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Mineral
Genesis |
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How
do minerals form? |
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The
Theory of Plate Tectonics There is a natural
order and beauty to the universe, and scientists spend most of their working
lives trying to discovery the fundamental rules that describe that order.
Mineralogists and geologists are no exception to this rule. By the
mid-sixties, most geologists accepted the hypothesis
or theory of plate tectonics.
This theory explained so much about the distribution of the different
kinds of rocks that make up the Earth that it literally rocked the foundations
of geology.
The idea
that the Earth's crust is broken up into tectonic plates that skate over the
surface of the Earth, was difficult for many people, geologists included, to
accept. The major argument against plate tectonics (or continental drift,
an earlier theory), was the difficulty of identifying a mechanism that cold move
entire continents large distances. It wasn't until early in the 1950's
that the evidence for plate tectonics
fell into place.
The Mechanism driving Plate Tectonics
Upwelling of hot partially
molten asthenospheric material
at spreading centers and the sinking of cold, dense oceanic plates at subduction
zones are the surficial expression of the convective forces that drive the
movement of the tectonic plates on the Earth's surface. Although,
geophysicists are still debating the existence of one or two layers of
convection cells in the mantle, it is agreed that the gravitational forces
responsible for mantle convection and subduction drive plate tectonics.
The outer 100 kilometers of the Earth that deforms rigidly is called the lithosphere.
It is this portion of the Earth that is fragmented into the tectonic plates that
flow over the hotter ductile (plastic) convecting asthenosphere.
 The
hot, ductile, peridotite (an example from the Basin and Range is shown to the
left) that makes up the asthenosphere rises close to the Earth's surface at the
mid-oceanic ridges. The upwelling of the hot low density asthenosphere
results in decompression melting of the peridotite (photograph to the left) to
generate basalt lavas. These basalt lavas separate, rise, and are erupted
to become new oceanic crust. The plates move away from each other over time, as
younger basaltic crust continues to plaster on to the edges of the plates.
The other example of divergent plate boundary occurs when a continental plate is
broken up by upwelling asthenosphere and subsequent magma injection. The
rift zone in East Africa and parts of the Arabian peninsula, and the Rio Grande
rift in New Mexico appear to be in the early stages of this process.
Subduction is the
result of two tectonic plates converging, as one plate, typically the oceanic
plate, is forced down or subducted under the less dense continental plate.
Continental plates are composed of a irregularly
stratified base of stratified mafic and intermediate igneous rocks, metamorphic
gneisses and schists, intruded and/or interstratified intermediate through
felsic plutonic and volcanic rocks, covered by an volumetrically insignificant
layer of sedimentary rocks and volcanic rocks. The continental rocks are
less dense than the sea-water altered basalts, gabbros and peridotites that make
up the oceanic plates. The gravitational pull of the denser oceanic plate
drives the subduction.
Occasionally, the
oceanic plate is thrust up over the less dense continental plate in a convergent
process called obduction. The peridotite, gabbros, and basalts exposed on
Cyprus and Oman ophiolite complexes are believed to be part of an obducted
oceanic plate. Convergence can also take place between two dominantly
oceanic plates.
The collision of
two continental plates or two oceanic plates are also examples of convergent
plate movement. When two continents collide, the two continental masses
buckle and are pushed upward and sideways. The collision of India into
Asia 50 million years ago caused the Eurasian plate to override the Indian
Plate. The continued slow convergence of the Indian and Eurasian plates
over millions of years produced the folded and deformed Himalayas, the highest
continental mountains in the world.
Transform
boundaries occur when two plates slide horizontally past each another. The
large faults or fracture zones that make up a transform boundary typically
connect two spreading centers. Most transform faults occur in oceanic
plates. One well-known continental transform fault is the San Andreas
fault. Faulting will not cause California to "fall" into the
ocean, but in time, the part of California located west of the San Andreas fault
will move north with respect to the rest of North American.
The average rate
of plate movement is slow. According to the United States Geological
Survey, the Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the
East Pacific Rise in the South Pacific about 3,400 km west of Chile, has the
fastest rate (more than 15 cm/yr). These rates may seem insignificant but
they are not considering the huge expanse of geologic time. 2.5 cm/year is
approximately the rate at which human fingernails grow.
The
Wonder of Plate Tectonics The theory of plate
tectonics is considered to be the greatest geologic discovery of this century
because it can explain so many different geologic phenomena. Plate
tectonics explains the reason for the global distribution of volcanoes and
earthquakes, the shape of continents, the global distribution of plant and
animal fossils, the location of marine fossils and sediments at the top of
mountains, and the distribution of minerals. The approximately 3,000
minerals on Earth exist because of the distribution of specific chemical
elements and the wide range in pressure and temperature conditions. Both
the distribution and range of geologic environments and elements are controlled,
in large part, by plate tectonics.
Minerals
crystallize and are stable at a range of specific temperature and pressure
conditions. Crystallization also requires certain elements to be
present. If the temperature and pressure conditions change beyond the
stability field of a mineral, the mineral becomes unstable. Given enough
time and enough energy, an unstable mineral will breakdown and change to another
mineral or minerals by reacting with other unstable minerals present to
form new minerals or phases (a phase is a homogeneous, physically distinct
portion of matter occurring in the same physical state in a heterogeneous
environment or system). Phases can be crystalline solids, liquids, gases,
or fluids (fluids are materials that have some of the properties of both liquids
and gases).
