Geology of the lithosphereThe lithosphere is the outer solid part of the earth, includingthe crust and uppermost mantle. The lithosphere is about 100km thick, although its thickness is age dependent (older litho-sphere is thicker). The lithosphere below the crust is brittleenough at some locations to produce earthquakes by faulting,such as within a subducted oceanic plate. Source: USGS THE BASE OF THE LITHOSPHERE IS DEFINED AS THE 1300oC ISOTHERM
The Earth loses heat through it’s surface.Volcanoes produce molten rock suggesting internal heat is much hotter than surfaceheat.Generally the temperature increases depth: 1km deep =30oC above surfacetemperatures.The rate at which temperature rises (geothermal gradient) varies.In some parts of the crust the temperature rises slowly whilst in other parts itrises rapidly.Steep geothermal gradients are found in areas of igneous activity: magma chambersor cooling batholiths provide heat sources at depth.Since the inside of the earth is hotter than the outside it follows that heat mustflow outwards.HEAT FLOW is measured in joules per square metre per second (J m-2 s-1 ).The steeper the geothermal gradient the greater the heat flow.Rocks which are relatively good conductors permit a high heat flow.Heat flow is measured at depths below @ 30m so that solar input does not affectresults.
Heat flow over the earthThere is a general relationship of high heat flow associatedwith the igneous activity of oceanic ridges and young mountainranges while lowest values are found in ancient shield areas.Area of Earth’s Surface Heat Flow (J m-2 s-1 )Whole Earth 0.069All Oceans 0.069All Continents 0.069Ocean Basins 0.053Oceanic Ridges 0.080Oceanic Trenches 0.049Ancient Shield areas 0.041(older than 570 Ma)Platform areas 0.062(younger than 570 Ma)Old mountain areas 0.060(570-225 Ma)Young mountain areas 0.074(younger than 225 Ma)
Average heat flows from continental and oceanic crust are about equal. Observed heat flow of continental crust could easily be produced by the rocks of the continental crust. Continental crust appears to have a higher concentration (compared to Oceanic crust) of long-lived radioactive isotopes—the decay of which is the most important source of the earth’s heat. Oceanic crust could not produce the observed heat flow so it would appear that the heat is comes from the underlying mantle. This suggests that the mantle under the oceans is hotter than the mantle under the continents. This suggests that convection currents exist under the oceans but not under the continents. The currents bring heat from greater depth, they turn and flow under the oceanic giving off heat which escapes through the crust. The fact that the highest levels of heat flow occur at the ocean ridges an decreases across the ocean basins (lowest values at ocean trenches) suggests that the currents rise under the ridges , flow under the ocean basins and sink under the ocean trenches.
Rock strength related to temperatureWe all know that rocks near the surface of the Earth behave in a brittlemanner. Crustal rocks are composed of minerals like quartz and feldsparwhich have high strength, particularly at low pressure and temperature.As we go deeper in the Earth the strength of these rocks initially in-creases. At a depth of about 15 km we reach a point called the brit-tle-ductile transition zone. Below this point rock strength decreasesbecause fractures become closed and the temperature is higher,making the rocks behave in a ductile manner. At the base of the crustthe rock type changes to peridotite which is rich in olivine. Olivine isstronger than the minerals that make up most crustal rocks, so the upperpart of the mantle is again strong. But, just as in the crust, increasingtemperature eventually predominates and at a depth of about 40 km thebrittle-ductile transition zone in the mantle occurs. Below this point http://www.tulane.edu/~sanelson/geol111/deform.htm
CC Diagram 4 Look at the four diagrams provided: Explain the distinction between the Lithosphere and Asthenosphere (quote from the diagrams) What is the difference in the melting points of wet and dry granite?
Read page 142—WebsterWith the aid of diagrams explain why localised melting in the asthenosphere generates basaltic magmas.Use geological terminology including: Benioff zone.Why could this scenario be classified as idealistic?
THE BASE OF THE LITHOSPHERE IS DEFINED AS THE 1300oC ISOTHERMWhat is meant by the term isotherm?Describe the pattern of lithospheric thickness using the diagram above:How might this relate to the age of the lithosphere?
