Download Earth`s Dynamic Syst..

Part I – Earth Materials
Chapter 1: Planet Earth
Major Concepts:
 A comparison of Earth with other inner planets reveals the distinguishing characteristics of our
planet and shows what makes it unique.
 Earth’s atmosphere is a thin shell of gas surrounding the planet. It is a fluid in constant motion.
Other planets have atmospheres, but Earth’s is unique because it is 78% nitrogen and 21%
oxygen.
 The hydrosphere is another feature that makes Earth unique. Water moves in a great, endless
cycle from the ocean to the atmosphere, over the land surface, and back to the sea again.
 The biosphere exists because of water. Although it is small compared with other layers of Earth,
it is a major geologic force operating at the surface.
 Continents and ocean basins are the largest-scale surface features of Earth.
 The continents have three major components, each of which reveals the mobility of Earth’s
crust:
o Ancient shields
o Stable platforms
o Belts of folded mountains
 The major structural features of the ocean floor are:
o The oceanic ridges
o The vast abyssal floor
o Long, narrow, and incredibly deep trenches
o Seamounts
o Continental margins
 Earth is a differentiated planet, with its materials segregated into layers according to density.
The internal layers classified by composition are:
o Crust
o Mantle
o Core
The major internal layers classified by physical properties are:
o Lithosphere
o Asthenosphere
o Mesosphere
o Outer core
o Inner core
Material within each one of these units is in motion, making Earth a changing, dynamic planet.
INTRODUCTION TO GEOLOGY
Geology is the science of Earth. It concerns all of Earth: its origin, its history, its materials, its
processes, and the dynamics of how it changes.
EARTH COMPARED WITH OTHER PLANETS
Among the inner planets (Mercury, Venus, Earth, the Moon and Mars), Earth is unique because
of its size and distance from the sun. It is large enough to develop and retain an atmosphere and a
hydrosphere. Temperature ranges are moderate, such that water can exist on its surface as liquid, solid
and gas.
The Solar System
The sun generates heat by nuclear fusion, and is the centre of the system. As seen from above
their north poles, the planets move counterclockwise about the Sun in slightly elliptical orbits; all orbit in
the same plane as the Sun’s equator. The sizes and compositions of the planets vary systematically with
distance from the Sun. The density of a planet or moon reveals these dramatic differences in
composition – the densities of the rocky inner planets are quite high (over 3g/cm3), while the gas- and
ice-rich outer planets have densities less than about 1.6g/cm3.
The solar system formed about 4.6 billion years ago. Only the inner planets are even vaguely
like Earth. The solar system consists of one star, a family of eight planets (almost 70 moons discovered
so far), thousands of asteroids, and billions of meteoroids and comets. All planetary bodies in the solar
system are important in the study of Earth because their chemical compositions, surface features, and
other characteristics show how planets evolve. They provide important insight into the forces that
shaped our planet’s history.
Earth
On Earth, huge quantities of water are in constant motion, in the sea, air, and on land. Cyclonic
storms pump vast amounts of water into the atmosphere. When this water becomes precipitation on
land, it flows back to the sea in great river systems that erode and sculpt the surface. Earth is just the
right distance from the Sun to let water exist as a liquid, a solid and a gas. Water in any of those forms is
part of the hydrosphere. The presence of water as a liquid on Earth’s surface enabled life to evolve.
Earth’s dynamism is in remarkable contrast to other planetary bodies. Its interior and surface
continually change as a result of its internal heat, due to natural radioactivity (the breakdown of
potassium, uranium and thorium). Many other planetary bodies have changed little since they formed
because they are no longer hot inside. Earth’s internal heat creates slow movements within the planet.
The Inner Planets
Mercury is so small that it was unable to generate and retain enough internal heat to sustain
prolonged geologic activity. It rapidly cooled and lost the ability to make volcanoes, and so its surface
has changed little in billions of years. Thus, it remains as a ‘fossil’ of the early stages in planetary
development. Mars is too cold and the atmospheric pressure too low for water to exist as a liquid.
Venus has more internal energy, which moves the crust and continually reshapes its surfaces. A thick
carbon dioxide-rich atmosphere holds in the solar energy that reaches the surface, leading to
temperatures of around 500oC.
EARTH’S OUTERMOST LAYERS
The outermost layers of Earth are the atmosphere, hydrosphere, and biosphere. Their dynamics
are especially spectacular when seen from space. One of the unique features of Earth is that each of the
planet’s major realms is in constant motion and continual change, even the seemingly immobile
lithosphere.
The Atmosphere
The atmosphere plays a part in the evolution of most features of the landscape and is essential
for life. The earliest atmosphere was very different from that of today and was essentially oxygen-free
and consisted largely of carbon dioxide and water vapour.
The Hydrosphere
The hydrosphere is the total mass of water on the surface of the planet. Water covers about
71% of the surface. About 98% of this water is in the oceans, while 2% is in streams, lakes, groundwater
and glaciers. All of Earth’s weather patterns, climate, rainfall, and even the amount of carbon dioxide in
the atmosphere are influenced by the water in the oceans.