An example may
illustrate these points. Picture a basalt erupting, cooling and then
crystallizing along the Juan de Fuca ridge. After crystallization,
sea-water will infiltrate and circulate through the cooling basalt. In
time, some of the fine-grained pyroxene and plagioclase minerals that make up
the basalt, and that are no longer within their stability field, will start to
break down at the lower temperature. The presence of water will
speed this reaction. The elements in the decomposing pyroxene and
plagioclase will recombine to produce clay minerals, chlorite, and other low
temperature minerals that are stable in the presence of water at the current
temperature and pressure conditions.
After some much
longer period of time (in the order of millions of years), the Juan de Fuca
plate that has been moving east away from the spreading center, is subducted
eastward beneath North America. The temperature and pressure conditions
change as the slab slowly descends into the Earth. At some point, the
pressure and temperature conditions change enough (pressure increases,
temperatures will decrease), and blueschist facies metamorphism of the altered
basalts occurs. High pressure minerals such as lawsonite, albite feldspar,
jadeite (a pyroxene group mineral), glaucophane (an amphibole group mineral),
and pyrope (Mg-rich garnet) form at the expense of the minerals in the hydrated
altered basalt.
If subduction
continues, temperature and pressures continue to increase, until the the T-P
conditions of the eclogite facies are reached. At this point, the hydrous
minerals in the blueschist will breakdown and recrystallize to form an eclogite
which consists of anhydrous pyrope and a Na-rich pyroxene called omphacite.
Pyrope and omphacite are stable at the higher temperature and pressure
conditions. The water that was released with the breakdown of the hydrous
minerals, will move upwards through the rock column. The presence of water
at some higher level in the upper mantle peridotite or lower crustal rocks may
instigate partial melting as some of the minerals become unstable at those
pressure and temperature conditions, triggering another series of mineral
breakdown and crystallization reactions.
The
Rock Cycle A dynamic balance exists between
constructive and destructive forces operating on Earth. Subduction and
continent-continent collision build mountains; weathering and erosion tear them
down. Rocks and minerals are constantly being crystallized, decomposed,
and mechanically broken down into smaller fragments or altered into other
minerals. The products of weathering and erosion are carried by water,
wind, ice, and air, as sediments to the ocean where they are deposited.
With continued accumulation, the sediments are compacted under the pressure of
overlying, younger sediments, and are transformed through the processes of
lithification into sedimentary rocks.
The sedimentary
minerals and rocks recrystallize under heat and pressure to produce metamorphic
minerals and rocks. Continued heating may result in melting to form magmas which
in term crystallize under various temperature and pressure conditions to form
igneous rocks of a wide variety of compositions. Magmas that
crystallize slowly at depth in magma chambers produce coarse-grained intrusive
or plutonic rocks. Magmas that erupt at the Earth's surface cool much more
rapidly and may form volcanic glasses (obsidian), fine-grained lavas, or
pyroclastic rocks depending on magma composition and water and gas
content. Pyroclastic rocks (also called tuffs) consist of consolidated
accumulations of material explosively erupted from volcanoes. This
material can range in size from tiny fragments of glass shards or minerals to
huge bombs larger than a car. Volcanic bombs the size of small houses were
erupted from Mayan Volcano, Philippines on March 1, 2000.
Minerals are
produced by three major categories of processes: igneous, sedimentary, and
metamorphic processes. Water plays an important role in each of
these processes. Water changes the temperature and pressures at which
igneous and metamorphic reactions occur. Water is capable of dissolving,
transporting, recombining, and precipitating minerals. Water may be
released during volcanism, plutonism, and metamorphism to form heated
hydrothermal solutions that precipitate ore minerals or alter and breakdown
preexisting minerals. Hot seawater circulating through oceanic ridge
basalts dissolve metals and ultimately erupt and deposit a fine precipitate of
sulfide mineral from fumaroles located on the ocean bottom called black
smokers. Meteoric waters concentrate ore minerals by the process of supergene
enrichment.
Water is by far the the most important agent in the deposition of the detrital
sediments that are compacted, consolidated and cemented into clastic
sedimentary rocks. The majority of limestones are the accumulated remains
of shell and coral fragments precipitated by invertebrate marine animals in
shallow seas. Evaporite deposits consist of layers of gypsum, halite and
halite that were chemically precipitated from evaporating salt-rich waters in
confined marine basins or playas (seasonal) lakes in arid or semi arid
climates. Brines (salty aqueous solutions) derived from the waters in
sedimentary basins dissolve metals and later precipitate them in Mississippi
Valley-type sulfide deposits in carbonate rocks. Low temperature aqueous
solutions precipitate carbonate and sulfates in solutions cavities in carbonate
and other sedimentary rocks.
Click on the
following links to learn more about igneous
processes and volcanism, metamorphism
and sedimentary processes, and hydrothermal
and aqueous solutions.
The plate tectonic
diagrams on these pages are taken from the online edition of This
Dynamic Earth: The Story of Plate Tectonics, by permission of the authors,
Jacquelyne Kious and Robert I. Tilling of the United States Geological
Survey. All diagrams are courtesy of the United States Geological
Survey. We recommend their site as an excellent source of more information
on plate tectonics.
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