The Earth

The Earth Geology 1

G. Bertotti

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Earth

2 - 4 September

The architecture of the Earth

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Tools to investigate the deep Earth Seismic waves (P and S)

P waves S waves

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A lot of information from space

The GRACE mission

Meteorites Mostly derived from particles which never aggregated to form a planet

Considered to be similar to the Earth interior

Gravity constrains distribution of masses in the Earth

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We start a trip from the deepest Earth to the surface

Fig 7.16b fowler

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Some data

A major discontinuity at ~3000km (Gutenberg discontinuity) forming the Core-Mantle-Boundary - molten outer core/solid inner core - solid mantle - compositions of mantle and core are very different

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The core

Composed of Fe(80%) + Ni resulting from the sinking of heavy elements towards the center of the Earth during the initial stages of the planet

Nickel (5%) Sulfur (5%)

Oxygen (5%) Iron

(85%)

OUTER CORE

The core provides the heat and magnetic field

Nickel (6%)

Iron (94%)

INNER CORE

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Very high T in the core; still the remnant of primary heat distributed by vigorous convective movements in the outer core

20% of the heat flow at the surface of the Earth comes from the CMB! This is not renewable

Fig 7.16b fowler

Heat

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Earth’s magnetic field Very intense movements take place in the outer core of the Earth. They are the source of the geomagnetic field

A simple dipole situation at and above the surface of the Earth

But inside…

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Strongly non-linear processes such as (outer core) convection cause periods of quiet separated by short abrupt changes The geomagnetic field these are polarity reversals, when the magnetic N becomes S and vice versa.

The reversals are random; therefore, also the duration of the chrons si random

A reversal lasts 1000-10000yr, but might be even faster

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Magnetic chrones last roughly 0.5Myr The very long Jurassic quiet zone

A very active situation during the last few Myr!

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Reversals in geological history

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In the mantle!

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Tools

Xenoliths: fragments of very deep rocks brought to the surface by volcanoes

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Seismics/seismology gravity

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From seismic data to tomography, a great tool to image the Earth

Using a large number of ray paths we determine the velocity deviations with respect to a given model

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The basics of tomography (applicable at very different scales)

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An example

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What controls the seismic velocity? What is the geological information we can extract from this data? It is a question of temperature or composition?

t – 2

he E

arth

•fairly homogeneous until 670km

• weird things at 100-200km •something very different in the uppermost few 10s of km. This is the crust

The mantle: between the CMB and the base of the crust (Moho)

CMB

ρ

Vs T

Tliq Vp

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The mantle has a fairly homogeneous composition

Is composed of dunites and peridotites.

Dominant minerals are olivine and pyroxens (and their high P pahses)

Overall chemical composition

There are variations at the level of detailed geochemistry

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With the exception of a thin interval at 100-200km depth (see later), the mantle is solid.

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The physical state of the mantle

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The mantle: fairly homogeneous but with discontinuities

at 670-700km

at 50-150km (very important)

at ~400km (less important)

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Discontinuities within the mantle The deeper ones are crucially controlled by the state of olivine, the dominant mineral in the mantle

Si04 cells change their organization: olivine changes to β spinel with a 5-10% increase in density.

Spinel changes to perovskite, with a 10% increase in density and corresponding increase in seismic velocities

perovskite

spinel

olivine

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The low-velocity zone (LVZ)

In the upper part of the mantle an anomalous interval with low seismic velocities: the base of the lithosphere (LAB)

The analysis of seismic waves shows a low velocity zone at >80- 100km.