The Biosphere
The biosphere is the part of Earth where life exists. Almost the entire biosphere exists in a
narrow zone extending from the depth to which sunlight penetrates the oceans (about 200m) to the
snow line in the tropical and subtropical mountain ranges (about 6000m above sea level). The main
factors controlling the distribution of life on Earth are temperature, pressure, and chemistry of the local
environment.
The biosphere has been a major geological force – essentially all the present atmosphere has
been produced by the chemical activity of the biosphere; while much of the rock in Earth’s crust
originated in some way from biological activity.
EARTH’S INTERNAL STRUCTURE
The solid materials of Earth are separated into layers according to composition and mechanical
properties. From outside in, the compositional layers are crust, mantle and core. Layers based on
physical properties are lithosphere, asthenosphere, mesosphere, outer core and inner core.
Studies of earthquake waves, meteorites that fell to Earth, magnetic fields and other physical
properties show that Earth’s interior consists of a series of shells of different compositions and
mechanical layers. Earth is a differentiated planet, consisting of internal layers of increasing density
toward the centre. Thus, gravity is the motive force behind Earth’s differentiated structure. Chemical
and mechanical properties define different sets of behavior.
Internal Structure Based on Chemical Composition
The crust is the outermost layer. Continental crust is thicker (as much as 75km), is composed of
less-dense granitic rock (about 2.7g/cm3), and includes the planet’s oldest rocks (billions of years old).
Oceanic crust is 8km thick, composed of denser basaltic rock (about 3.0gc/m3), is comparatively
undeformed by folding, and is geologically young (200 million years or less).
The mantle is about 2900km thick and constitutes the great bulk of Earth (82% of its volume and
68% of its mass). It is composed of silicate rocks (Silicon and Oxygen), and also contains abundant iron
and magnesium. Its density increases with depth from about 3.2g/cm3 to nearly 5g/cm3.
The core is a central mass about 7000km in diameter. Its density increases with depth but
averages about 10.8g/cm3. It makes up 16% of Earth’s volume but 32% of its mass. It is mostly metallic
iron.
Internal Structure Based on Physical Properties
The lithosphere is the solid, strong and rigid outer layer; it includes the crust and the uppermost
part of the mantle. Earth’s lithosphere varies greatly in thickness.
The asthenosphere is the area where temperature and pressure cause the material to melt to a
plastic state.
The mesosphere is the area where high pressure offsets the effect of high temperature, forcing
the rock to be stronger than in the overlying layer.
The core can be divided into the solid inner core and the liquid outer core. The outer core is
2270km thick, with the inner core being about 1200km. The core is extremely hot, and heat loss rom
the core and the rotation of Earth probably cause the liquid outer core to flow. This circulation
generates Earth’s magnetic field.
MAJOR FEATURES OF THE CONTINENTS
Continents consist of three major structural components – shields, stable platforms and folded
mountain belts. Continental crust is less dense, thicker, older, and more deformed than oceanic crust.
The difference in elevation of continents and ocean basins reflects their fundamental difference
in composition and density, with the lower density of the continental crust causing it to be more
buoyant. The average elevation of the continents is 0.8km above sea level, and the average elevation of
the seafloor is about 3.7km below sea level. From a regional perspective, the geologic differences
between continents are mostly in size and shape, and in the proportions of shields, stable platforms and
folded mountain belts.
The shield is the extensive flat region of a continent, in which complexly deformed ancient
crystalline rocks are exposed. These rocks (highly deformed igneous and metamorphic rock) formed
over 1 billion years ago and are also called the basement complex. Shields have been relatively
undisturbed for more than a half-billion years. A shield is a regional surface of low relief (relief is the
elevation difference between the low and high points) that generally has an elevation within a few
hundred metres of sea level. Shields have a complex internal structure and arrangement of rock types.
Most of the rock in shields was formed several kilometers below the surface. They are now exposed
only because the shields have been subjected to extensive uplift and erosion.
When the basement complex is covered with a veneer of sedimentary rocks, a stable platform is
created. The layered sedimentary rocks are nearly horizontal and commonly etched by dendritic
(treelike) river patterns. These areas have been relatively stable throughout the last 500 or 600 million
years. The shield and stable platform can be grouped together in what is called a craton.
Folded mountain belts typically occur along continent margins, and are long, linear zones in
Earth’s crust where the rocks have been intensely deformed by horizontal stress during collision
between lithospheric plates. They generally have also been intruded by molten rock. The topography
can be high and rugged, or worn down to a surface of low relief.
MAJOR FEATURES OF THE OCEAN BASINS
Oceanic crust differs strikingly from continental crust in rock types, structure, landforms, age
and origin. The major features of the ocean floor are the oceanic ridge, the abyssal floor, seamounts,
trenches, and continental margins.
Submarine topography is as varied as that of the continents. Oceanic crust is mostly basalt, a
dense volcanic rock, and the major topographic features are related volcanic activity. Oceanic crust is
young, in geologic terms (less than 150 million years). It has not been deformed into folded mountain
belts.
The oceanic ridge extends continuously from the Arctic Basin, down the centre of the Atlantic
Ocean, into the Indian Ocean, and across the South Pacific. It is a broad, fractured rise, generally more
than 1400km wide, with peaks rising as much as 3000km above their surroundings. A crack-like rift
valley runs along the axis of the ridge throughout much of its length, which totals about 70,000km.
