04_Geology_Origin of earth.pdf

7/30/2019 04_Geology_Origin of earth.pdf

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Earth—Its Place in Our Solar System

• Some 4.6 billion years ago, various planetesimals in oursolar system gathered enough material together toform Earth and the other planets. Scientists think thatthis early Earth was probably cool, of generally uniformcomposition and density throughout, and composed

mostly of silicates, compounds consisting of siliconand oxygen, iron and magnesium oxides, and smalleramounts of all the other chemical elements.



Subsequently, when the combination of meteorite

impacts, gravitational compression, and heat from

radioactive decay increased the temperature of Earth

enough to melt iron and nickel, this homogeneous

composition disappeared and was replaced by a series

of concentric layers of differing composition and

density, resulting in a differentiated planet



• This differentiation into a layeredplanet is probably the mostsignificant event in Earth’s history.Not only did it lead to the formationof a crust and eventually continents,

but it also was probably responsiblefor the emission of gases from theinterior that eventually led to theformation of the oceans andatmosphere.



• This orderly division results from density differences betweenthe layers as a function of variations in composition,temperature, and pressure.

• The core has a calculated density of 10–13 grams per cubiccentimeter (g/cm3) and occupies about 16% of Earth’s totalvolume. Seismic (earthquake) data ndicate that the coreconsists of a small, solid inner region and a larger, apparentlyliquid, outer portion. Both are thought to consist mostly of ironand a small amount of nickel.

• The mantle surrounds the core and comprises about 83% of Earth’s volume. It is less dense than the core (3.3–5.7g/cm3)and is thought to be composed mostly of peridotite, adark, dense igneous rock containing abundant iron andmagnesium.



• The mantle can be divided into three distinct zonesbased on physical characteristics. The lower mantle issolid and forms most of the volume of Earth’s interior.

The asthenosphere surrounds the lower mantle. It hasthe same composition as the lower mantle, butbehaves plastically and flows slowly. Partial meltingwithin the asthenosphere generates magma (molten

material), some of which rises to the surface because itis less dense than the rock from which it was derived.The upper mantle surrounds the asthenosphere. The

solid upper mantle and the overlying crust constitute

the lithosphere, which is broken into numerousindividual pieces called plates that move over theasthenosphere, partially as a result of underlyingconvection cells.





• The crust, Earth’s outermost layer, consists of twotypes. Continental crust is thick (20–90 km), has anaverage density of 2.7 g/cm3, and contains

considerable silicon and aluminum. Oceanic crust isthin (5–10 km), denser than continental crust (3.0g/cm3), and is composed of the dark igneous rocksbasalt and gabbro.





Earth’s Systems

• Scientists whostudy Earth haveidentified four mainEarth systems: the

geosphere,atmosphere,hydrosphere, andbiosphere. Each

system is unique,yet each interactswith the others.



• Geosphere. The area from the surface of Earth down to itscenter is called the geosphere. The geosphere is dividedinto three main parts: the crust, mantle, and core. The rigid

outer shell of Earth is called the crust. There are two kindsof crust—continental crust and oceanic crust. Just belowthe crust is Earth’s mantle. The mantle differs from thecrust both in composition and behavior. The mantle rangesin temperature from 100°C to 4000°C — much warmer than

the temperatures found in Earth’s crust. Below the mantleis Earth’s core.

• Atmosphere. The blanket of gases that surrounds ourplanet is called the atmosphere. Earth’s atmospherecontains about 78 percent nitrogen and 21 percent oxygen.The remaining 1 percent of gases in the atmosphereinclude water vapor, argon, carbon dioxide, and other tracegases. Earth’s atmosphere provides oxygen for living things,protects Earth’s inhabitants from harmful radiation fromthe Sun, and helps to keep the planet at a temperaturesuitable for life.



• Hydrosphere. All the water on Earth, including thewater in the atmosphere, makes up the hydrosphere.About 97 percent of Earth’s water exists as salt water,

while the remaining 3 percent is freshwater containedin glaciers, lakes and rivers, and beneath Earth’s surfaceas groundwater. Only a fraction of Earth’s total amountof freshwater is in lakes and rivers.

• Biosphere. The biosphere includes all organisms onEarth as well as the environments in which they live.Most organisms live within a few meters of Earth’ssurface, but some exist deep beneath the ocean’s

surface, and others live high atop Earth’s mountains.All of Earth’s life-forms require interaction with at leastone of the other systems for their survival.



Earth’s Composition

• Matter. Atoms are the basic building blocks of allmatter.

• Atoms. Matter is anything that has volume andmass. Everything in the physical world thatsurrounds you is composed of matter. On Earth,matter usually occurs as a solid, a liquid, or a gas.All matter is made of substances called elements.An element is a substance that cannot be broken

down into simpler substances by physical orchemical means. For example, gold is still goldwhether it is a gold brick, coins, or a statue.





What elements are most abundant?

• Astronomers have identified the two most abundantelements in the universe as hydrogen and helium. Allother elements account for less than 1 percent of allatoms in the universe, as shown in the figure



• Analyses of the composition of rocks and minerals on Earthindicate that the percentages of elements in Earth’s crustdiffer from the percentages in the universe. As shown inthe figure, 98.5 percent of Earth’s crust is made up of only

eight elements. Two of these elements, oxygen and silicon,account for almost 75 percent of the crust’s composition.This means that most of the rocks and minerals on Earth’scrust contain oxygen and silicon.

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04_Geology_Origin of earth.pdf