The change is gradual and spread of few tens of kms

The depth and the amplitude of the transition are variable

SNA=shield N America ATL=Atlantic TNA=tectonic N America

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Experimental petrology and inferred geothermal gradients indicate that this is a zone of partial melting

It is only a few % but enough to change mechanical properties of the mantle in a fundamental manner

small melt pockets in mantle rocks

The LVZ is the asthenosphere! What is above it is the lithosphere (we neglect complications in the lithosphere, for the moment)

The lithosphere is a more rigid layer “floating” on rocks able

to flow at higher rates

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asthenosphere

lithosphere

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The consequences of having a rigid layer on top of a softer layer: glacial rebound

• Scandinavia is uplifting • >200m in the last 6000yr. • uplift rates are in the order of 1cm/year

ice caps ~20.000 y ago

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The uplift is caused by the rebound of the Erath after the end of the last glaciation (ca. 8000 years ago)

lithosphere

lithosphere

lithosphere

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What do we need

•a somewhat stronger layer overlying a softer layer

•downward movement reached a maximum

•upward movement continues at present even if the glaciers are not there any more

•In mechanical terms this corresponds to an upper “rigid” layer overlying a viscous lower layer

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Everything simple in the lithosphere?

(Mohorovicic)

A lower part with high seismic velocities

An upper part (30km in this case) with low seismic velocities

a sharp decrease of velocities (moving upward) (different from the LAB) Moho

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Mantle Tec Major Faul

NO ZONE plex Series tonites ts

Basic Com Kinzigitic

20 Km 45° 30’ N

Finero

8° 00’ E

46° 00’ N

Maggiore Lake

Balmuccia

Baldissero

8° 30’ E

IVREA VERBA

Orta lake

Varallo

In some localities the Moho is exposed!

Above the Moho, gabbros and metamorphic

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rocks (light and slow)

Beneath the Moho homogeneous peridotie (heavy and fast)

The Moho is a material boundary, that is, a thin zone which separates different kinds of rocks!

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The Moho: harzburgite in contact with layered gabbro

The Moho in Oman Layered gabbro

peridotite

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Above the Moho, the crust

Two very different types of crust: the continental and oceanic crust

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NB: Oceanic and continental crust can lie on the same lithospheric mantle.

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The oceanic crust •Simple structure of three layers: sediments, basalts and intrusive rocks (gabbros) • thickness is very homogeneous (beside near the ocean ridges) • no oceanic crust older than 180Myr

Warning: the geologic definition does not coincide with the geographic one: you can have sea on continental plates (e.g. the NL) and, rarely, oceanic crust forming emerged lands

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Some details

We will see later how this is formed

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The continental crust A very complex internal structure

Variable:

- Thickness (from ~10km to >60km)

- Composition variable but in general silica-rich

- age of rocks (up to >3.4Gyr)

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A summary

The lithosphere: a major element for fundamental and applied geology

crust

lith. mantle

asthen.

Moho

LAB

The crust-lithospheric mantle transition (Moho) is a material boundary. Its position can only be changed by thickening and thinning

The lithosphere- asthenosphere transition (LAB) is a thermal boundary the position of which depends on the geothermal gradient.

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There is movement in the system!

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We know very well how plates are presently moving (=kinematics) Very Long Baseline Interferometers and GPS measurements are used

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Displacement vectors of GPS stations world-wide

Large regions with coherent displacement vectors separated by sharp boundaries

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Looking at relative movements : 12 major blocks (plates) moving with respect to each other and with little internal movements

JUAN DEflXA l Pl.ATE

t1J I

PACtF1C PlATE

.. Tra1uform lcn.1lr ....

ANTARCTIC Pl.A1E 0 1,.500

[al

Uncer1oin plule boundary Oirocrion ol plu r molion Ir 1011 rrolion ra' ) in mm/yr)

Oivorgant !spreading ndge offael by lronsfc>1m fouhsl

TU Delft 36

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The lesson: 12 major plates can be defined in the outer part of the Earth which move with respect to each other and which display little internal deformation

How representative is this situation for the geological past?

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• Glacial deposits of the same age found in areas vey far away from each other

• Glacial deposits founds at “absurd” latitudes

The explanation: the absolute and relative position of the continents was different from the present day one and together they formed the mega-continent Pangea

limit of Late Paleozoic glaciers

Late Paleozoic glacial deposits

Late Paleozoic glacial deposits

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A similar conclusion is reached looking at Paleozoic (600-250Myr ago) plants and animals

We think that plate movements have taken place since ~500-600Ma

We need a system which allows for large movements over a large amount of time

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The tectonic plates correspond to the lithosphere!

crust

lith. m antle

asthen.

