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Geology is the study of the planet earth, inlcuding it’s composition, origin, and history. Most of the solid part of the earth is composedo f rocks. Because the earth is composed of rock, an understanding of their nature, compostition origin, and histories is central to understanding the Earth history. (Raymond, 2002 p1)
Chapter 1 Rock and Earth Structure
Contents • Introduction • Three class of rock • Rock distribution in the earth • Earth structure and petrotectonic assemblages • summary
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)
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 actve fluid or gas.
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 seimetary 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.
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.
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 interudes to form masses of siliceous igneous rock (granitoid). Invading magma cause hihg temperature, to low to high pressure metamorphic rocks. • Sediments may acumulate in the basin along the arc-trench (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 percolatin 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.
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.
