Geology is the study of the planet earth, including it’s composition, origin, and
history. Most of the solid part of the earth is composed of rocks. Because the earth
is composed of rock, an understanding of their nature, compositions origin, and
histories is central to understanding the Earth history. (Raymond, 2002 p1)
Road to you by: Koist team
Chapter 1
Rock and Earth Structure
Contents
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Introduction
Three class of rock
Rock distribution in the earth
Earth structure and petrotectonic assemblages
summary
Introduction
• Rock is a solid aggregate mineral grains, or a solid naturally occurring mass
composed of mineral grains, glass, altered organic matter, and
combination of those components. (e.g: igneous rock, metamorphic rock,
sedimentary rock)
• Whereas, stone is informal term to describe the little piece of rock in
specific type. (e.g: sandstone, mudstone, wackestone, etc)
• Petrology is the knowledge used to denote overall study of rock including
petrography (study of description and classification of rock e.g: under thin
section), and petrogenesis (the study of histories and origin of rocks).
• Petrography is a tool that basically observational science, to observe the
features of rock including the color, mineralogy, and textures of the rock,
to classify the type and subdivision of the observed rock.
• Petrogenesis analysis, is a work to observe the histories and genetic of the
rock. Which requires various combination of experimental and theoritical
and uses inductive and deductive reasoning to arrive at conclusions about
the origin and histories of the rock (e.g: requires tectonics study,
petrographical analysis, chemical and geochemical analysis, etc)
Three class of rock
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There are three main type of rocks: igneous, sedimentary, and metamorphic rocks.
Igneous rock form by crystallization of melted rock material. The melted rock
materials referred to as magma. The heat of magma coming from: the rest of
accretion of the planets left over 4-5 bilion years old, heat released from
radiactive decay of earth element in earth mantle, and heat released from the
plate movement over the astenosphere.
Sedimentary rocks form under surface condition consist of accumulation of:
chemical and biochemical precipitates, fragments or grain of minerals and fossils,
combinations of these kinds of minerals. Like igneous they may also experience
the crystallization process but it’s occur at the surface pressure and temperature
especially from precipitation of aqueous solution.
Metamorphic rocks are kind of rock that formed originally as igneous or
sedimentary rock that transformed to become a new rock as response to Pressure
and Temperature. The new rock has changed mineralogically, texturally or both
without undergoing melting. In addition to the process there are also another
causes like directed stress, and chemically active fluid or gas.
Rock Distribution on Earth
• On the land, sedimentary rock exposed totally of 66 % of all rocks
(Blatt and Jones 1975). Due to the distribution laterally on the
surface regardless the volume in vertically.
• Contrary in oceanic crystalline (igneous and metamorphic) rock
become a major exposure.
• Much of the solid material underneath the crust is metamorphic
rock predominantly, as response to the heat and pressure on the
upper mantle is really high.
• Considering the volume of earth as a whole (1.083x1021 m3), the
core comprises about 16.2% by volume, mantle makes up 83.12%
and the crust only 0.6% of the volume. Regarding the crust
composition sedimentary material made up about 4.8% (the rest is
crystalline) which means that sedimentary material (including
sedimentary rock on earth surface) only comprise 0.029% of a
whole earth volume.
Earth Structure and
petrotectonic assemblages
• The earth structures is distinguished from multiple zones
that marked by seismic wave velocity called discontinuity.
This phenomena unveiled by geophysical study of earth.
• First zone separate the earth core and mantle is Lehmann
discontinuity
• Outward the zone is Gutenberg discontinuity, is separate
the inner mantle and outer mantle which rich in
magnesium. and a zone which has a more plastic than layer
above and below it revealing low velocity zone of seismic
wave (now is called asthenosphere).
• Upper this zone (separate asthenosphere and crust) there
is a mohorivicic Discontinuity.