Moho

LAB

The lithosphere is lighter than the asthenosphere and can float on it The (rigid) lithosphere lies on the softer and more deformable substratum of the asthenosphere

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Remember: the lithospheric mantle is relatively homogeneous underneath continents and oceans; the crusts are, on the contrary very different.

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Different types of plate margins

AFRICAN Pt.ATE PACIR: Pt.ATE s

SOUJ M\ERICAN PlAIE

r[ :13 t - Jll.

1 i

T U Delft 43

CA r n t [spu'!ladirwa n orfse1 by lronJOrm foull)I

... Unconoln plole boundary

Oiroctton ol plore motion It 1011 molion 10• 1 in rnrn/ytl

ANlARCTIC Pl.AlE 0

Different processes at plate margins

T U Delft 44

CONVEAGENT TRANSFORM PLATE BOUNDllR Y PLA:t'E BOUNDAR'1

DIVERGENT PLAT E BOUNDAR't'

CONIJERQENT PLAT E BOUNDAR't'

CO NTINENTAL RIFT ZONE ('tOUtHl PLAT E BO UNDAR'l' I

7 F"Et(CH

ISLANC A RC SHIELD VOLCANO

STRATO VOLCANO

.... LITHOSPHERE

AS THENOSPH EA E SUll::lUCTING P ATE

HOT SPO T

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The areas where lithosphere is created: mid-oceanic ridges

• The largest mountain belt on Earth • elevation of 3-5km above sea floor

• very rugged topography, flattening moving away from the axis 45

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The rift valleys: a very active places! Distribution of earthquakes in the Earth

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Oceanic ridges: Very intensive volcanism

Warm bodies (magma chambers) underneath oceanic ridges

Not everybody finds it a bad place to live!

http://media.marine-geo.org/video/vigorous- hydothermal-flow-sully-2005

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Earthquakes in the Central Atlantic

Nicely visible in Iceland

And very active extension

Accommodate extension

Summarizing the processes 2 A thin dike erupts, spilling lava on

the ocean floor in characteristic "pillows:'

Hot mantle rock rises, decompresses, and melts to a mush of crystals and basaltic magma.

Dikes Dikes intruding dikes 3 As the basalt mush cools, dikes

intrude dikes to form sheeted dikes. Remnants of the spreading center move away laterally.

! ! ! = ! 4 Sediments are deposited on the 0 km spreading seafloor. ll'llllllilllll . !

Oceanic crust

S A gabbro layer is formed adjacent to the magma chamber.

Mantle

- l , Spreading 2

center

,

1 6 8

6 In the magma chamber, crystals settle out of the magma, forming the peridotite layer.

Peridotite layer

Figure4.15 Understanding Earth, Sixth Edition

- - © 2010 W. H. Freeman and Company

.....,.

TU Delft 49

With time, the newly formed oceanic rocks move away from the ridge

TU Delft 50

Transionn boundary

Cruse ma!ilng n ll'KI same d1reet.oo; no ·ra.u11 motion

Plate B=---------...--.-=:=J

;::;;;:::_ MCtion along strike-s.fip fault

Crus1 [

LJ\Jmsph&re

Plafemo1Mm

Earttlqua M e?'Centers Copyright IO 2006 Pearson Prentice Ha Inc. Fig 12.29

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The depth of the ocean floor changes!

The further away, the deeper the ocean

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high topography

sea floor flattens away from ridge

fault-controlled topography

Ocean floor bathymetry

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Searching for a motor for vertical movements (subsidence)

Variations of ocean floor depths nicely fit the Parson-Sclater equation: fast at the beginning and flattening then out Rates depend on age!

