DEFINITION OF STRUCTURAL GEOLOGY

The study of the deformation of the

Earth’s crust or lithosphere. We know that the surface of the

Earth is actively deforming from such evidence as earthquakes

and active volcanism, and from rocks at the surface of

the Earth that have been uplifted from great depths. The rates

of processes (or timescales) of structural geology are very

slow compared to ordinary events. For instance, the San

Andreas fault moves only a couple of centimeters a year,

which is considered relatively fast for a geological process.

Even this process is discontinuous near the surface, with

major earthquakes happening every 50–150 years, but perhaps

with more continuous flowing types of deformation at

depth. Mountain ranges such as the Alps, Himalaya, or those

in the American west are uplifted at rates of a few millimeters

a year, with several kilometers height being reached in a few

million years. These types of processes have been happening

for billions of years, and structural geology attempts to

understand the current activity and this past history of the

Earth’s crust.

Structural geology and tectonics are both concerned with

reconstructing the motions of the outer layers of the Earth.

Both terms have similar roots—structure comes from the

Latin struere, meaning “to build,” whereas tectonics comes

from the Latin tektos, meaning “builder.”

Motions of the surface of the Earth can be rigid body

rotations, where a unit of rock is transported from one place

to another without a change in size and shape. These types of

motions fall under the scope of tectonics. Alternatively, the

motions may be a deformation, involving a change in the

shape and size of a unit of rock, and this falls under the field

of structural geology.

During motions on faults or with the uplift of mountains,

rocks break at shallow levels of the crust and flow like

soft plastic at deeper levels of the crust. These processes occur

at all scales ranging from the scale of plates, continents, and

regional maps, to what is observable only using electron

microscopes.

Structural geology and tectonics have changed dramatically

since the 1960s. Before 1960, structural geology was a

purely descriptive science, and since then it has become an

increasingly quantitative discipline, especially with applications

of principles of continuum mechanics and with increasing

use of laboratory experiments and the microscope to

understand the mechanisms of deformation.

Tectonics has also undergone a recent revolution (since

the understanding of plate tectonics in the 1960s) and the

framework it provided for understanding the large-scale

deformation of the crust and upper mantle. Both structural

geology and tectonics have made extensive use of new tools

since the 1960s, including geophysical data (e.g., seismic

lines), paleomagnetism, electron microscopes, petrology,

and geochemistry.

Most studies in structural geology rely on field observations

of deformed rocks at the Earth’s surface and proceed

either downscale to microscopic observations or upscale to

regional observations. None of these observations alone provides

a complete view of structural and tectonic processes, so

structural geologists must integrate observations at all scales

and use the results of laboratory experiments and mathematical

calculations to make better interpretations of our observations.

To work out the structural or tectonic history of an area,

the geologist will usually proceed in a logical order. First, the

geologist systematically observes and records structures

(folds, fractures, contacts) in the rock, usually in the field.

This consists of determining the geometry of the structures,

including where they are geographically, how are they oriented,

and what are their characteristics. Additionally, the structural

geologist is concerned with determining how many

times the rocks have been deformed and which structures

belong to which deformation episode.

The term attitude is used to describe the orientation of a

plane or line in space. Attitude is measured using two angles:

one measured from geographic north, and the other from a

horizontal plane. The attitude of a plane is represented by a

strike and a dip, whereas the attitude of a line is represented

by a trend and plunge. Strike is the horizontal angle, measured

relative to geographic north, of the horizontal line in a

given planar structure. The horizontal line is referred to as

the strike line and is the intersection of a horizontal plane

with the planar structure. It is easily measured in the field

with a compass, holding the compass against the plane, and

keeping the compass horizontal. Dip is the slope of the plane

defined by the dip angle and the dip direction. It is the acute

angle between a horizontal plane and the planar structure,

measured in a vertical plane perpendicular to the strike line.

It is necessary to specify the direction of the dip.

To understand the processes that occurred in the Earth,

structural geologists must also understand the kinematics of

formation of the structures; that is, the motions that occurred

in producing them. This will lead to a better understanding of

the mechanics of formation, including the forces that were

applied, how they were applied, and how the rocks reacted to

the forces to form the structures.

To improve understanding of these aspects of structural

geology, geologists make conceptual models of how the structures

form, and test predictions of these models against

observations. Kinematic models describe a specific history of

motion that could have carried the system from one configuration

to another (typically from undeformed to deformed

state). Kinematic models are not concerned with why or how

motion occurred, or the physical properties of the system

(plate tectonics is a kinematic model).

Mechanical models are based on continuum mechanics

(conservation of mass, momentum, angular momentum, and

energy), and our understanding of how rocks respond to

applied forces (based on laboratory experiments). With

mechanical models we can calculate theoretical deformation

of a body subjected to a given set of physical conditions of

forces, temperatures, and pressures (an example of this is

the driving forces of tectonics based on convection in the

mantle). Mechanical models represent a deeper level of

analysis than kinematic models, constrained by geometry,

physical conditions of deformation, and the mechanical

properties of rocks.

It is important to remember that models are only models,

and they only approximate the true Earth. Models are

built through observations and provide predictions to test the

model’s relevance to the real Earth. New observations can

support or refute a model. If new observations contradict

predictions, models must be modified or abandoned.

Structural Geology and the Interior of the Earth

Structures at the surface of the Earth reflect processes occurring

at deeper levels. We know that the Earth is divided into

three concentric shells—the core, mantle, and crust. The core

is a very dense iron-nickel alloy, comprised of the solid inner

core and the liquid outer core. The mantle is composed of

lower density, solid magnesium-iron silicates, and is actively

convecting, bringing heat from the interior of the Earth to the

surface. This heat transfer is the main driving mechanism of

plate tectonics. The crust is the thin low-density rock material

making up the outer shell of the Earth.

Temperature increases with depth in the Earth at a gradient

of about 54°F per half a mile (30°C/km) in the crust

and upper mantle, and with a much smaller gradient deeper

within Earth. The heat of the Earth comes from several different

sources, including residual heat trapped from initial

accretion, radioactive decay, latent heat of crystallization of

the outer core, and dissipation of tidal energy of the Sun-

Earth-Moon system.

Heat flows out of the interior of the Earth toward the

surface through convection cells in the outer core and mantle.

The top of the mantle and the crust is a relatively cold and

rigid boundary layer called the lithosphere and is about 61

miles (100 km) thick. Heat escapes through the lithosphere

largely by conduction, transport of heat in igneous melts, and

in convection cells of water through mid-ocean ridges.

Structural geology studies predominantly only the outer

12–18 miles (20–30 km) of the lithosphere, putting into perspective

that we are inferring a great deal about the interior

of the Earth by examining only its skin.

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