Some answers to the exercises in Tutorial 1
Question 1
This question is about lithospheric heat flow and strength.
(a) Briefly explain why a continental geotherm has an upper straight portion, a curved portion
and a lower straight portion.
Upper straight portion is conductive geotherm (Fig. 2.6 p45). Lower straight region is adiabatic convective
geotherm. Curved portion is the thermal boundary layers where cond and conv geotherms meet.
(b) Discuss the factors that facilitate lithosphere failure under extension.
High heat flow (Fig.3.10). Long time scale (Fig. 3.9). Mineralogy. Applied stress. Strain softening at high
strain rates.
(c) Briefly explain why, for a given set of reasonable geological conditions, oceanic lithosphere
is stronger than continental lithosphere under extension.
Oceanic lithosphere is topped by basaltic/gabbroic crust that contains plagioclase but (unlike
continental crust) no quartz. Moreover because oeceanic crust is thin, olivine-bearing mantle
peridotite extends to shallow depths (equivalent to plag-rich lower continental crust) in oceanic
regions than in continental plates. Reference to Block 1 Fig. 3.8 shows that under realistic geological
sytrain rates (10-16–10-14 s-1) plagioclase retains resists ductile failure to higher temperatures than
quartz, and olivine is still stronger.
Or, as written on p.69 of Block 1, continental lithosphere, which is dominated in its upper region by quartz,
will deform at much lower temperatures than oceanic lithosphere, which is dominated by plagioclase in the
upper part and olivine below about 101km. Thus, oceanic lithosphere is much stronger than continental
lithosphere given the same conditions.
Question 2
(a) Describe and account for the variation of surface heat flow with age for oceanic lithosphere.
(a) Heat flow falls exponentially from more than 250 mW m-2 for the youngest oceaniclithosphere to about 50
mW m-2 for the oldest. The relationship is not linear, but is proportional to the inverse of the square root of the
age up to about 55 Ma. For older oceanic lithosphere, the relationship is more complex, but flattens out to
about 50 mW m-2 by between 100 and 170 Ma. Convection is an important mechanism facilitating heat flow in
young oceanic lithosphere. Up to 30% of the heat is transferred hydrothermally and more than a quarter of this
occurs in lithosphere younger than 1 Ma. By 65 Ma, the sealing age for oceanic lithosphere, heat flow is
essentially all conductive.
(b) Describe and account for the variation of surface heat flow with age for continental
lithosphere.
(b) The heat flow through continental lithosphere depends of the province. There is also a general trend with
age and heat flow falls off rapidly from more than 120 mW m-2 for the youngest lithosphere, to about 50 mW
m-2 for lithosphere older than 100 Ma. The radiogenic heat produced in the upper crust decreases slowly with
age up to 2500 Ma. The heat from magmatic and metamorphic events decreases more rapidly up to 1500 Ma
after which it is insignificant. The background heat from the lower crust and mantle remains constant with age.
(c) Compare estimates of the temperature at the base of the continental and oceanic lithosphere.
(c) Oceanic and continental lithosphere have the same heat flow through the mantle. Any differences seen at
the surface in terms of heat flow are caused by variations in lithosphere thickness and/or additional heat
generated within the lithopshere. Convective replacement of cold material by hotter material beneath the base
of the lithospheric plates maintains a basal isotherm of 1350oC - 1450oC at 125 km - 95 km.
(d) How and why does one of the models for oceanic lithosphere fit the observed heat flow,
ocean depth and age data better than the others?
(d) There are two types of models which have been developed to understand the relationship between oceanic
lithosphere age, heat flow and depth. The half space model assumes that heat flow falls off with the reciprocal of
the square root of age and depth falls off with the square root of age. This model fits the depth and heat flow data
quite well for young oceanic lithosphere. The plate model assumes young oceanic lithosphere to act as a cooling
boundary layer of fixed thickness and older lithosphere to be thicker and to cool more slowly. This model fits the
data quite well for ages over about 70 Ma. The Global Depth and Heat flow plate model is a hybrid of the plate
and half space models. It fits the data best for all ages of oceanic lithosphere because it takes the best features of
each of the other models. It assumes a plate thickness of 95 km and a basal temperature of 1450 oC, which is
thinner and hotter than earlier and simpler models.