Typical pattern of cooling related processes

Rocks cool and the lithosphere becomes thicker

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Moving away of plates and magma generation at ridges allows for dating of the oceanic crust

2 million years ago

1.35 million - - - -=- years ago

•t

oday ' ' ' ' ' ' ' ' ' ' ' t 4

t-brmal magnetic polarity

D Reversed magnetic , polarity b

+ +

2.5 1.65 CIJ Normal polarity

c:::::J Reversed polarity

.7 .7 1.65 2.5 Millions of years ago

T U Delft 53

- + + - + + - + + -

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Ages of oceanic crust

Keep for later: no oceanic crust older than 180Myr! The accretion is not uniform through time

180 120 68 33 0Ma

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Conservative plate boundaries (strike-slip, transcurrent, transform) =

Displacement is parallel to the boundary

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M\ERICAN PlAIE

Where are they?

EURASIAN PlATE

PACIR: Pt.ATE s sour r[ :13 t - Jll.

1 i

(at

T U Delft 57

... Unconoln CA r nt plole boundary [spu'!ladirwa n Oiroctton ol orfse1 by lronJOrm plore motion foull)I It 1011 molion

10• 1in rnrn/ytl

ANlARCTIC Pl.AlE 0

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In the oceanic

domain

Link two segments of the oceanic ridges

The East Pacific rise

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In the continental domain

Izmit (1999)

Major earthquakes are associated with transform faults in continents

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Secondary structures develop when the trace of the fault is not straight

The Dead Sea pull-apart basin

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The areas where lithosphere is consumed

PACK PlAtE : A f N f t A

50UJ M\ERICAN PIAIE

t{ 33 t _,..

[o)

... Tran:dorm foulr .... Uncenoln

01vnrgaru - - • plole bound cry (sp1ead1ng ndQ.@ . I orhe1 by tronJOrm rouIbl

Dirocrion ° plar mohon Ir Ioli rnolion 1a• " in rnm/yrl

ANTARCTK'. PlA1E o 1..soo

T U Delft 62

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Convergence, protracted over long time, is accommodated by subduction

Three major processes occur

• volcanism fluid-driven melting

• Earthquakes (friction between plates)

• accretion

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Volcanism in subduction zones: subduction is needed

Fluids are carried at depth by the subducting slab Once they reach a depth of ~100km they are expelled and move upward thereby melting overlying rocks

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The earthquake map

TU Delft 65

DEPTH (km) 0 0

0["') ". . -' 0IO

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Earthquake: huge friction is generated between the upper and the lower (subducting) plate

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Convergence during the inter- seismic stage is accommodated by warping of the upper plate. Stress is accumulated.

During the earthquake, the upper plate suddenly rebounds to its previous position.

When submarine a tsunami is generated

The March 2011 Japan earthquake

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RUSSJF-. CAN .a.DA

Predicwd tsunamm wave heights

-

- - - -

1 -

0 FEET

6 1 1 a... 5

Epicenter 2 4S ?M Jooa l fime 12:46AM ET

U NI T ED S TA T E S

A U S T RA L IA

T U Delft 68

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Implies friction down to large depths

km

Earthquakes occur down to >500km depth (Wadati-Benioff zone)

longitude (̊)

Earthquakes underneath Indonesia

thermal structure of subduction zone

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Two end-member settings of subduction zones: Andean- and Pacific-type

0km

Peru

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10 0

20 0 North Hebrides

300 200 100 0km

Kurile-Kamtchatka

Dips of different subduction zones

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Andean type

• shallow dipping slab

• high mountains

• little to no deformation in the upper plate

• major, not-too-shallow earthquakes

Subduction zone NW US - (Stern 2002)

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Pacific type •steep slabs

• low relief-mountains •major extension in the upper plate (back-arc basins). Can lead to the opening of an ocean

Two famous examples: Japan and the Tyrrhenian Sea Steep subduction zones and associated belts are (very) curved

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Using a large number of ray paths we determine the velocity deviations with respect to a given model

Remember tomography? a great tool to image the Earth

The basics of tomography (applicable at very different scales)

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Examples

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What controls the seismic velocity? What is the geological information we can extract from this data?

