Earth s Cmpsitin and Structure: A Jurney t the Center f the Earth
Earth s Surface Our experience with Earth is limited t its surface. Yet Earth has a cmplicated interir. Earth is characterized by An internally generated magnetic field. A layered interir Slid and liquid layers. A gaseus envelpe. i.e. atmsphere The Black Canyn f the Gunnisn, CO
The Slar System Human perceptins have changed. Early histry Planets as mving lights. 1600s 1 st telescpes saw hazy spheres. Tday A cmplex, evlving system. Structure Histry Space prbes have phtgraphed and analyzed planets. Scientists have hypthesized likely rigins f the slar system.
Earth and the Slar System What wuld slar system visitrs ntice? Magnetic field. Atmsphere. Surface features. Cntinents. Oceans. Plar ice caps. Evidence f humanity? Structures.» Dams.» Great Wall f China.» Cities.» Rads / canals. Electric lights.
The Celestial Neighbrhd Interstellar space: a ~vacuum with a virtual absence f matter. The amunt f matter greatly increases appraching the Sun. The Sun ejects matter utward int space as the slar wind. Slar wind: Magnetically & electrically charged particles. Stream utward in all directins. Cnsists f Prtns (+ charge). Electrns ( charge). Only a small percentage f the slar wind impinges upn Earth.
Five Key Characteristics Abut Earth s Structure: 1. Earth has a diple magnetic field that deflects slar wind and prtects earth s surface frm slar radiatin 2. Earth has a stratified atmsphere, mainly cmpsed f nitrgen (N 2 ) and xygen (O 2 ) 3. Earth is made f a variety f minerals, glasses, melts, fluids and vlatiles, all left behind during birth f the slar system 4. The Earth has layers: a thin silicate crust, a thick irn- & magnesium silicate mantle, and a thick metallic cre 5. Physically, the earth can be divided int a rigid uter lithsphere and a plastic/ductile asthensphere
Earth s Magnetic Field Gedynam - The Earth s magnetic field is prduced by the gedynam -Flw in the liquid irn uter cre creates a magnetic field Magnetic field - regin affected by frce emanating frm a magnet - grws strnger as separating distance decreases - attracts r repels magnetically charged r mving electrically charged bjects - cmpasses wrk because Earth is a large magnet
Earth s Magnetic Field Magnetic field - Like a bar magnet, Earth s magnetic field is a diple, (has bth a N and S ple) - Slar wind cntains electrmagnetic particles that are deflected by earth s field. These particles distrt the shape f earth s magnetic field in space - Van Allen belts tw belts in the inner magnetic field where high energy csmic rays are trapped. Prtects us frm slar radiatin!
Nrthern & Suthern Lights Frm because f ur diple magnetic field!
Aurrae Sme ins escape Van Allen belts. These ins are pulled t the magnetic ples. The ins create light in the upper atmsphere. Spectacular aurra fllw slar flares. Aurra brealis Nrthern lights. Aurra australis Suthern lights.
Earth s Atmsphere Distinct layers f gas surrund the slid prtin f the earth. Cmpsitin is ~unifrm regardless f altitude 78% N 2 21% O 2 All thers ~1% Ar, CO 2, CH 4, H 2 O, Ne, CO, SO 2 Sme ther Planets have atmspheres t! Nne have N 2 & O 2 as dminant gasses Earth was xygen-free until ~2.5 Ga
Earth s Atmsphere Pressure decreases with increasing altitude Reflects # f mlecules/vlume Lwer pressure = less mlecules/vlume Air pressure @ sea level = 14.7 lb/in 2 = 1 bar Pressure is caused by the weight f verlying material Upper atmsphere has less material abve it Pressure is lwer 99% f atmsphere is belw 50 km, the rest is between 50 and 500 km.
Earth s Atmsphere Earth s Atmsphere is divided int distinct layers based n altitude Exsphere (very thin ~500 km) Atmsphere merges with space Thermsphere (>90 km) Where space shuttles rbit Messphere (50-90 km) Meters burn up here Stratsphere (12-50 km) Stable air; gd fr jets Trppause (11-12 km) Trpsphere (0-11 km) Mixing layer All weather is limited t this layer Trp = Greek fr turning
Earth s Atmsphere Trpsphere A well-mixed layer dminated by cnvectin f air masses Cnvectin Methd f heat transfer in a fluid Think lava lamp! Cld is mre dense = sinks Ht is less dense = rises This prcess results in circular cnvectin cells Als causes pressure gradients which create wind! Als applies t the interir f the Earth this guy likes cnvectin
Earth s Cmpnents Earth s surface = ~30% land, ~70% water unlike any ther knwn planet Hydrsphere = includes ceans, lakes, seas, rivers, & grundwater Crysphere = glaciers, snw, and sea ice Earth s surface is nt flat; it has tpgraphy Ignring ceans, Earth s surface is dminated by tw distinct elevatins: Mst land is 0-2 km abve sea level Mst f the sea flr is 3-5 km belw sea level
Earth s Cmpnents Earth s elemental cmpsitin reflects mstly heavier elements nt blwn away by slar wind during frmatin f the slar system Mst abundant elements Fe, O, Si, Mg Mst cmmn minerals cnsist f silica (SiO 2 ) mixed in varying prprtins with ther elements such as Fe, Mg, Al, Ca, K, Na Felsic = mre silica (less Fe/Mg) & less dense E.g. Granite Mafic = less silica (mre Fe/Mg) & mre dense E.g. Gabbr / Basalt 30% Range: Felsic / Intermediate / Mafic / Ultramafic 35% 15% Bulk Earth cmpsitin 10% 10%
Earth Materials Elements cmbine in a variety f Earth materials. Organic cmpunds Carbn-cntaining cmpunds. Mst are residue frm nce-living creatures. Include wd, peat, lignite, cal, and il. Gelgically rare (decmpses in cntact with xygen).