Seismology and the LithospshereSeismic waves are waves of energy that travel through the earth, for example as aresult of an earthquake, explosion, or some other process that imparts low-frequency acoustic energy. The propagation velocity of the waves depends on den-sity and elasticity of the medium. Velocity tends to increase with depth, and rangesfrom approximately 2 to 8 km/s in the Earths crust up to 13 km/s in the deep man-tle.Earthquakes create various types of waves with different velocities; when reachingseismic observatories, their different travel time enables the scientists to locatethe epicenter. In geophysics the refraction or reflection of seismic waves is usedfor research of the Earths interior, and artificial vibrations to investigate subsur-face structures.
Read page 119 of McLeishDraw and label diagram 6.8Add bullet points about the velocities of earthquake waves
Body wavesBody waves travel through the interior of the Earth. They follow raypaths refractedby the varying density and modulus (stiffness) of the Earths interior. The densityand modulus, in turn, vary according to temperature and composition. This effect issimilar to the refraction of light waves.P-wavesP waves (primary waves) are compressional waves that are longitudinal in nature. Pwaves are pressure waves that are the initial set of waves produced by an earth-quake. These waves can travel through any type of material, and can travel at nearlytwice the speed of S waves.: 1450 m/s in water and about 5000 m/s in granite.S-wavesS waves (secondary waves) are shear waves that are transverse in nature. Thesewaves typically follows P waves during an earthquake and displaces the ground per-pendicular to the direction of propagation. S waves can travel only through solids, asfluids (liquids and gases) do not support shear stresses. S waves are slower than Pwaves, and speeds are typically around 60% of that of P waves in any given material. Both waves get slower as the density of the medium increases. P waves get faster as the compressibility decreases. S waves get faster as rigidity increases. (Liquids have a zero rigidity)
Read pages 121/122/123 of McLeish:1. Draw and explain diagram 6.132. Draw and explain 6.14 A) Why do the waves bend? B) Why do the waves speed up with depth?
3. What is the Moho? (this is covered in more detail in the next few pages)4. What is the thickness of oceanic and continental crust?5. What happens to P and S waves just below the Moho?
Using the diagram above describe the relationship between velocity and depth of the seismic P wave:
Using the diagrams above describe the relationship between velocity and depth of the seismic Swaves, suggest an explanation for this occurrence:
How can such data be used to show crustal layering?What can be deduced about the internal structure of the crust?How can such data be used to calculate the thickness of the crust?Why are earthquakes rare occur in the asthenosphere?
Extra Reading and research can be found at COCORP andBIRPS:Below is an extract from the COCORP website -The Consortium for Continental Reflection Profiling (COCORP)pioneered the use of multichannel seismic reflection profilingfor the systematic exploration of the continental lithosphere.COCORP-type profiles routinely probe to the base of the crustand frequently deeper. COCORP has collected over eleven thou-sand kilometers of profiling at thirty sites in the UnitedStates. Among the best known of COCORPs US results are itsdemonstration of large-scale, low-angle thrust faulting in theAppalachians; confirmation of a thrust origin for Laramidebasement uplifts; delineation of the variable character of thecontinental Moho, including new evidence for its post-orogenicre-equilibration, its multi-genetic origin (including phasechanges) and possible role as a structural detachment; magma"bright spots" beneath Cenozoic rifts of the western US;crustal-scale detachment faults in the eastern Basin andRange; mapping of major buried Precambrian layered sequencesin interior US; and definition of crustal shear zones markingmajor Proterzoic sutures of the buried craton.The success of COCORP stimulated major deep seismic explo-ration programs in over twenty countries, including Great Brit-ain (BIRPS), France (ECORS), Germany (DEKORP) and Canada(LITHOPROBE).
Using Figures 2a and 2b complete Figure 2c to show the Lithosphere and Astheno-sphere.Looking back at the thickness of the lithosphere how do you think depth of thelow velocity zone correlates to the distance from the oceanic ridges?
