The Earth’sinterior structure - Lecture 1

How do we know what’s inside the Earth?• samples from inside the Earth (not many)• indirect observations of the Earth’s interior seismic, gravity, magnetic, and heat flow measurements• today (mostly): seismic studies of Earth’s interior

EARTH’S INTERIOR: SAMPLES

Plummer 1st Cdn edition, Ch. 4 pp 107-109, plus Box 4.4

(page 117)

KTB borehole -- drilled to a depth of 10 km in GermanyRussians have drilled a 12-km-deep borehole

Convecting Mantle

Kimberlites: samples of the upper mantle

Kimberlites: contain samples of the upper mantle (from >150 km deep)

Box 4.4, page 118 (Plummer 1st Canadian edition)

Mantle xenoliths: samples of the upper mantle from volcanoes

http://www.ees.nmt.edu/condie/MantleXenolith_NM.jpg http://www.calstatela.edu/faculty/acolvil/interior.html

EVIDENCE FROM SEISMIC WAVES

Plummer 1st Cdn. edition, Ch. 4 pp. 109-112

Refracted and Reflected Waves

Fig. 4.01

Seismic Reflection

Seismic Reflection

T53

Crustal Seismology: Seismic Reflection

Lithoprobe Seismic ReflectionNovember 2000

Vibroseis trucksStewart-Cassiar HwyNorthwest BC

pp 110-111, Canadian edition of text

Active sourcereflection seismology:

Petroleum industry’sprimary explorationtechnique

• energy reflects off sharp boundaries between rocks with strong contrasts in density and seismic velocity

• reflection is widely used by oil industry to find hydrocarbon traps in sedimentary basins

Seismic Reflection

Fig. 4.2

Seismic Refraction

Fig. 4.3

Effect of material change on paths of Refracted and reflected waves

layer with slow v

layer with faster v

layer with even faster v

With no change in properties, no refractions, no reflections

Slower

Faster

Curvature(refraction)

of the energy

Velocitygraduallyincreasingwith depth

seismic raypath bends

• energy is transmitted through layers and takes a curved path back to the surface without a reflection

• this happens because seismic velocity increases gradually with depth

• seismic velocities in the Earth generally increase with depth due to effects of pressure

Seismic Refraction

EARTH’S INTERNAL STRUCTURE

Plummer et al. Canadian Edition Ch. 4 pp. 112-119

Fig. 4.5

• Earth structure dominantly radial due to – pressure: rearrange atoms to form denser

minerals, for example, in the mantle

– compositional differentiation (denser elements sink to the center)

• main compositional layers are:– crust (0 to 30 km, on average)

– mantle (30 to 2900 km)

– core (2900 to 6370 km)

Earth’s Radial Structure

• some important rheologic layers are:– solid lithosphere (0 to 150 km)

– gooey asthenosphere (150 to 300 km)

– liquid outer core (2900 to 5150 km)

– solid inner core (5150 to 6370 km)

Earth’s Radial Structure

Figure 4.6

P-wave velocities in crust and upper mantleMUCH lowervelocities nearEarth’s surface

Table 4.1 in Canadian edition of text

Fig. 4.7

P- and S- wave velocities inside the Earth

• upper mantle (30-660 km)– 8 km/s : peridotite

– seismic velocity variations due to lithosphere, asthenosphere (gooey), olivine phase changes

• lower mantle (660-2900 km)– steady increase in velocity

– probably same composition as upper mantle

Wait! How do we know this??

We measure the arrival times of different seismic waves, and compare them with what we would expect from calculations for a model Earth.