4 Earth_s Layered Structure

8/4/2019 4 Earth_s Layered Structure

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Earth’s Layered

StructureChapter 8, Section 4



Layers Defined by Composition

If Earth were made up of the samematerials throughout its interior, seismicwaves would travel in straight lines atconstant speeds

Seismic waves reaching seismographslocated farther from an earthquake travelat faster average speeds, due to increased

pressure at depth Earth’s interior consists of three major

zones defined by its chemical composition– the crust, mantle, and core



Paths of Seismic Rays



The Crust

Crust–

thin, rocky, outer layer of Earth;divided into oceanic and continental crust

The oceanic crust is roughly 7 kilometersthick and composed of the igneous rocks

basalt and gabbro

The continental crust is 8-75 kilometersthick (avg. 40 km) and consists mostly of

the granitic rock granodiorite The rocks of the oceanic crust are younger

then the rocks of the continental crust



Mantle and Core

Mantle – a solid, rocky shell thatextends to a depth of 2890 km

Over 82% of Earth’s mass iscontained in the mantle

The boundary between the crust andmantle represents a chemicalcomposition change

Core – a sphere composed of aniron-nickel alloy



Layers Defined by PhysicalProperties

Earth’s interior has a gradual increase intemperature, pressure, and density withdepth

Depending on the physical environment(temperature and pressure), a materialmay behave like a brittle solid, a putty, ora liquid

Earth can be divided into layers based onphysical properties – the lithosphere,asthenosphere, outer core, and inner core.



Lithosphere and Asthenosphere

Lithosphere – relatively cool, rigidshell consisting of the crust andupper most mantle

Averages about 100 kilometers inthickness

Asthenosphere – soft,

comparatively weak layer beneaththe lithosphere

The rocks within the asthenosphereare close enough to their melting

points that they are easily deformed



Lower Mantle

From a depth of 660 km down tonear the base of the mantle

More rigid layer that is still prone togradual flow

At the bottom of the mantle, rocksbehave more like those in theasthenosphere



Inner and Outer Core

The core, which is composed of aniron-nickel alloy, is divided into tworegions with different physical

properties

Outer Core – liquid layer 2260 kmthick, flow produces Earth’s magnetic

field

Inner Core – sphere with a radiusof 1220 km, compressed into a solid

state by immense pressure



Earth’s Layered Structure



Discovering Earth’s Layers

Moho–

boundary separating the crust fromthe mantle where the velocity of seismic wavesabruptly increases

Seismic waves from even small earthquakes

can travel around the world The outer core causes P waves that travel

through it to arrive several minutes afterexpected and are bent by 100o away from the

earthquake (shadow zone) It was further shown that S waves do not travel

through the outer core, making geologistsconclude that the outer core is composed of a

liquid



Discovering Earth’s Composition

Early seismic data and drilling technologyindicate that the continental crust ismostly made of lighter, granitic rocks

The crust of the ocean floor has a baslaticcomposition

The composition of the mantle and core isknown from more indirect data

Scientists use lava and meteorites to inferthe composition of the mantle and core

Earth’s core is thought to be mainly denseiron and nickel, similar to metallicmeteorites. The surrounding mantle isbelieved to be composed of rocks similar

to stony meteorites



Earth’s Interior Showing P and S

Wave Paths



Assignment

Read Chapter 8, Section 4 (pg. 233-237)

Do 8.4 Assessment #1-6 (pg. 237)

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4 Earth_s Layered Structure