Great fracture systems trend perpendicular to the ridge.
The abyssal floor is the broad, relatively smooth deep-ocean basin area. The abyssal hills are
relatively small ridges or hills (rising up to 900m), covering 80-85% of the seafloor, making them the
most widespread landforms on Earth. Near the continental margins, land-derived sediment completely
covers the abyssal hills, forming flat, smooth abyssal plains.
The deep-sea trenches are the lowest areas on Earth’s surface. The Mariana Trench in the
Pacific Ocean is the deepest part of the oceans (11000m below sea level). The trenches are always
adjacent to chains of volcanoes called island arcs, or to coastal mountain ranges of the continents.
Isolated peaks of submarine volcanoes are seamounts. Some rise above sea level to form
islands.
The zone of transition between a continent and an ocean basic is a continental margin. The
submerged part of a continent is called a continental shelf. Geologically, the continental shelf is part of
the continent. Currently, they form 11% of the continental surface. The continental slope is a long,
continuous slope from the outer edge of the continental shelf to the deep-ocean basin and marks the
edge of the continental rock mass.
Chapter 2: Geologic Systems
Major concepts:
 A natural system is a group of interdependent components that interact to form a unified whole
and are under the influence of related forces. The materials in a system change in an effort to
reach and maintain equilibrium.
 Earth’s system of moving water, the hydrologic system, involves the movement of water – in
rivers, as groundwater, in glaciers, in oceans, and as water vapour in the atmosphere. As water
moves, it erodes, transports, and deposits sediment creating distinctive landforms and rock
bodies.
 Radiation from the Sun is the source of energy for Earth’s hydrologic system.
 A system of moving lithospheric plates – the plate tectonic system – explains Earth’s major
structural features. It operates from Earth’s internal heat.
 Where plates move apart hot material from the mantle wells up to fill the void, and creates new
lithosphere. The major features formed where plates spread apart are continental rifts, oceanic
ridges, and new ocean basins.
 Where plates converge, one slides beneath the other and plunges down into the mantle. The
major features formed at convergent plate margins are folded mountain blets, volcanic arcs, and
deep-sea trenches
 Where plates slip horizontally past one another, transform plate boundaries develop on long,
straight faults. Shallow earthquakes are common.
 Far from plate margins, plumes of less-dense mantle material rise to shallow levels, feeding
within-plate volcanoes and producing minor flexures of the lithosphere.
 Earth’s crust floats on the denser mantle beneath. The crust rises and sinks in attempts to
maintain isostatic equilibrium.
GEOLOGIC SYSTEMS
A system is a group of interdependent materials that interact with energy to form a unified
whole. Most geologic systems are open; that is, they can exchange matter and energy across their
boundaries.
Two types of systems are important in geology. A closed system exchanges only heat (no
matter), while an open system exchanges both heat and matter with its surroundings. Most geologic
systems are open systems, for example a river system. A cooling lava flow is an example of a closed
system. Earth itself is a system – a closed system since the end of the heavy meteorite bombardment
some 4 billion years ago. Subsystems also exist – a river system is pat of the larger hydrologic system.
DIRECTION OF CHANGE IN GEOLOGIC SYSTEMS
In all natural systems change occurs in the direction necessary to establish and maintain
equilibrium – a condition of the lowest possible energy.
The change and rearrangement of materials on and in Earth does not occur at random, but in a
definite, predictable way. Each component of this system is connected to another in such a way that
even a small change causes change in the rest of the system. Since changes in natural systems have a
universal tendency to move toward a state of equilibrium – a condition of the lowest possible energy –
it is possible to predict the direction of change in a natural system.
In all such transformations, some energy is lost, generally as heat. A fundamental natural law
holds that any system tends to ‘run down’, and gradually lose energy of the sort that can cause change.
The most stable state is always the one with the lowest energy. Although this equilibrium state is the
preferred state of all systems, there are many intermediate or metastable (for example a boulder sitting
in a slight depression on a hillside, where a small force would not be enough to displace it, but a larger
force would) states, adding to the complex problem of understanding earth’s dynamic systems.
Systems, Equilibrium, and Geology
The major geologic systems are the hydrologic system and the tectonic system.
THE HYDROLOGIC SYSTEM
The hydrologic system is the complex cycle through which water moves from the oceans, to the
atmosphere, over the land, and back to the oceans again. Water is in the hydrologic system – moving as
surface runoff, groundwater, glaciers, waves, and currents – it also erodes, transports and deposits
surface rock material.
The system operates as energy from the Sun evaporates water in the oceans. Most of the water
vapour condenses and returns directly to the oceans as rain. Atmospheric circulation carries the rest
over the continents, as rain, sleet, hail or snow. Water can run back into the oceans through the river
systems, while some will seep into the ground, moving through pore spaces in soil and rocks, becoming
available to plants, and continuing to slowly seep into streams and lakes. As water moves, it erodes and
transports rock material, then deposits it as deltas, beaches, and other accumulations of sediment.
Mercury and the Moon have no hydrologic system, and as such, their surfaces have remained
unchanged for billions of years.
The hydrologic system also contains within it a huge amount of energy.
