9. our living earth

9. Our Living Earth• Earth’s atmosphere, oceans & surface• Earth’s interior & earthquakes• Earth’s plate tectonics activity• Earth’s magnetic field & magnetosphere• Earth’s evolving atmosphere• Earth’s human population & biosphere

The Earth: A Portrait From Space

Earth Data (Table 9-1)

Earth From An Apollo Spacecraft

Earth’s Environmental Spheres• Earth’s spheres

– Geosphere Rock & metallic Earth materials– Hydrosphere Water as ice, liquid & humidity– Atmosphere ~78% nitrogen & ~21% oxygen– Biosphere All living things (biomass)

• Earth’s ecosystem– Matter flows

• A closed system for most practical purposes– Meteoroids enter daily, spacecraft leave occasionally

– Energy flows• An open system for most practical purposes

– Sunlight brings extremely large amounts of energyon one side

– Radiant heat in extremely large amounts leaveson all sides

Rocks• Definition

– Consolidated mixture of one or more minerals• Monomineralic rocks have many crystals of 1 mineral• Polymineralic rocks have many crystals of >2 minerals

• Making rocks– Igneous processes Fiery origins– Sedimentary processes Cemented small

particles– Metamorphic processes Changed by

heat/pressure• Destroying rocks

– Physical / mechanical weathering– Chemical weathering

Rock Cycle: Materials & Processes• Materials

– Magma solidifies & becomes…

– Igneous rock weathers & becomes…

– Sediment lithifies & becomes…

– Sedimentary rock metamorphoses & becomes…

– Metamorphic rock melts & becomes…

• Processes– Solidification produces igneous rock– Weathering produces sediment– Lithification produces sedimentary rock– Metamorphism produces metamorphic rock– Melting produces magma

Rock Cycle

Magma: Source of Igneous Rocks• Earth’s interior is hot

– Residual heat of formation ~ 4.6 billion years ago– Decay of radioactive isotopes

• Earth’s interior is mostly solid or “plastic”– Solid: Rigid / brittle under intense pressure– Plastic: Flows slowly under intense pressure

• Localized areas are hot enough to melt rocks– Magma temperatures vary ~ 600°C to ~ 1,400°C– Iron turns red at ~ 600°C & melts at ~ 1,500°C

• Magma has ~ 10% greater volume than source– Same mass Greater volume Lower density⇒ ⇒

Some Common Igneous Rocks

Sedimentary Rock Categories• Organic Remains of plants & animals

– Coal Fossilized fern leaves

• Clastic Broken rock & mineral fragments– Sandstone, shale & limestone

• Bioclastic Broken shell fragments– Coquina Limestone “fossil hash”

• Chemical Crystallization from water solution– Gypsum A common “evaporite” mineral

Some Clastic Sedimentary Rocks

Three Metamorphic Processes• Heat Absolutely essential

– Hot enough for atoms & molecules to slowly migrate– Cool enough so that nothing melts

• Pressure Common but not essential– Subduction zones Pacific Northwest– Regional subsidence Mississippi Delta

• Fluids Only near active volcanoes– Volcanically active areas Eastern Oregon

Foliated Metamorphic Rocks: Gneiss

Oregon’s Metamorphic EnvironmentPortland

Astoria

Earth’s “Chemical” Differentiation

Characterizing Earth’s Interior• Chemical composition Mineral composition

– Low density minerals Crust• Granite continents & basalt ocean basins

– Intermediate density minerals Mantle• Peridotite

– High density minerals Core• Iron & nickel

• Physical condition Solid / plastic / liquid– A function of temperature & pressure

• Temperature increases slowly with depth• Pressure increases rapidly with depth

– Solid Lithosphere Old & cool enough– Plastic Asthenosphere Lubricating layer– Solid Mantle Very slightly plastic– Liquid Outer core Temperature wins– Solid Inner core Pressure wins

Earth’s Interior Facts & Evidence• Some basic facts

– Overall average density ~ 5.5 g . cm–3

– Surface average density ~ 2.7 to 3.0 g . cm–3

– Interior must have higher density materials• Much higher atomic number Metals⇒• Greater compression due to greater pressure