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Andean or Pacific type?

Which of the two mode develops depends on the competition between horizontal velocity and gravity (=weight of the subducting plate)

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Accretionary wedge During subsidence, rocks from the lower plate are moved taken away form the lower plate and incorporated in the upper plate.

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The Nankai orogenic wedge (Taiwan) o 21 CDP

1.5 - - - - - - - - - - - - - I 2 .0

w·est

11121 1121 7321 7721 1121

east 2 0

J.O 3 0

4 0

fl 0

7 0 1 0

e.o...1.::..;..:;;.:;:.::;::::::o;..=..::.o.;;;;..;:.:;.:..;,..;;.l;;'-'"!:::.;;.;.1:.::...:..;:.;;;;..;:;..;:;::;:;:...:.:.:..;;::.;..-..; ...,;;.;.;::.;; ...:...;;:;;;.;....ii;.,;::..::;;.....:, :.:;:..;..;;.;c::;.;.;;.;;....;.....;;..::;;;.;;;;.;;::.....;.;.:..::.;.;;;;.:....:....;.......;...::..;.;;;o.;;;:...:....-; ....:::.; :...;.;;....., :..:::..:;:.....;...:;;..;:.;:.;;:.....;.;::;:.;....;.;......:.;...;.o ..........;; ;.;;.L.e o

1.6 I 5

2.0

4 0

Sout h China Sea - - -

3.0 3 0

+-+se1amogenic fault

1 0

\ \

'\\ \ OOST

OOST \ Jt

.I('

megathr ust. decollement

-;-•.O f-- (-o 5.0 50

eo

1 0

T U Delft 77

eo..&.. -,. ...- -.- -... .;;.;;;.;;.;;;;.

;;;.;;.;;;.; ..,........;s.tepdown.....,. C!!l-...

...-

......1... ao

- 10 0 10 20 30 40 50 6 0 ( km )

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Summary of plate tectonics

Everything working well?

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3 – 4 September

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Real world plate tectonics

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Summary of plate tectonics

Everything working well?

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Everything seems to be perfect, we have the permanent machine!

(Mickey Mouse) plate tectonics The lithosphere of the Earth is broken in ~12 major ± rigid blocks which move with respect to each other; action takes place at plate boundaries

The machine goes on forever without modifications (no history).

Implies very simple convection patterns

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Divergent boundaries are born

The East African rift

This is continental rifting

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Looking carefully…. serious problems

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de

pth

(k m

)

Continental rifting produces crustal thinning and creates accommodation space

0

10

20

30

40

0 50 100 150 distance (km)

200 Middle - Upper Triassic ?Devonian - Lower Triassic Basement

250 Neogene Paleogene Cretaceous Jurassic

b)

? ?

Moho

In tra man tlereflections

High velocity body (+8 km/s)

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Moho

Note: at present there is no extension across the North Sea. Rifting died

Had extension continued we would have had

• two plates

• two plate margins

the North Sea

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Not only, rifts can begin and die. In some situations the forces can change and the system is set under compression

The oceanic ridge and the passive continental margin will progressively enter the subduction zone

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The continental collision marks a slow-down of the convergence between the two plates and, often, the end of subduction.

Any idea why?

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The Alps

A great example of collisional belt (between Africa and Europe).

•The convergent boundary is nearly dead, • the two plates are becoming one.

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Subducting plates should all be of oceanic nature

Subducting plates should be infinitely long

If the Mickey Mouse plate tectonic does not work well, we conclude that also the underlying simple convection model is inadequate

How does convection work?