Major Subsystems of the Hydrologic System
Atmosphere-Ocean System
The hydrologic system and the atmosphere work together to create the climate system
which in turn controls how the hydrologic system operates in a certain area
River Systems
Most water precipitated onto land returns to the oceans through the river system.
Water flows through rivers at a great speed, so although they may not be deep, a large
volume of water passes through them, and most of the landscape is dominated by
features formed by running water. Rivers provide the fluid medium that transports
huge amounts of sand, silt and mud to the oceans, such as the Nile Delta. 0.0001% of
Earth’s water is in rivers.
Glacial Systems
Glacial systems form in cold climates where the amount of ice melting in summer is less
than the amount of snow deposited in winter. Glacier systems greatly modify the
normal hydrologic system because the water that falls upon the land does not return
immediately to the ocean as surface runoff. Water resides in a glacier for an average of
10,000 years. 2% of Earth’s water is in the form of ice.
Groundwater Systems
20% of water not in the ocean occurs as groundwater. As it moves, it dissolves soluble
rocks and creates caverns and caves that can enlarge and collapse to form surface
depressions called sinkholes. Sinkholes may become filled with water and form circular
lakes.
Shoreline Systems
This is the transport of water and sediment by waves along coastlines. The effects of
shoreline processes are wave-cut cliffs, shoreline terraces, deltas, beaches, bars and
lagoons.
Eolian (Wind) Systems
The circulation of the atmosphere forms the eolian system. In the broadest sense, the
wind itself is part of the hydrologic system, a moving fluid on the planet’s surface.
THE TECTONIC SYSTEM
The tectonic system involves the movement of the lithosphere, which is broken into a mosaic of
separate plates. These plates move independently, separating, colliding, and sliding past one another.
The margins of the plates are sites of considerable geologic activity, such as seafloor spreading,
continental rifting, mountain building, volcanism, and earthquakes.
The theory of plate tectonics was developed in the 1960s and provides a master plan of Earth’s
internal dynamics. In geology, tectonics is the study of the formation and deformation of Earth’s crust
that results in large-scale features. A fundamental tenet of plate tectonics is that the segments, or
plates, of the rigid lithosphere are in constant motion relative to one another and carry the lighter
continents with them.
Plates of oceanic lithosphere form as hot mantle material rises along mid-oceanic ridges; they
are consumed in subduction zones, where one converging plate plunges downward into the hotter
mantle below. The descent of these plates is marked by deep-sea trenches that border island arcs and
some continents. Where plates slide by one another, large fractures form. The movement and collision
of plates accounts for most of Earth’s earthquakes, volcanoes, and folded mountain belts, as well as for
the drift of its continents.
Each plate is as much as a few hundred kilometers thick, and they slide over the more mobile
asthenosphere at rates of 1 – 10cm per year. The basic source of energy for tectonic movement is
Earth’s internal heat, transferred by convection.
Major Subsystems of the Tectonic System
Divergent Plate Boundaries
The plates move apart at divergent plate boundaries, which coincide with midoceanic
ridges. Hot molten material from the deeper mantle wells up to fill the void. Some of
this material erupts on the seafloor as lava. The molten rock solidifies and forms new
lithosphere. The most intense volcanism on Earth occurs at divergent plate boundaries,
but it is largely concealed below sea level.
Transform Plate Boundaries
These occur where plates horizontally slide past one another. Shallow earthquakes are
common along all transform boundaries, but volcanic eruptions are uncommon. Most
transform plate boundaries are on the seafloor, though the San Andreas Fault system in
California is an example of one on a continent.
Convergent Plate Boundaries
This is where plates move together, producing more complicated geologic activity than
that at transform plate boundaries. Intense compression builds high folded mountain
belts. Earthquakes and volcanoes dramatically outline convergent plate margins. If
oceanic plate dives beneath a continent, the molten rock may form a chain of volcanoes
on the continental margin. In convergence between two plates with oceanic crust,
strings of volcanic islands, called island arcs will form. As each subducting plate moves
downward, earthquakes are produced. Continental crust and island arc crust is less
dense than oceanic crust and resists subduction back into the dense mantle. This kind
of crust becomes intensely compressed and folded at convergent plate margins. Ex:
Andes Mountains
Within-Plate Tectonics and Mantle Plumes
Plumes of hot rock rising from the mantle may create isolated volcanoes and gently
warp the interior of a plate, such as the geysers of Yellowstone National Park in
America. Earthquakes related to this sort of activity are also common.
Plates and Plate Motion
1. The seven major lithospheric plates are:
 North American
 South American
 Pacific
 Australian
 African
 Eurasian
 Antarctic
2. The divergent plate boundaries are marked by oceanic ridges, which extend from the Arctic
south through the central Atlantic and into the Indian and Pacific oceans. Movement of the
plates is away from the crest of the oceanic ridge.
3. The North American and South American plates are moving westward and interacting with
the Pacific, Juan de Fuca, Cocos, and Nazca plates along the west coast of the Americas.
4. The Pacific plate is moving northwestward away from the oceanic ridge toward a system of
deep trenches in the western Pacific basin.
5. The Australian plate includes Australia, India, and the northeastern Indian Ocean. It is
moving northward, causing India to collide with the rest of Asia to produce the high
Himalaya Mountain ranges and the volcanic arc of Indonesia.