• Some suggestive evidence– Asteroids orbiting the Sun

• Range of materials from rock to iron/nickel• Proportions would produce a planet like Earth

– Meteorites found on Earth• Range of materials from rock to iron/nickel• Proportions would produce a planet like Earth

Earth’s Layers: The Lithosphere

Earth’s Layers: Crust/Mantle/Core

Earthquake Focus & Epicenter

The focus is also called the hypocenter

Seismic (Earthquake) Waves• Body waves

– Source location: Focus• Place of maximum underground shaking• Place where the earthquake begins

Usually ! ! !– Varieties

• Compressional waves P-waves Primary waves

• Transverse waves S-waves Secondary waves

• Surface waves– Source location: Epicenter

• Place of maximum surface shaking• Place directly above the focus Usually ! ! !

– Varieties• Compressional waves Sideways jolting• Transverse waves Up & down jiggling

Compressional & Transverse Waves

Compressional Seismic Waves

Transverse Seismic Waves

Body Seismic Waves

Surface Seismic Waves

Seismicity & Earth’s Internal Structure

Plate Tectonics• Tectonic plates = Lithospheric plates

– Rigid & brittle• “Glide” over the asthenosphere

– Sizes vary greatly• Micro plates Juan de Fuca plate• Macro plates Pacific plate

• Three kinds of tectonic plates– Oceanic plates Basaltic composition– Continental plates Granitic composition– Composite plates Both basalt & granite

Mantle Convection & Plate Motion• Thermal gradient: Hotter at core than at crust

– Results in a density gradient• Heat sources

– Planetesimal impact Dominant as a protoplanet

– Radioactive decay Ongoing exponential decay

– Gravitational collapse Minimal as a protoplanet• Point of origin

– Thought to be the core-mantle boundary• Shape

– Elongated “curtains” of rising material

A Model of Mantle Convection

A Global View of Mantle Convection

Tectonic Plates

Tectonic Plate Boundary Processes

Divergent Plate Boundaries

Convergent Plate Boundaries

Transform Plate Boundaries

Mid-Atlantic Ridge Spreading Zone

Ridge offset by transform faults

Effects of Plate Motion: Volcanoes• Divergent tectonic plate boundaries

– Most rising magma spreads out under lithosphere• Lithosphere warms Lowers density Floats higher⇒ ⇒• Penetrates the lithosphere, causing eruptions

• Convergent tectonic plate boundaries– Highest density plate subducts

• Ocean ⇒⇐ ocean collision– Oldest (i.e., coldest & densest) basaltic plate subducts– Basaltic to andesitic lavas build gently curving line of volcanoes

• Ocean continent collision⇒⇐– Basaltic (therefore most dense) oceanic plate subducts– Andesitic to rhyolitic lavas build gently curving line of volcanoes

Plate Motion Effects: Earthquakes• Divergent tectonic plate boundaries

– All activity is near the Earth’s surface• Virtually all earthquakes are shallow

– Most rock is relatively warm & soft• Absence of brittle rock reduces earthquake strength

• Convergent tectonic plate boundaries– Ocean – ocean boundaries

• Deep & strong earthquakes are very common– Ocean – continent boundaries

• All depths & strong earthquakes are very common• Transform tectonic plate boundaries

– Ocean – ocean boundaries• Absence of brittle rock reduces earthquake strength

– Ocean – ocean boundaries• Presence of brittle rock increases earthquake strength

Plate Motion Effects: Mountains• Volcanoes

– Usually occur at convergent & divergent boundaries• At least one plate must have basaltic oceanic crust

– Factors contributing to solid rock melting• Thermal gradient ⇒ Deeper is hotter• Friction ⇒ Subducting slab ⇔ country

rock• Addition of water ⇒ Under-sea subduction

trenches• Folded mountains

– Occur primarily at convergent boundaries• Both plates must have granitic continental crust

– Thrust faulting is also very common• Significant crustal shortening

Plate Motion Effects: Geography• Continent ⇔ ocean configuration very dynamic

– Three probable Pangaea episodes• All major landmasses gather into one supercontinent• Remaining 70% of Earth’s surface is one super-ocean

