CRUSTAL DEFORMATION and

MOUNTAIN BUILDING

JOEMAR J. CABRADILLA

Introduction

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

A. Theories of Diastrophism

1. Theory of IsostacyFrom 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 upward.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 this theory.

5. Expansion TheoryThe theory which states that the earth is

gradually expanding. Expansion of the Earth would change the continent’s position.

B. Crustal Deformation Processes

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 including

– Joints – Faults– Folds

– Foliation

Deformed vs. Undeformed

In an undeformed sequence, strata occurs in horizontal layers, just like it was deposited.

• 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

Deformed vs. Undeformed 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 strong foliations.

DeformationIn general you can say that a rock has been

deformed if it has:

translated (moved) from its original position. changed in orientation (folding, rotation and/or

tilting). changed in shape (distortion)

Deformation TermsStrain: A change in size and/or shape due to the application of stress. Descriptive terms: shortening/contraction, stretching/extension, shear. Strain = change in length / original length

Stress: A force exerted over some area that causes rocks to undergo strain. Descriptive terms: compression, tension, shear. Stress = force / area

Types of Stress

Pressure: When stress is the same in all directions. Causes volumetric change, no shape change. E.g. water or air pressure.

Compression: A stress that causes contraction.

Tension: A stress that causes extension.

Shear: A stress that causes shearing

a. Folds

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, including:

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 becomes.3. Pressure must not exceed the internal strength of the rock. If it does, fracturing occurs.4. Deformation must be applied slowly.

Folding Types and Terms

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 layers.

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 large .

Tight Fold: A fold that is narrow, i.e. the angle between limbs is small.

Plunging Fold: The hinge line is non-horizontal

Non-plunging Fold: The hinge line is horizontal

Doubly Plunging Folds form domes and basins.

Dome: An upside-down bowl-shaped fold

Basin: A right-side-up bowl-shaped fold

1. Monocline

The simplest type of fold which involves a slight bend in otherwise parallel layers of rock.

2. Anticline

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.

Types of Folds

3. Syncline

A type of fold where the rock layers are warped downward. Both anticlines and synclines are the result of compression stress. 

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 5) 

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).

What Causes Folding?Folds can form due to tectonic compression or shear. Folds can also form near buried or curved faults.

b. 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.

Types of Faults

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 FaultIt 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 fault.

2. Reverse FaultIt 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 foothills.

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.

a. Mountain Formation

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.  

1. Accumulation of Sediments

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 movement. 

2. Orogenic Stage

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.

3. Isostatic Rebound

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.

c. Kinds of Mountain

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 harder rock.

d. Life History of Mountains

YOUNG MATURE OLDHigh ElevationStill growing

Elevation is not very high, peak lowering Stopped growing

Low elevation

Rugged, irregular skylineGood scenerysnowcapped

Rounded tops covered with vegetation

Monad rocks stand out

Steep slope with base talus

Uniform gentle slopes with covered talus

Region rather flat with low rolling hills

YOUNG MATURE OLD Young rushing streams often torrential with deep ravinesNarrow valley

Mature, slow flowing streams water gaps

Move slowlyHave low banks

Avalanches, earthquakes and landslides commonly occur

Avalanche rare, earthquake unknown

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