6. The African plate includes the continent of Africa, plus the southeastern Atlantic and
western Indian oceans. It is moving northward and colliding with the Eurasian plate.
7. The Eurasian plate, which consists of Europe and most of Asia, moves eastward.
8. The Antarctic plate includes the continent of Antarctica, plus the floor of the Antarctic
Ocean. It is unique in that it is nearly surrounded by oceanic ridges.
GRAVITY AND ISOSTASY
Gravity plays a fundamental role in Earth’s dynamics. It is intimately involved with
differentiation of the planet’s interior, isostatic adjustments of the crust’s elevation, plate tectonics, and
downward flow of water in the hydrologic system.
The gravitational adjustment of Earth’s crust is isostasy. For example, if rock is removed from a
mountain range by erosion, it will lighten the load, causing the deep crust to move upward. Earth’s
lithosphere therefore continuously responds to the force of gravity as it tries to maintain a gravitational
balance.
Isostasy occurs because the crust is more buoyant than the denser mantle beneath it. Denser
crustal material sinks deeper into the mantle than does less-dense crustal material. As a result of
isostatic adjustment, high mountain belts and plateaus are commonly underlain by thicker crust that
extends deeper into the mantle than do areas of low elevation. Any thickness change in an area of the
crust causes an isostatic adjustment.
Chapter 3: Minerals
All of Earth’s dynamic processes involve the growth and destruction of minerals as matter
changes from one state to another.
Major Concepts:
 An atom is the smallest unit of an element that possesses the properties of the element. It
consists of a nucleus of protons and neutrons and a surrounding cloud of electrons.
 An atom of a given element is distinguished by the number of protons in its nucleus. Isotopes
are varieties of an element, distinguished by the different numbers of neutrons in their nuclei.
 Ions are electrically charged atoms, produced by a gain or loss of electrons.
 Matter exists in three states – solid, liquid, gas – the differences among the three are related to
the degree of ordering of the atoms.
 A mineral is a natural solid possessing a specific internal atomic structure and a chemical
composition that varies only within certain limits. Each type of mineral is stable only under
specific conditions of temperature and pressure.
 Minerals grow when atoms are added to the crystal structure as matter changes from the
gaseous or the liquid state to the solid state. Minerals dissolve or melt when atoms are
removed from the crystal structure.
 All specimens of a mineral have well-defined physical and chemical properties (such as crystal
structure, cleavage or fracture, hardness and density).
 Silicate minerals are the most important minerals and form more than 95% of Earth’s crust. The
most important silicates are feldspars, micas, olivines, pyroxenes, amphiboles, quartz, and clay
minerals. Important nonsilicate minerals are calcite, dolomite, gypsum, and halite.
 Minerals grow and are broken down under specific conditions of temperature, pressure, and
chemical composition. Consequently, minerals are a record of the changes that have occurred
in Earth throughout its history.
MATTER
An atom is the smallest unit of an element that possesses the properties of the element. It
consists of a nucleus of protons and neutrons and a surrounding cloud of electrons. There are three
states of matter: gas, liquid, and solid. Each state is distinguished by unique physical properties.
Processes in Earth’s dynamics mostly involve the changing of matter from one state to another.
Most rock bodies are mixtures or aggregates of minerals. A mineral is a naturally occurring
compound with a definite chemical formula and a specific internal structure.
Atoms
The atomic mass of an element is the sum of the number of neutrons and protons. The
distinguishing feature of an element is the number of protons in the nucleus of each of its atoms, often
called the atomic number. Electrons in the outer shells control the chemical behavior of each element.
Isotopes
An isotope is when some atoms of a given element contain a different number of neutrons.
Some isotopes are unstable, emitting particles and energy as they experience radioactive decay to form
new, more stable isotopes.
Ions
The attraction between positive ions and negative ions is the bonding force that sometimes
holds matter together. Ionic size and ionic charge control how elements fit together to make solid
minerals.
Bonding
An atom is most stable if its outermost shell is filled to capacity with electrons. The noble gases,
with a full outer shell, will not normally combine with other elements. An ionic bond is a bond between
ions of opposite electrical charge, such as Na+ and Cl-. These typically form between elements far from
each other in the periodic table. A covalent bond is where two elements (usually close together in the
periodic table) bond together to share electrons to attain stability, such as O2, or bonds between Carbon
and Hydrogen in organic materials. Many bonds in natural substances are intermediate between
covalent and ionic bonds. Sometimes, electrons are ‘pulled’ closer to the nucleus of one ion than to the
other, leading to one part of the molecule having a slight charge, as in the Si-O bond. A metallic bond is
where atoms contribute one or more outer electrons that move relatively freely throughout the entire
aggregate of ions. This is responsible for the special characteristics of metals, including their high
electrical conductivity and ductile behavior.
States of Matter
The principal differences between solids, liquids, and gases involve the degree of ordering of the
constituent atoms. In a typical solid, atoms are arranged in a rigid framework, however in crystalline
solids, the atomic structure consists of a regular, repeating, three-dimensional pattern known as a
crystal structure. In amorphous solids the atomic arrangement is random.