The Mohorovičić discontinuity (Croatian pronunciation: [mɔhɔˈrɔvitʃitɕ]) (MOE-HOE-ROE-vee-cheech), usually referred to as the Moho, is the boundary between theEarths crust and the mantle. Named after the pioneering Croatian seisomologistAndrija Mohorovičić, the Moho separates both oceanic crust and continental crustfrom underlying mantle. The Moho mostly lies entirely within the lithosphere; onlybeneath mid-ocean ridges does it define the lithosphere – asthenosphere boundary.The Mohorovičić discontinuity was first identified in 1909 by Mohorovičić, when heobserved that seismograms from shallow-focus earthquakes had two sets of P-wavesand S-waves, one that followed a direct path near the Earths surface and the otherrefracted by a high velocity medium.The Mohorovičić discontinuity is 5 to 10 kilometres (3–6 mi) below the ocean floorand 20 to 90 kilometres (10–60 mi) beneath typical continents, with an average of 35kilometres (22 mi) beneath them.Two paths of a P-wave, one direct and one refracted as it crosses the MohoImmediately above the Moho the velocities of primary seismic waves (P-waves) areapproximately those of basalt (6.7 – 7.2 km/s), and below they are that of peri-dotitic or dunitic Earth-materials (7.6 – 8.6 km/s). That suggests the Moho marksa change of composition, but the interface appears to be too even for any believablesorting mechanism within the Earth. Near-surface observations suggest such sortingproduces an irregular surface. Some history of suggestions that the boundary marksinstead a phase change controlled by a temperature gradient in the Earth. (SeeMcLeish page 122/123 for further detail)
The Moho is a high velocity medium. Which P wave would travel fastest—the wavethat passed through the Moho or the wave that travelled within the overlying crust?From Specimen paper:3. Describe and explain how a study of earthquake body waves providesevidence for the variation in thickness and mechanical properties of thelithosphere and asthenosphere.Use the text books, classwork and research to plan the answer to the above essay.The mark scheme below should help you.3. Holistic approach (12+1 marks description) (12+1 marks explanation)Define earthquake body waves.Use of/explanation of formulae for P and S wave velocities.Incompressibility v density changes - P-waveRigidity v density - S-waveDepth profile of P and S waves to show velocity variation.Depth of the Low velocity zone with distance from spreading centre.Thickening of the lithosphere with age and distance from ridge.Diagrams creditedDefinition of the difference between Lithosphere and Asthenosphere in terms ofmechanicalproperties.Lithosphere = plate. Cold (<1300o C isotherm) -Brittle thus fractures.Asthenosphere = partially molten (5 %) - Ductile thus flowsTotal 25 marksYou will have 35 minutes to write the response under timed conditions in class.You may on this first occasion use your notes.Poor responses will have to repeat the timed task after college until an accept-able standard is achieved.
Oceanic crust is the part of Earths lithosphere that surfaces in theocean basins. Oceanic crust is primarily composed of mafic rocks, whichis rich in iron and magnesium. It is thinner than continental crust, gener-ally less than 10 kilometers thick, however it is denser, having a meandensity of about 3.3 grams per cubic centimeter.Although a complete section of oceanic crust has not yet been drilled,geologists have several pieces of evidence that help them understandthe ocean floor. Estimations of composition are based on analyses ofophiolites (sections of oceanic crust that are preserved on the conti-nents), comparisons of the seismic structure of the oceanic crust withlaboratory determinations of seismic velocities in known rock types, andsamples recovered from the ocean floor by submersibles, dredging(especially from ridge crests and fracture zones) and drilling. Oceaniccrust is significantly simpler than continental crust and generally can bedivided in three layers.Layer 1 is on an average 0.4 km thick. It consists of unconsolidated orsemiconsolidated sediments, usually thin or even not present near themid-ocean ridges but thickens farther away from the ridge. Near thecontinental margins sediment is terrigenous, meaning derived from theland, unlike deep sea sediments which are made of tiny shells of marineorganisms, usually calcareous and siliceous, or it can be made of volcanicash and terrigenous sediments transported by turbidity currents.Layer 2 could be divided into two parts: layer 2A – 0.5 km thick upper-most volcanic layer of glassy to finely crystalline basalt usually in theform of pillow basalt, and layer 2B – 1.5 km thick layer composed of dia-base dykes.Layer 3 is formed by slow cooling of magma beneath the surface andconsists of coarse grained gabbros and cumulate ultramafic rocks. Itconstitutes over two-thirds of oceanic crust volume with almost 5 kmthickness.
Label the Moho in the above diagram.Note the diagrams below show symmetry. You can be asked to complete half a diagram in the exam.