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The simple-minded convection models does not seem to work very well

Sources of figures http://www.studyblue.com/notes/note/n/chapter-8-earths-interior/deck/1294532 http://bc.outcrop.org/images/earthquakes/press4e/ http://www.somebits.com/weblog/2010/01/ http://foter.com/photo/murnpeowie-meteorite/ http://www.nasa.gov/mission_pages/Grace/multimedia/pia04235.html http://professoralexeinowatzki.webnode.com.br/geologia/estruturas-da-terra/ http://beforeitsnews.com/earthquakes/2013/04/spring-disappears-from-northern-hemisphere-the-winter-that-wont-end-2457450.html http://thewatchers.adorraeli.com/2011/03/15/about-geomagnetic-reversal-and-poleshift/ http://www.es.ucsc.edu/~glatz/geodynamo.html\ http://www.pinterest.com/nancyshogren/geology/ http://plate-tectonic.narod.ru/plum1photoalbum.html http://www.teara.govt.nz/en/photograph/9075/dunite

Sources of figures http://serc.carleton.edu/NAGTWorkshops/mineralogy/optical_mineralogy_petrography.html http://bc.outcrop.org/GEOL_B10/lecture19.html http://www.physicalgeography.net/fundamentals/10h.html http://www.kennislink.nl/publicaties/klimaatverandering http://gore.ocean.washington.edu/classpages/oman/geology_auto.html http://instruct.uwo.ca/earth-sci/200a-001/12occrst.htm http://scitechdaily.com/scientists-probe-earths-core-to-gain-a-better-understanding-of-planet-formation/ http://celebrating200years.noaa.gov/magazine/vlbi/Figure2.html http://www.astronautforhire.com/2009/09/theres-space-on-kauai.html http://www.hwsw.hu/hirek/34748/Navigacio_Galileo_GPS_muhold_finanszirozas.html http://users.indigo.net.au/don/ee/about.html http://www.dailygalaxy.com/my_weblog/images/2008/07/13/morenoglacier.jpg

Sources of figures http://wetenschap.infonu.nl/wetenschappers/90252-beroemde-wetenschappers-harry-hammond-hess.html http://svtmarcq.over-blog.com/article-naissance-d-une-theorie-la-derive-des-continents-1ere-s-91041389.html http://georneys.blogspot.nl/2010/12/famous-ocean-floor-map.html http://www.keesfloor.nl/artikelen/zenit/berlusconi/index.htm http://geoscience.wisc.edu/~chuck/Classes/Mtn_and_Plates/volcano.html http://quizlet.com/25506653/world-geography-chapter-4-section-1-flash-cards/ http://www.getdomainvids.com/keyword/atlantic%20ocean%20depths/ http://www.pacificislandtravel.com/nature_gallery/platetectonics.html http://www.turkish-media.com/forum/topic/107167-yerbilimleri-insan-din/ http://serc.carleton.edu/nnn/mathstatmodels/examples/SFSxl2.html http://www.geomapapp.org/gallery/Midoceanridgesgallery.html http://gomyclass.com/geology10/files/lecture12/html/web_data/file63.htm

Sources of figures http://esc135.wix.com/platentectoniek#!theorie http://www.kennislink.nl/publicaties/plaattektoniek http://all-geo.org/highlyallochthonous/2010/01/tectonics-of-the-haitian-earthquake/ http://sciencythoughts.blogspot.nl/2012/12/earthquake-beneath-eastern-black-sea.html http://heavenawaits.wordpress.com/the-garden-of-eden-and-the-%E2%80%9Cpit%E2%80%9D-%E2%80%93-tioran-land-drift-theory/ http://www.onweer-online.nl/forum/topic/28856/wat-is-de-zwaarste-aardbeving-die-de-vs-kan-treffe/ http://volcano.oregonstate.edu/vwdocs/vwlessons/volcano_types/subd1.htm http://wikitravel.org/nl/Krakatau https://umdrive.memphis.edu/hdeshon/www/courses.html http://safety.worldnomads.com/Chile/55134/Chile-earthquakes-advice-for-travelers#axzz2nOGv7xN9 http://www.nytimes.com/interactive/2011/03/11/world/asia/maps-of-earthquake-and-tsunami-damage-in-japan.html?_r=0

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