– The present situation• Major continental landmasses are relatively

stable• Major ocean basins are very dynamic

– Atlantic Ocean is increasing in size– Pacific Ocean is decreasing in size

Earth’s Magnetic Field• Basic physical processes

– Slow circulation of the liquid metallic outer core– Rapid axial rotation of once per day

• Basic properties– Combined magnetic field of many smaller “cells”– Reverses on average ~ 0.5 million years

• May be in the initial stages of a reversal now– Not perfectly aligned with Earth’s rotational axis

• True of almost every planet in the Solar System• Magnetic declination

– Deviation of magnetic North [compass] away from true North• Magnetic inclination

– Angle between Earth’s surface & Earth’s magnetic field lines

Visualizing Earth’s Magnetic Field

Earth’s Magnetosphere• Basic physical processes

– Earth’s relatively strong magnetic field– The ever-changing solar wind

• Ionized hydrogen atoms Free protons & electrons

• This is an electric current Generates a magnetic field

– Strong interaction between two magnetic fields• Basic properties

– Earth’s magnetosphere shaped like a teardrop• Blunt side faces Sun, pointed side faces opposite Sun

– Solar wind gusts produce striking effects• Geomagnetic storms Disrupt radio

signals– Occasionally strong enough to disrupt electric power distribution

• Aurorae Ionize atmospheric atoms

– Occasionally strong enough to be seen in Florida & Texas

Visualizing Earth’s Magnetosphere

Aurora Australis From Space

So-Called “Greenhouse” Effect

Earth’s 3-D Atmospheric Circulation

Earth’s Vertical Atmospheric Structure

Terrestrial Planetary Atmospheres• Venus

– ~100 times more atmosphere than Earth– ~ 96.5% CO2 & ~3.5% N2

• Runaway global warming– Very large amount of CO2 & relatively close to

the Sun• Earth

– ~ 78% N2 & ~21% O2• Moderate global warming

– Very small amount of CO2 & moderately close tothe Sun

• Mars– ~100 times less atmosphere than Earth– ~ 95.3% CO2 & ~2.7% N2

• Minimal global warming– Very small amount of CO2 & relatively far from

the Sun

Source of Planetary Atmospheres• Volcanic outgassing

– Venus• Abundant with no oceans to assimilate gases

– Earth• Abundant with oceans to assimilate gases

– Mars• Absent with no oceans to assimilate gases

• Comet impacts– Very common in the young Solar System– Very rare in today’s Solar System

Growth of Earth’s Atmospheric O2

Human Population & the Biosphere• Earth’s rapidly increasing human population

– Burning fossil fuels returns CO2 to the atmosphere• General upward trend Increasing use of fossil fuels• Seasonal fluctuations Summer CO2 uptake by plants

– Removing forest cover (deforestation)• Reduces CO2 uptake

– Partially offset by ocean absorption

• Radically changes local climate– Much hotter & much drier

– Body heat contributes to “urban heat island” effect• Only recognized very recently

Earth’s Growing Human Population

Northern Hemisphere CO2 Increase

Earth’s Changing Temperatures

Earth’s Antarctic Ozone Hole

• Earth’s environmental spheres– Geosp., hydrosp., atmosp. & biosp.

• The rock cycle– Five materials & five processes

• Magma as Earth’s initial condition– Three basic rock types

• Igneous, sedimentary & metamorphic• Earth’s internal structure

– Chemical & physical classifications– Interactions between temp. & pressure– Information from seismic waves

• Compressional & transverse waves• Surface & body waves

• Plate tectonics– Driven by mantle convection– Plate boundary types & properties

• Convergent, divergent & transform– Effects of plate tectonic activity

• Location of continents & ocean basins• Volcanic & earthquake activity

• Earth’s magnetic field– Basic causes

• Rotation & outer core convection• Geomagnetic field reversals

– Generation of the magnetosphere• Interactions with the solar wind• Aurora Borealis & Aurora Australis

• Terrestrial planetary atmospheres– Venus, Earth & Mars compared

• Atmospheric gases & amounts• Closeness to the Sun

– Atmospheric structure & circulation• Earth’s human population

– Rapid growth in numbers– Use of fossil fuels

• CO2 returned to Earth’s atmosphere• Global warming & ozone depletion

Important Concepts