THE NATURE OF MINERALS
A mineral is a natural inorganic solid with a specific internal structure and a chemical
composition that varies only within specific limits. All specimens of a given mineral, regardless of where,
when or how they were formed have the same physical properties. Minerals also have restricted
stability ranges.
Minerals are the solid constituents of Earth. For a substance to be considered a mineral, it must
meet the conditions listed above, and explained below.
Natural Inorganic Solids
Only naturally occurring inorganic solids are minerals.
The Structure of Minerals
The component atoms of a mineral have a specific arrangement in a definite geometric pattern
– their internal structure. The law of constancy of interfacial angles states that although the size or
shape of a mineral’s crystalline form may vary, similar pairs of crystal faces always meet at the same
angle.
Cleavage planes are planes of weakness in the crystal structure and are not necessarily parallel
to the crystal faces. Cleavage planes constitute a striking expression of the orderly internal structure of
crystals.
Different structural arrangements of exactly the same elements produce different minerals with
different properties. This ability of a specific chemical substance to crystallize in more than one type of
structure is known as polymorphism.
The Composition of Minerals
In some minerals, two or more kinds of ions can substitute for each other in the mineral
structure – ionic substitution. This causes changes in hardness and colour, for example, without
changing the internal structure. Some minerals that can undergo ionic substitution are feldspar, mica
and olivine.
The Physical Properties of Minerals
All specimens of a given mineral have the same physical and chemical properties. If ionic
substitution occurs, variation in physical properties also occurs, but because ionic substitution can occur
only within specific limits, the range in physical properties also can occur only within specific limits.
Significant and readily observable physical properties of minerals are crystal form, cleavage,
hardness, density, colour, lustre and streak.
If a crystal is allowed to grow in an unrestricted environment, it develops natural crystal faces
and assumes a specific geometric crystal form. The shape of a crystal is a reflection of the internal
structure and is an identifying characteristic for many mineral specimens. Cleavage is the tendency of a
crystalline substance to split or break along smooth planes parallel to zones of weak bonding in the
crystal structure. If the bonds are especially weak in a given plane, perfect cleavage occurs with ease. If
the differences in bond strength are not great, cleavage is poor or imperfect. Some minerals have no
weak planes in their crystalline structure, so they do not have cleavage and break along various types of
fracture surfaces; for example quartz which breaks by conchoidal fracture.
Hardness is a measure of a mineral’s resistance to abrasion – in effect, making it a measure of
the strength of the atomic bonds in a crystal. The Mohs hardness scale places diamonds at number 10,
with the softest mineral, talc, at number 1. Density is the ratio of the weight of a substance to its
volume. It depends on the kinds of atoms making up the mineral and how closely they are packed in the
crystal structure.
Colour is also important, though not diagnostic. Colour can be affected by impurities in the
mineral. Lustre describes the appearance of light reflected from a mineral’s surface, and is described as
metallic or nonmetallic. The lustre of a mineral is controlled by the kinds of atoms and bonds that link
them together. Many minerals with covalent bonds have a shiny lustre (adamantine lustre), as in
diamonds, while ionic bonds create more vitreous lustre, as in quartz. Streak refers to the colour of a
mineral in powder form and is usually more diagnostic than the colour of a large specimen.
Magnetism is a natural characteristic of only a few minerals, but is an important physical
property of rocks that is used in many investigations of how Earth works.
Stability Ranges
A mineral is stable if it exists in equilibrium with its environmental conditions, mainly pressure,
temperature and composition. Changes in these conditions can induce minerals to break down and
form new species that are stable under the new conditions. A mineral existing outside of its stable
range is called metastable; this occurs if the reactions to form new minerals from preexisting minerals
are very slow.
THE GROWTH AND DESTRUCTION OF MINERALS
Minerals grow as matter changes from a gaseous or liquid state to a solid state or when one
solid recrystallizes to form another. They break down as the solid changes back to a liquid or a gas. All
minerals came into being because of specific physical and chemical conditions, and all are subject to
change as these conditions change. Minerals, therefore, are an important means of interpreting the
changes that have occurred in Earth throughout its history.
Crystal Growth
Growth is accomplished by crystallization, which occurs by the addition of ions to a crystal face.
An environment suitable for crystal growth includes proper concentration of the kinds of atoms or ions
required for a particular mineral, and proper temperature and pressure.
In a restricted environment, a crystal may not grow to form its ideal crystal shape. Where a
growing crystal encounters a barrier, it stops growing. A crystal growing from a liquid in a restricted
space assumes the shape of the confining area, and well-developed crystal faces do not form. Crystal
growth in restricted spaces is common for rock-forming minerals. An interlocking texture resulting from
this competition for space is especially common in igneous rocks, which form by crystallization from
molten rock material.
Destruction of Crystals
Crystals can be destroyed by melting, or by ‘prying’ away atoms and carrying them away with a
solvent; usually (in geologic processes) water. Recrystallization due to changing environmental
conditions also destroys crystals, by rearrangement.
SILICATE MINERALS
More than 95% of Earth’s crust is composed of silicate minerals, a group of minerals containing
silicon and oxygen linked in tetrahedral units, with four oxygen atoms to one silicon atom. Several
fundamental configurations of tetrahedral groupings are single chains, double chains, two-dimensional
sheets, and three-dimensional frameworks.
