A seamount forms on young oceanic lithosphere near a mid-ocean ridge, and another seamount of equal mass forms on old oceanic lithosphere far from the ridge. Which seamount will cause greater lithospheric flexure (bending)?
AThe seamount on old lithosphere, because old lithosphere is weaker and more easily bent
BThe seamount on young lithosphere, because young lithosphere is thinner and more easily bent
CBoth will flex equally, because flexure depends only on the load mass, not on lithospheric age
DThe seamount on young lithosphere, because hot rocks are more buoyant and respond more dramatically to loading
Young oceanic lithosphere near a mid-ocean ridge is hot, thin, and has a small elastic thickness (Te). A small Te means the plate is mechanically weak and bends significantly under topographic loads. Old, cold oceanic lithosphere has a large Te because more of the plate is below the brittle-ductile transition temperature, contributing to its rigidity. The same load on old lithosphere produces much less flexure. This is why the Hawaiian chain shows progressively stronger subsidence signatures on older lithosphere as the Pacific Plate moves northwest over the hot spot.
Question 2 Multiple Choice
In the yield strength envelope (strength vs. depth diagram) for continental lithosphere, what controls the transition from increasing strength to decreasing strength as depth increases?
AThe transition from sedimentary to metamorphic rock types
BThe change from brittle frictional failure (governed by confining pressure) to ductile creep (governed by temperature)
CThe Moho discontinuity, where crustal composition changes to mantle composition
DThe increase in grain size with depth, which causes rocks to become more ductile
In the shallow crust, rock strength increases with depth because brittle frictional failure is governed by Byerlee's law: strength scales with confining pressure (which increases with depth). But temperature also increases with depth (the geotherm), and once temperature is high enough to activate ductile creep in the dominant mineral phases (quartz at ~300°C, feldspar at ~400°C, olivine at ~600–700°C), strength plummets exponentially with further temperature increase. The depth of peak strength is the brittle-ductile transition — a thermal boundary, not a compositional one.
Question 3 True / False
Old, cold oceanic lithosphere is mechanically stronger and stiffer than young, hot oceanic lithosphere near a mid-ocean ridge.
TTrue
FFalse
Answer: True
Lithospheric strength is controlled primarily by temperature: the cooler the rock, the deeper the brittle-ductile transition and the greater the integrated strength. Old oceanic lithosphere has been cooling since it formed at the ridge — its geotherm is lower, the seismogenic zone extends deeper, and its elastic thickness is larger. Young lithosphere at the ridge crest is nearly at asthenospheric temperatures and has very little mechanical strength. This is why subducting old oceanic slabs can transmit stresses over large distances and generate deep seismicity, while young slabs buckle and deform.
Question 4 True / False
Earthquakes can occur throughout the full thickness of the lithosphere, including in the ductile lower portion.
TTrue
FFalse
Answer: False
Earthquakes require brittle failure — sudden shear fracture or frictional sliding on faults. This only occurs in the brittle portion of the yield strength envelope, which corresponds to the seismogenic zone. In the ductile portion (deeper, hotter rocks), deformation occurs by creep — slow, continuous flow that releases strain gradually without generating seismic waves. The seismogenic depth limit corresponds closely to the brittle-ductile transition temperature for the dominant mineral. In most continental crust this is the upper 15–20 km; deeper earthquakes occur in the strong upper mantle beneath some old cratons.
Question 5 Short Answer
Why does the yield strength envelope for continental lithosphere sometimes show a 'jelly sandwich' pattern — strong upper crust, weak lower crust, and strong upper mantle?
Think about your answer, then reveal below.
Model answer: Continental crust is rich in quartz and feldspar, which become ductile at lower temperatures (~300–400°C) than olivine (~600–700°C). The lower crust reaches quartz/feldspar ductile temperatures while the uppermost mantle is still cool enough to remain brittle and strong. This temperature contrast between the weak lower crust and the strong olivine-rich upper mantle creates a strength minimum sandwiched between two stronger layers.
The key is that lithospheric strength depends on both temperature (the geotherm) and mineralogy (which mineral is deforming). In oceanic lithosphere, olivine dominates from the surface down, so there is one main brittle-ductile transition. Continental crust has a compositional boundary at the Moho: felsic minerals (quartz, feldspar) in the crust have lower ductile transition temperatures than mafic minerals (olivine) in the mantle. So the lower crust goes ductile before the upper mantle does, creating the 'jelly' weak layer between two 'bread' strong layers.