination of fault and a fold in a rock
an overthrust fault. (Figure 6).
Folds can form due to tectonic compression or shear.
Folds can also form near buried or curved faults.
Faults form in rocks when the stresses overcome the
internal strength of the rock resulting in a fracture. It is defined
as the displacement of once connected blocks of rocks along a
fault plane. This can occur in any direction with the blocks
moving away from each other. Faults occur from both tensional
and compression forces.
There are several kinds of faults which are named
according with the type of stress that acts on the rock and by
the nature of the movement of the rock blocks or either side
of the fault plane.
1. Normal Fault
It occurs when
tensional forces act in
opposite directions and
cause one slab of the rocks
to be displaced up and the
other slab down. This type
is sometimes called gravity
2. Reverse Fault
It develops when compression forces exist.
Compressions causes one block to be pushed up and over
the other block.
a. Reverse Fault b. Thrust Fault
3. Graben Fault
A fault produced when tensional stresses result in
the subsidence of a block of rock. On a large scale these
features are known as rift valleys.
4. Horst Fault
Is the development of two reverse faults causing a
block of rock to be pushed up.
5. Transform Fault/Strike-slip fault
These faults are vertical in nature and are
produced where the stresses are exerted parallel to each
other (e.g. San Andreas Fault in California)
Many faults are mixed in slip type, called oblique-slip faults.
C. The Earth’s Mountains
A mountain is any landmass on Earth's surface that
rises abruptly to a great height in comparison to its
surrounding landscape. By definition, a mountain rises
1,000 feet (305 meters) or more above its surroundings
and has steep sides meeting in a summit that is much
narrower in width than the mountain's base. Any
highland that rises no higher than 1,000 feet (305 meters)
above its surroundings, has a rounded top, and is less
rugged in outline than a mountain is considered a hill.
High hills at the base of mountains are known as
Mountains cover approximately one-fifth of
Earth’s land surface. Although rare, a mountain can
exist singly (Mount Kilimanjaro – Northeast Tanzania)
but most often it occurs in groups called mountain
range --- a succession of many closely spaced
mountains covering a particular region of the Earth.
Mountain belts consist of several mountain ranges
that run roughly parallel to each other. The North
American Cordillera, the Himalayas, the Alps, and the
Appalachians are all examples of mountain belts that
are composed of numerous mountain ranges.
Geologists have developed a general model to
explain how most mountain ranges form. This model
suggests that mountain building involves three stages: (1)
accumulation of sediments, (2) an orogenic period of
rock deformation and crustal uplift, and (3) a period of
crustal uplift caused by isostatic rebound and block-
faulting. The latter two stages of this model involve
tectonic convergence of crustal plates which provides the
compressional and tensional stresses that produce rock
deformation, uplift, and faulting.
Mountain belts normally contain numerous layers of
sedimentary and volcanic igneous rocks. These
accumulations can be several kilometers in thickness. Most
of these accumulations were originally deposited in a
marine environment. The beds of the sedimentary rocks are
composed of particles that came from nearby terrestrial
landmasses. These particles were released from rocks by
weathering and then transported by erosional forces to the
edge of the terrestrial continental crust. Beyond the edge of
the continents, these sediments are lithified to form shales,
limestones, and sandstones that make up the continental
shelves, slopes, and rise. Accumulations of volcanic rock
develop along convergent boundaries where subduction is
causing magma plumes to form plutons and volcanoes. The
volcanoes are usually spatially organized in a line, called an
island arc, that runs at right angles to the direction of crustal
In the orogenic stage of mountain building, the accumulated
sediments become deformed by compressional forces from the collision
of tectonic plates. This tectonic convergence can be of three types: arc-
continent, ocean-continent or continent-continent. In an ocean-
continent convergence, the collision of ocean and continental plates
causes the accretion of marine sedimentary deposits to the edge of the
continent. Arc-continent convergence occurs when an island arc
collides with the edge of a continental plate. In this convergence, the
ocean plate area between the arc and the continent is subducted into the
asthenosphere and the volcanic rocks and sediments associated with
the island arc become accreted to the margin of the continent over time.