Silicate minerals are complex in both chemistry and crystal structure, but all contain a basic
building block called the silicon-oxygen tetrahedron. Four large oxygen ions are arranged to form a foursided pyramid with a smaller silicon ion bonded between them – a tetrahedron.
Silicon-oxygen tetrahedrons combine to form minerals in two ways. The first is when oxygen
ions of the tetrahedrons form bonds with other elements, such as iron or magnesium. The more
common way is by the sharing of an oxygen ion between two adjacent tetrahedrons, enabling them to
form a larger ionic unit, like joining beads to form a necklace. The sharing of oxygen ions by the silicon
ions results in several fundamental configurations of tetrahedral groups:
1. Isolated tetrahedrons (ex: olivine)
2. Single chains (ex: pyroxene)
3. Double chains (ex: amphibole)
4. Two-dimensional sheets (ex: micas, chlorite, clays)
5. Three-dimensional frameworks (ex: feldspars, quartz)
The unmatched electrons of the silicate tetrahedron are balanced by various metal ions, such as
ions of calcium, sodium, potassium and iron. Considerable ionic substitution can occur in the crystal
structure, ex: sodium can substitute for calcium. Thus, minerals of a major silicate group can differ
chemically from one another, but have a common silicate structure.
ROCK-FORMING MINERALS
Fewer than 20 kinds of minerals account for the great bulk of Earth’s crust and upper mantle.
The most common silicate minerals are feldspars, quartz, micas, olivine, pyroxenes, amphiboles, and
clay minerals. Important nonsilicates are calcite, dolomite, halite, and gypsum.
Most of Earth’s crust and upper mantle are composed of silicate minerals in which the common
elements combine with silicon and oxygen. These are hard to identify, due to the rarity of welldeveloped crystal faces, and their small size.
Felsic Silicate Minerals
Felsic minerals, sometimes called sialic (due to their high silicon and aluminium content),
includes the major constituents of continental crust: feldspars and quartz. Felsic minerals have low
densities. Feldspars are the most abundant minerals in granite, a common crustal rock. Quartz usually
grows in the spaces between the other minerals, and typically lacks well-developed crystal faces as a
result of this. Quartz is abundant in all three major rock types. The pure form is colourless, but slight
impurities produce a variety of colours. Quartz has the simple composition SiO2.
Micas are potassium aluminium silicates. Two common varieties occur in rocks: muscovite and
biotite. Mica is abundant in granites and in many metamorphic rocks and is also a significant
constituent of many sedimentary rocks.
Mafic Silicate Minerals
Mafic minerals contain much magnesium and iron, generally range from dark green to black,
and have high densities. Mafic minerals are common in Earth’s mantle and in oceanic crust. They
generally crystallize at higher temperatures and have higher densities than felsic minerals.
Olivine is a green, glassy material; composed of isolated Si-O tetrahedrons linked together by
magnesium or iron ions. Pyroxenes are found in many mafic rocks in the crust and mantle. Amphiboles
have the same chemical composition as pyroxenes, with the difference being that amphiboles contain
hydroxyl ions. This mineral is common in many igneous and metamorphic rocks. A dangerous form of
amphibole is asbestos.
Clay Minerals
Clay minerals are an important part of the soil, and form at Earth’s surface, where air and water
react with various silicate minerals, breaking them down to form clay and other products. Like the
micas, the clay minerals are sheet silicates. A common clay mineral is kaolinite.
Nonsilicate Minerals
Most of these are carbonates or sulfates and typically form at low temperatures and pressures
near Earth’s surface.
Calcite is composed of calcium carbonate, usually transparent or white, and common at Earth’s
surface. Besides being the major constituent of limestone, calcite is the major mineral in the
metamorphic rock marble.
Dolomite is widespread in sedimentary rocks.
Halite and gypsum are the two most common minerals formed by evaporation of seawater or
saline lake water. Halite is NaCl. Gypsum is CaSO4.2H2O.
Oxide minerals lack silicon as well and include several economically important iron oxides, such
as magnetite and hematite. Magnetite is one of the few minerals that is naturally magnetic.
Chapter 4: Igneous Rocks
Major Concepts:
 Magma is molten rock that originates from the partial melting of the lower crust and the
upper mantle, usually at depths between 10 and 200km below the surface.
 The texture of a rock provides important insight into the cooling history of the magma. The
major textures of igneous rocks are:
 Glassy
 Aphanitic
 Phaneritic
 Porphyritic
 Pyroclastic
 Most magmas are part of a continuum that ranges from mafic magma to silicic magma.
 Silicic magmas produces rocks of the granite-rhyolite family, which are composed of quartz, Kfeldspar, Na-plagioclase, and minor amounts of biotite or amphibole.
 Basaltic magmas produce rocks of the gabbro-basalt family, which are composed of Caplagioclase and pyroxene with lesser amounts of olivine and little or no quartz.
 Magmas with composition intermediate between mafic and silicic compositions produce rocks
of the diorite-andesite family.
 Basalt, the most abundant type of extrusive rock, typically either erupts from fissures to produce
relatively thin lava flows that cover broad areas or erupts from central vents to produce shield
volcanoes and cinder cones. Volcanic features developed by intermediate to silicic magmas
include viscous lava flows, ash-flow tuff, composite volcanoes, and collapse calderas. The
abundance of water in silicic magma is critical to its development and eruption.