OphiolitesOphiolites are pieces of oceanic plate that have been thrusted (obducted) onto theedge of continental plates. They provide models for processes at mid-ocean ridges.Ophiolites are an assemblage of mafic and ultramafic lavas and hypabyssal rocksfound in association with sedimentary rocks like greywackes and cherts. They arefound in areas that have complex structure. Cross-sections simplified from R.C.Coleman, 1981, Journal of Geophysical Research, v. 86, p. 2497-2508.Ophiolites have been found in Cyprus, New Guinea, Newfoundland, California, andOman. The Samail ophiolite in southeastern Oman has probably been studied in thegreatest detail. The rocks probably formed in the Cretaceous not far from the what isnow the Persian Gulf. The rocks were later thrust (pushed uphill at a low angle)westward onto the Arabian shield.Why do Ophiolites give evidence for the composition of oceanic crust and uppermantle?
This is an American diagram in UK we spell: DYKESOphiolites are characterized by a classic sequence of rocks. This sequence is wellexposed at the Samail ophiolite. The base of the sequence is sedimentary rocks ofthe Arabian shield, not part of the ophiolite, on which the oceanic plate was pushed.From base to top the ophiolite is made of: peridotite, layered gabbro, massive gab-bro, dykes, and volcanic rocks. At Samail this entire sequence is 15 km thick. Thebasal peridotite is made of a rock called harzburgite (made mostly of the mineralsolivine and enstatite). Within the peridotite are many dikes of gabbro and dunite.The peridotite is deformed. The peridotite is overlain by dunite (an intrusive igneousrock made mostly of the mineral olivine) that grades upward to gabbro (an intrusiveigneous rock made mostly of plagioclase and clinopyroxene - augite). The sequenceis capped by dikes and volcanic rocks (pillow basalts that erupted on the oceanfloor). Sequence of rocks simplified from R.C. Coleman (1981).This Oman classic ophiolite complex should be studied carefully.You can be examined on this or a similar sequence.
From a tectonic perspective, the peridotite is depleted mantle that was under the magma chamberat the mid-ocean ridge crest. The gabbro layer is related, in some way, to the crystallization of themagma chamber (probably with repeated injections of magma). The dikes and volcanic rocks areformed by magma in transit to or at the surface. Cross-section simplified from Pallister, J.S., andHopson, C.A., 1981, Journal of Geophysical Research, v. 86, p. 2593-2644.To learn more about gabbro and other igneous rocks review the Geotour Igneous Rocks Module orvisit the petrography homepage.To see a classic bit of oceanic crust that has been thrusted up on a continent (an ophiolite ) visit theOman Virtual Field Trip.Source of Information:McBirney, A.R., 1985, Igneous petrology: Freeman Cooper & Co.Space for your own notes:/research:Where can ophiolites be found in Britain?Why do ophiolites not typically show magnetic anomalies?
How can ocean drilling be used to provide evidence for thecomposition of oceanic crust and upper mantle?Give an example of an ocean core.How might the evidence compare with that of ophiolites?
OceanicWhat do these two diagrams show? Continental
Complete this diagramExtra : Read pages 174/176 McLeish
What does the above diagram indicate about the plate movements inthe Atlantic and Pacific Oceans?Work out the rate of movement for each ocean.What does the diagram below indicate?
What is the relationship between the direction of magnetism and the table above?
How can the diagrams above help to provide a frame of reference for calculating the rate of seafloorspreading?How can radiometric dating and ocean drilling be used to date magnetic anomalies?
Hot SpotsRead page 44/45 of the OCR bookCopy the formula to calculate the rates of sea floor spreading.Answer questions 1,2 and 3 from page 45.
Oceanic lithosphere consists mainly of mafic crust andultramafic mantle (peridotite) and is denser than thecontinental lithosphere, for which the mantle is associated withcrust made of silicic rocks. Oceanic lithosphere thickens as itages and moves away from the mid-ocean ridge. Thisthickening occurs by conductive cooling, which converts hotasthenosphere into lithospheric mantle, and causes theoceanic lithosphere to become increasingly thick and densewith age. Oceanic lithosphere is less dense thanasthenosphere for a few tens of millions of years, but after thisbecomes increasingly denser than the asthenosphere. Thegravitational instability of mature oceanic lithosphere has theeffect that, at subduction zones, the oceanic lithosphereinvariably sinks underneath the overriding lithosphere, whichcan be oceanic or continental. New oceanic lithosphere isconstantly being produced at mid-ocean ridges and isrecycled back to the mantle at subduction zones. As a result,oceanic lithosphere is much younger than continentallithosphere: the oldest oceanic lithosphere is about 170 millionyears old, while parts of the continental lithosphere are billionsof years old.Write a definition for each of the following terms:MAFICULTRAMAFICSILICIC
What conclusions can be drawn from the diagram above?