Earth Materials Elements cmbine in a variety f Earth materials. Minerals Inrganic crystalline slids. Cmprise rcks and, hence, mst f the Earth. Mst rcks n Earth are silicates (based n Si and O). Glasses Nn-crystalline mineral-like matter. Cl t quickly t frm structure Rcks Aggregates f minerals. There are many types. Igneus Cled frm a liquid (melt). Sedimentary Debris cemented frm pre-existing rck. Metamrphic Rck altered by pressure and temperature.
Earth Materials Metals Slids made f metallic elements. Melts Rcks that have been heated t a liquid. Magma Mlten rck beneath the surface. Lava Mlten rck at the surface. Vlatiles Materials that turn int gas at surface temps. H 2 O, CO 2, CH 4, and SO 2 Vlatiles are released frm vlcanic eruptin.
A Layered Earth We live n the thin uter skin f Earth. Early perceptins abut Earth s interir were wrng. Open caverns filled with magma, water, and air. Furnaces and flames. We nw knw that Earth is cmprised f layers. The Crust. The Mantle. The Cre. Outer Cre. Inner Cre. Sme basic rules f physics give sme clues Frm Miltn s Paradise Lst
Earth s Density Earth s Density gives us clues abut its internal structure Density = Mass/Vlume Measures hw much mass is in a given vlume. Expressed in units f mass/vlume e.g. g/cm 3 Ice flats why? Estimates f earth s mass and vlume give a whle earth density f ~5.5 g/cm 3 Typical rcks at the surface f the Earth have a density f 2.0-2.5 g/cm 3 What des this require f the density f material in the Earth s interir?
Earth s Density Earth s shape as a clue t the internal structure f the Earth If density increased gradually and unifrmly twards the center, a significant prtin f Earth s mass wuld be near the uter edges. Then centrifugal frce (nt centripetal) wuld cause the planet t flatten int a disk. This has (bviusly) nt happened
Earth s Layers Earth s shape as a clue t the layering f the earth If the Earth cnsisted f a thin slid shell ver a thick liquid center, then the surface wuld rise and fall with tides like the cean This des nt happen; nly the ceans rise and fall. Thus, the Crust des nt flat ver a liquid interir
A Layered Earth Earthquake clues - Earthquake energy transmitted as seismic waves that pass thrugh Earth. Seismic waves have been used t prbe the interir. Wave velcity changes with density. Velcity changes give depth f layer changes. Changes with depth. Pressure. Temperature. Mre n this in Chapter 10 and Interlude D!
The Earth (and ther planets) have layered interirs. Crust Cntinental Oceanic Mantle Upper Lwer Cre Earth s Interir Layers Outer Liquid Inner Slid W. W. Nrtn
The Crust The utermst skin f Earth with variable thickness. Thickest under muntain ranges (70 km 40 miles). Thinnest under mid-cean ridges (3 km 2 miles). The Mhrvičić discntinuity r Mh is the lwer bundary. Separates the crust frm the upper mantle. Discvered in 1909 by Andrija Mhrvicic. Marked by a change in the velcity f seismic P waves.
Tw Types f Crust Cntinental crust Underlies the cntinents. Avg. rck density abut 2.7 g/cm 3. Avg. thickness 35-40 km. Felsic cmpsitin. Avg. rck type = Granite Oceanic crust Underlies the cean basins. Density abut 3.0 g/cm 3. Avg. thickness 7-10 km. Mafic cmpsitin Avg. rck type = Basalt/Gabbr
Tw Types f Crust Crustal density cntrls surface psitin. Cntinental crust Less dense; flats higher. Oceanic crust Mre dense: flats lwer.
Crustal Cmpsitin 98.5% f the crust is cmprised f just 8 elements. Oxygen is (by far!) the mst abundant element in the crust. This reflects the imprtance f silicate (SiO 2 -based) minerals. As a large atm, xygen ccupies ~93% f crustal vlume.
Bulk Earth Cmpsitin 35% 30% 10% 10% 15%
Earth s Mantle Slid rck layer between the crust and the cre. 2,885 km thick, the mantle is 82% f Earth s vlume. Mantle cmpsitin = ultramafic rck called peridtite. Belw ~100-150 km, the rck is ht enugh t flw. It cnvects: ht mantle rises, cld mantle sinks. Three subdivisins: upper, transitinal, and lwer.
The Cre An irn-rich sphere with a radius f 3,471 km. 2 cmpnents with differing seismic wave behavir. Outer cre Liquid irn-nickel-sulfur 2,255 km thick Density 10-12 g/cm 3 Inner cre Slid irn-nickel ally Radius f 1,220 km. Density 13 g/cm 3 Flw in the uter cre generates the magnetic field.
Lithsphere-Asthensphere The Crust, Mantle, Cre bundaries defined by cmpsitin but smetimes we want t divide the layers f the Earth by their behavir r physical prperties Lithsphere The brittle prtin f Earth s interir. Behaves as a nn-flwing, rigid material. The material that mves as tectnic plates. Made f 2 cmpnents: crust and upper mantle. Asthensphere The ductile prtin f Earth s interir. Shallwer under ceanic lithsphere. Deeper under cntinental lithsphere. Flws as a sft ductile slid. Cntains a small percentage f melt (< 2%)
Bundaries Between Layers The Crust-Mantle bundary = Mh defined by seismic discntinuity indicating significant change in cmpsitin. Brittle-ductile transitin Defined by a significant change in rck physical prperties (viscsity) Als defined as the depth belw which earthquakes d nt ccur. Lithsphere Crust