 Masses of igneous rock formed by the cooling of magma beneath the surface are called
intrusions or plutons. The most important types of intrusions are batholiths, stocks, dikes, sills,
and laccoliths.
 The wide variety of magma compositions is caused by variations in
 The composition of the source rocks
 Partial melting
 Fractional crystallization
 Mixing
 Assimilation of solid rock into the molten magma
 Most basaltic magma is generated by partial melting of the mantle at divergent plate boundaries
and in rising mantle plumes. Most intermediate to silicic magma is produced at convergent
plate boundaries. Partial melting of continental crust at rifts and above plumes can also
produce silicic magma.
THE NATURE OF IGNEOUS ROCKS
Igneous rocks form from magma – molten rock material consisting of liquid, gas, and crystals. A
wide variety of magma types exist, but important end members are:
o Basaltic magma – typically very hot (from 900oC to 1200oC), and very fluid
o Silicic magma – cooler (less than 850oC) and highly viscous
Magma is hot, partially molten rock material. Most magmas are not entirely liquid but are a
combination of liquid, solid and gas. Crystals may make up a large portion of the mass. Like most fluids,
magma is less dense than the solid from which it forms, and because of buoyancy, it tends to migrate
upward through the mantle and crust. Magma can intrude intot he overlying rock by injection into
fractures, it can dome the overlying rock, or it can melt and assimilate the rock it invades. Magma
eventually cools and crystallizes to form igneous rocks. Magma that solidifies below the surface forms
intrusive rock. When magma reaches the surface without completely cooling and flows out over the
landscape as lava, it forms extrusive rock.
Most terrestrial magmas consist largely of molten silicates, with the principal elements in
magmas being O, Si, Al, Ca, Na, K, Fe and Mg. SiO 2 and H2O largely control the physical properties of
magma, such as its density, viscosity (more SiO2 makes it more viscous, lower temperatures make it
more viscous), and the manner in which it is extruded.
Mafic magmas contain about 50% SiO2 and have temperatures ranging from about 100oC to
o
1200 C. Mafic minerals such as olivine and pyroxene, crystallize from such magmas. Silicic magmas
contain between 65% and 77% SiO2 and generally have temperatures lower than 850oC. Felsic minerals,
such as feldspars and quartz, are the dominant minerals that crystallize from these magmas.
Water vapour and carbon dioxide are the principal gases dissolved in a magma, they are
volatiles. Dissolved water tends to decrease the viscosity of magma. Magmas rich in volatiles also tend
to erupt more violently because of the explosive expansion of gas bubbles.
Igneous rocks on continents are mostly formed at convergent plate margins, while they are not
common in the stable platform, although they may form in association with a mantle plume. The
oceanic crust is almost entirely igneous rock formed at an oceanic rift.
TEXTURES OF IGNEOUS ROCKS
The texture of a rock refers to the size, shape, and arrangement of its constituent mineral
grains. The major textures in igneous rocks are glassy, aphanitic, porphyritic, and pyroclastic. Texture is
important because the mineral grains bear a record of the energy changes involved in the rock-forming
process and the conditions existing when the rock originated. Texture is very different to composition,
and it is texture that provides the most information about how a rock was formed.
Glassy Texture
This is produced by very rapid cooling, which could result from lava flowing into water,
or being blown into a much cooler atmosphere. The randomness of the ions in a hightemperature melt is ‘frozen in’ because the ions do not have time to migrate and
organize themselves in an orderly, crystalline structure.
Aphanitic Texture
This results from relatively rapid cooling, but not nearly as rapid as the cooling that
produces glassy textures. Ions have time to collect and organize themselves. Viewed
under a microscope, many crystals of feldspar and quartz are recognizable. Many
aphanitic and glassy rocks have numerous small spherical or ellipsoidal cavities, vesicles,
produced by gas bubbles trapped in the solidifying rock.
Phaneritic Texture
Grains in rocks of this texture are large enough to be recognized without a microscope,
and the equigranular texture shows a uniform rate, very slow, cooling, which must occur
far beneath the surface. Rocks with phaneritic textures are in fact only exposed after
erosion has removed thousands of metres of covering rock. Pegmatites are intrusive
igneous rocks with especially coarse grains.
Porphyritic Texture
This refers to igneous rocks which have grains of two distinct sizes. The larger, wellformed crystals are phenoscrysts, while the smaller crystals constitute the matrix or
groundmass. This texture occurs in either aphanitic or phaneritic rocks, and usually
indicates two stages of cooling – an initial stage of slow cooling during which the larger
grains developed, followed by a period of more rapid cooling, during which the smaller
grains formed.
Pyroclastic Texture
Under a microscope, rocks of this texture have grains which are broken fragments
rather than interlocking crystals. It is produced when explosive eruptions blow crystals
and still molten magma into the air as ash. If the fragments are still hot when they are
deposited, they will be welded (fused) together by the weight of the overlying rock.
TYPES OF IGNEOUS ROCKS
Igneous rocks are classified on the basis of texture and composition. The major kinds of igneous
rocks are granite, diorite, gabbro, rhyolite, andesite, and basalt.