Firstly describe the pattern of earthquakes, then try to explain the trend shown. Lookback at the previous page.
Label a likely location for Ophiolites.Explain why the volcano could produce basalt, andesite or granite.
LABEL THE DIRECTION OF PLATE MOVEMENTExplain how an accretionary prism is formed.
LEARN THIS DIAGRAM FOR ESSAYS!!Use page 39 of the OCR text book to complete the above diagram.Use notes from page 40 Oceanic—continental convergent plate margin toexplain the processes below.
Create a mind map for the information on page 40 OCR book:Oceanic –Oceanic convergent plate marginInclude the diagram of the cross section.
Draw diagram b from page 39 of the OCR text book and write 10 bullet points toexplain the process. Make sure you give specific examples throughout.
Make notes on the ppt point presentation given in class. Use page 43 of the OCR book to supplement yournotes. GL5, Theme 4, Key Idea 2: The J. Tuzo Wilson Cycle – Ocean Basin Evolution Continental rift formation – constructive plate margin
Oceanic-continental destructive plate margin e.g. Andes, South America
Continental-continental destructive plate margin e.g. India/Tibet and the Alps Age of crust is calculated using radio isotope dating
Past examples of ocean formation and closure. Iapetus ocean, closed causing the Caledonian orogeny (Siliurian to Early Devonian, roughly 440-390Ma) Tethys ocean closed as India collided with Eurasia forming the Himalayas and Africa collided with Europe forming the Alps (end Cretaceous, early Tertiary)(see movie) GEOLOGY GL5 THEMATIC UNIT 4 GEOLOGY OF THE LITHOSPHERE A.M. MONDAY, 24 June 20024. Describe and explain the evidence for ocean basin evolution (formation, growth and destruction) as proposed in the J. Tuzo Wilson cycle. 
Why does the continental lithosphere resist subduction?What does this suggest about the age of continental crust?
What conclusions can be drawn about the distribution of the ages ofthe rocks in the diagrams above?What do you think an inside out continent would be?(Extra reading :Super continent by Ted Neild
Partial Melting and granite magmatismIn pairs try to answer Q1 2008 paper.Then write 10 bullet points for your notes
ISOSTASYUse Webster page 17 and your own research to explain isostatic uplift and gravitation collapse. Make sure you usediagrams!!
Use the internet to research isostasy.Use published lithosphere sections and densities to compare the relative weights andheights of columns of continental and oceanic lithosphere.
What is delamination?How might this explain the Tibetan Plateau?What impact could this have on the direction of stress and subsequentfaulting in the Tibetan Plateau region?
Read 157,158,159 of WebsterUse diagrams and bullet points to show how the fold mountains were formed during the closing of the Iapetus Ocean.
4. (a) Describe how fold mountains are formed.(b) Evaluate the importance of isostasy during their formation.  2010You will write this under timed conditions 35mins. (Remember to include diagrams) You must achieveyour target grade or you will have repeat the exercise under timed conditions in your lunchtime.Use the space below to plan your response:
What type of faults are above?What is the direction of stress?
Use Webster page 100/157/158 to explain the formation of the Moine Thrust.(Note the folding takes place before the faulting.)
http://www.see.leeds.ac.uk/structure/faults/stress/Research and make notes from this website on the direction of stress in faults. Use diagrams!!! Then answer Q1 2004 .
4. (a) Describe how forces acting on continental lithosphere may cause brittle or ductiledeformation.(b) Evaluate the importance of the depth in the lithosphere on the types of deformationproduced. Research for homework -to be completed under timed conditions.Use the past papers to complete the data response and essay questions you have not yet done.This is good for revision!!!Please see me if you get stuck and use the mark schemes to help you understand what the exam-iners are looking for.(There are one or two essay questions that would not now be in GL5 but in GL4 instead.)