# Lecture 8

• Direct study of rocks
• Surface rocks
• Drill holes
• Lava

Xenoliths - rocks taken from mantle through magma, and carried to the surface

### Earthquakes

• Earthquakes occur as a result of built up strained energy in blocks of rock subjected to tectonic forces
• Earthquakes are therefore shaking ground accompanied from the sudden release of energy
• Caused by fracturing rocks, due to the strain energy that builds up as a result of plate motion

Fault - rock fracture along which relative movement occurs

Focus - initial point of rupture along a fault

• Focal depth influences earthquake impacts

Epicenter - point on the surface directly above the focus

• Seismic body waves move throughout the body of the planet

### What Can We Conclude by Tracking These Body Waves as They Make Their Way Throughout the Planet?

• The waves as they make their way through the planet, it is going to encounter layering,
• The waves will be bent when passing from the crust to the liquid lower layers
• IT IS REFRACTING

P Waves - primary waves

S Waves - secondary waves

Surface Waves - waves that do not go through the body, instead travel along surface

• S waves cannot travel through liquids, therefore they are blocked by the outer core
• This is one of the most powerful evidences that the core is liquid

### Core

• Fe - Ni metal
• Inner core ﻿$\rightarrow$﻿ solid
• Outer core ﻿$\rightarrow$﻿ liquid
• Peach analogy to regard the layers of the earth, the skin of the peach being the crust

### Mantle

• Made of relatively dense rock
• Not as dense as core

### Asthenosphere

• Weak zone
• Near melting point

### The Crust

• Oceanic crust
• Basalt
• Dense, thin
• Continental crust
• Granite
• Less dense, thick
• Crust + top mantle = lithosphere

### Why Do Continents Move Around?

• Mantle convection drives the plate motion
• Hot rock (low density) rises from depth and cool rock (high density) sinks from the surface
• Hot mobile mantle + thin, cool, brittle lithosphere
• Breaks into plates