1 Seismic Classification of Materials Basis of Classification: Ability to transmit shear. SolidsLiquids P-waves √P-waves √ S-waves √S-waves X Cup of CementCup of Tea Stir cement –cup spins S tir tea – cup motionless Shear force transmittedShear force not to cup.transmitted to cup.

2 P- and S-wave velocities (α and β) P-wave S-wave Liquid Viscous Solid BrittleSolid Solid

3 Physical Basis of Plate Tectonics The Upper and Lower Mantle are Viscous solids. The Lithosphere is a Brittle solid. Distinction between Viscous and Brittle behavior is fundamental to understanding Plate Tectonics.

4 Conventional terminology Basis of Classification: Response to applied force Solids: maintain their form (rigid, hard) Liquids: deform easily (fluid, soft) Classification of Earth Materials Look here F t

5 Solids (maintain form) No relative displacement of internal particles

6 Liquids (deform easily) Relative displacement of internal particles ✓

7 Solids are hardLiquids are soft Relative displacements of internal particles indicate the internal flow properties or Rheology of a material. ▪ ▪ ▪▪ ▪▪ ▪ ▪ ✓ x

8 Rheology: flow properties of material Rheology Behaviour Example Brittle fractures glass, chalk Elastic stretches rubber band & rebounds Viscous deforms toothpaste ( or ductile)

9 Rheological Classification Basis of Classification: Net response to short term forces. i.e., A force is applied and then removed. Look at net response after force is removed. Look here t F applied removed 1 2

10 BrittleElasticViscous ForceResponse AppliedFractureDisplacement Displacement RemovedNo ChangeOscillationsNo Change Net R.D.’s X X √ Rheological Classification

11 Viscoelasticity Most materials have: an Elastic rheology on a short time scale, and a Viscous rheology on a long time scale. e.g. spring with weight is elastic on time scale of a few seconds or minute, but slowly stretches on a time scale of several years.

12 The Mantle responds as an elastic material to seismic wave disturbances. –Time scale of < 1 second. The Mantle responds as a viscous material to long term geologic (or tectonic) forces –Time scale of hundreds of millions of years. Mantle Viscoelasticity

13 “Short” and “Long” are relative terms. Forces acting for the same duration may be “short” for one material but “long” for another material with different physical properties. Since most materials have different physical properties at different temperatures => Forcing for the same duration may result in different rheological responses from the same material when at different temperatures.

14 E.g. Block of tar – cold=> brittle, splits -warm => viscous, deforms E.g. Toffee – cold => brittle, cracks - warm => viscous Mantle Rocks: Below 100km, Warm tar Top 100 km, Cold tar “Short” and “Long” are relative terms.

15 Tectonic Forces Buoyancy Forces acting for millions of years => Mantle Response: Viscous flow in the ‘warm’ interior. Brittle fracture in the ‘cold’ outer 100 km. => The Lithosphere is a Rheological Boundary –Same material above and below, but different rheology due to temperature profile with depth:

16 Temperature Profile in Earth 100 km Depth Temperature (C) Brittle Viscous T < 1300 C Crust Mantle T > 1300 C Lithosphere