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Pressure and temperature increase with depth. About 20°C/km (called geothermal
gradient) (Raymond, 2002 p3-4). on volcanically active areas (island arc) the
temperature gradient increases at rate 30-50 °C/km in contrast near ocean
trenches the temperature rate may be as low as 5 to 10 °C/km. this response to
expressions for pressure formula P= ρgh
The earth crust due to the convective motion of magma within the mantle
disaggregate to several plates that moving relative to each other on earth surface.
The movement could be convergent, divergent, and transform.
Convergent movement margin example is subduction zone. This movement create
destruction of the plate, whereas the divergent movement create a constructive
plate (that allow magma from mantle to rise and solidified made up a new plate
(commonly in oceanic)).
Tranform fault mark shear boundary as the response to transform movement
where the plates pass one into another.
The site where three plate boundaries come together called triple junction.
Each type of plate boundary as well as tirpple junction and site within plates, give
rise to distinctive suites of rocks called petrotectonic assemblages (Dickinson
1971).
• At spreading centers, one or two types of volcanic rock form and
these acompanied by various sedimetary and metamorphic rocs.
Initiantion of the spreading process results in formation of graben
or rifts zone.
• In continents these zone serve as basin for accumulation of sand
and gravel. Intrusion of magma cause local metamorphism of low
pressure, high temperature type.
• In ocean floor spreading centers, volcanism is followed by
circulation of seawater through hot volcanic rocks and into
surrounding rocks. Resulting in low pressure chemical alteration or
metamorphism.
• Sedimentation in the oceanic areas produces thin layer of very fine
grained sediment. Locally however gravels derived from volcanic
rocks are deposted.
• In convergent plate boundary zones have wide range of
rock types. Volcanic rocks of diverse compositions
develop in volcanic mountain chains called arc, that
form on the overriding plate. At depth magma intrudes
to form masses of siliceous igneous rock (granitoid).
Invading magma cause high temperature, to low to
high pressure metamorphic rocks.
• Sediments may acumulate in the basin along the arctrench (called fore arc basin) in oceanic crust. The
subducted sediments and underlying oceanic crust and
mantle either become metamorphosed by high
pressure, low temperature condition.
• Shearing at transform fault boundaries forms
metamorphic rocks. Because rocks are
deformed rapidly, they may show extensive
breaking or stretching of mineral grain.
Seawater percolating into shear zone aids in
the metamorphic process.
• Triple junction assemblages are not easly recognized as they consist of
composite assemblage representing various types of plate boundaries that
form the tripe junction. But considering the rock assemblage for each
plate movement can be inducted to reconstruct the triple junction
assemblage, and several work have been carried to observe the triple
junction assemblage.
• Likewise Intraplate (within plate) volcanism and sedimentation process
yield variety of products. Silica poor rock dominante the volcanic and
plutonic site within the ocean plates. A variety of thin sedimentary rocks
also forms here. Within continents at intraplate sites, unusual rocks,
including diamond bearing types, form alongwith a diversity of common
volcanic rock types.
• Continental sites of sediment formation are quite varied, as are the
resulting rocks.
• Distinct sediments form river, lake, glacial, and other environment occurs.
• Along the tectonically quiet coastal areas, deposits sediments yield
sandstone, shale, and limestone.
Summary
• Earth is divided into core, mantle, and crust. The core is made up of
material, mantle consist of magma, and the crust made up of rock
material produced by the crystallization of magma and recycled of the
product. Magma is a mobile material that move through convection
energy transfer.
• The crust is the outer part of the earth which all rock forming process
happened. And disaggregated into several plates.
• Studying the rock is the core of petrology to understand the earth process
and history.
• Three kind of rocks in the crust are: igneous rock, sedimentary rocks, and
metamorphic rocks.
• Due to the magma convection movement governs the earth crust plates
movement. Could be convergent, diverent, and transform and the
encounterd of three of them (called trippple junction) the motion and the
physical of these process called tectonic process. The boundary between
the plate movement and the composition of rocks within it made up to a
tectonic assemblages which varied in type for one assemblage to another.