This type of collision may have been responsible for the creation of the
Sierra Nevada mountains in California during the Mesozoic Era. The
final type of convergence occurs when an ocean basin closes and two
continental plates collide. Continent-continent convergence mountain
building is responsible for the formation of the Himalayas, Ural, and
Appalachian mountain systems.
At the end of plate convergence, mountain building
enters its final stage. This stage is characterized by crustal
uplift because of isostatic rebound and block-faulting.
Isostatic rebound involves the vertical movement of
continental crust that is floating in the plastic upper mantle.
As erosion removes surface materials from mountains, the
weight of the crust in this region becomes progressively less.
With less weight, the continental crust makes an isostatic
adjustment causing it to rise vertically (float higher) in the
mantle. This process also causes tensional forces to exist in a
horizontal direction breaking the continental crust into a
number of blocks. Each block moves vertically to compensate
for the tensional forces producing normal and graben faults.
1. Folded Mountains
Folded mountains happen when there is a
pushing together of part of the earth's crust from the
ends, causing it to fold and ripple in the middle. The
same phenomenon can be seen if you push the ends of a
rug together; it ripples in the center. In the dipping
strata of the folds, weak beds are eroded first, leaving
resistant beds of syncline and monoclonal ridges.
2. Volcanic Mountains
The formation of these mountains is triggered by a small
crack in the Earth's crust which, in turn, is attributed to the
movement of tectonic plates. This crack allows the magma to
escape to the surface wherein it cools down and forms various
volcanic structures.) Along with molten rock, volcanic ash and
other gases are also introduced into the Earth's atmosphere
during a volcanic eruption. When this volcanic material comes
to the surface of the Earth, it settles along the vent wherein it
cools over the course of time, and eventually results in the
formation of a mountain. While the entire process seems pretty
simply, it takes several thousand of years and numerous layers
of solidified lava for the formation of volcanic mountains. Such
activities is also seen in the ocean, wherein the build-up of
volcanic matter continues for years together and finally results
in formation of islands when it reaches the ocean surface.
3. Fault-block Mountains
Fault blocks are very large blocks of rock, sometimes
hundreds of kilometers in extent, created by tectonic and
localized stresses in the Earth's crust. Large areas of bedrock are
broken up into blocks by faults. The largest of these fault blocks
are called crustal blocks. Large crustal blocks broken off from
tectonic plates are called terranes.
Because most stresses relate to the tectonic activity of
moving plates, most motion between blocks is horizontal, that is
parallel to the Earth's crust by strike-slip faults. However vertical
movement of blocks produces much more dramatic results.
Landforms (mountains, hills, ridges, lakes, valleys, etc.) are
sometimes formed when the faults have a large vertical
displacement. Adjacent raised blocks (horsts) and down-
dropped blocks (grabens) can form high escarpments. Often the
movement of these blocks is accompanied by tilting, due to
compaction or stretching of the crust at that point.
4. Erosion Volcanic Mountains
These are single or group
mountains that have been
formed by the erosion of streams.
Earth forces raise rocks to a high
altitudes, therefore deepening
the valleys and isolating their
peaks. The Sierra Madre
Mountain range in Luzon has
been formed because of the deep
eroded area occupying very wide
places with many peaks.
5. Upwarped Mountains/Dome Mountains
Dome mountains are created when a large amount of
magma pushes up from below the Earth’s crust, but it never
actually reaches the surface and erupts. And then, before it can
erupt, the source of the magma goes away and the pushed up
rock cools and hardens into a dome shape. Since the dome is
higher than its surroundings, erosion works from the top
creating a circular mountain range.
These kind of mountains don’t usually get as high as folded
mountains because the force of the magma underneath doesn’t
push hard enough. Over a long period, the magma cools to
become cold, hard rock. The result is a dome-shaped mountain.
Over long periods of time, erosion wipes away the outer layers
of the mountain, exposing the dome-shaped cooled magma of
YOUNG MATURE OLD
Elevation is not
very high, peak
Steep slope with
flat with low
YOUNG MATURE OLD
Have low banks
A particular slide catching your eye?
Clipping is a handy way to collect important slides you want to go back to later.