Topic Xii

“Earth DynamicCrust”

I. Evidence of Crustal Movement:

A. Original Horizontality: assumes that sedimentary rock is deposited in flat layers.

What causes these layers to shift?

B. Types of Crustal Activity:

1. Faulting: cracks within Earth’s crust.

2. Folding: bending of rock layers

3. Tilting: rock layers pushed on an angle.

II. Earthquake: the vibration or shaking of earth’s crust caused by seismic waves.

A. Types of Seismic Waves:

1. Primary Waves: “P” waves are compressional waves that travel fast through both liquids and solid rock.

2. Secondary Waves: “S” waves are shear waves that travel slowly through solid rock only.

III. Earthquake Location:

A. Focus: the point below earth’s surface where the earthquake originates.

B. Epicenter: the point on Earth’s surface straight above the focus.

C. Calculating Epicenter Distance: requires the difference in arrival times of “P” and “S” waves to a seismic station.

* The further the seismic station is from the epicenter the greater the difference in arrival time between the two waves.

D. Epicenter Location: requires 3 seismic station readings. Where all 3 intersect is the epicenter.

E. Measuring Earthquake Strength:

1. Mercalli Scale: measures the earthquake’s intensity based on the description of the damaged caused.

*larger number indicates greater damage.

2. Richter Scale: a magnitude scale used to describe the amount of energy released by an earthquake.

a. Each step increase represents an increase of 10 times the next lower number.

Ex. A magnitude 7 is 10 times greater than a 6, but 100 times greater than a mag. 5.

IV. Earth’s Layers: Earth is divided into four major zones: crust, mantle, outer core and inner core.

A. Crust: “lithosphere” outer most layer and relatively thin (10-65 kilometers).

1. Continental Crust:

*thicker

*mostly granite

*low density 2.7 g/cm3

2. Oceanic Crust:*thin*mostly basalt*more dense 3.0 g/cm3

B. Mantle: located below the crust. Separated from the crust by the Moho interface.

1. Asthenosphere: “upper mantle” flows like a thick plastic fluid.

2. Stiffer Mantle: thicker and more solid.

C. Outer Core: made up of liquid iron.

D. Inner Core: made up of solid iron and nickel.

1. Meteorites: are composed of similar material (Fe and Ni) as Earth’s inner core.

2. Shadow Zone: a dynamic band around Earth that receives no “P” or “S” from an earthquake epicenter.

a. “S” waves are absorbed by the liquid outer core.

b. “P” waves are refracted (bent) due to the different material densities of Earth’s layers.

V. Continental Drift: theory proposed by Alfred Wegener stating that Earth’s crust is shifting.

A. Wegener’s Evidence:

1. Continental coastlines fit like a jigsaw puzzle.

2. Similar rocks, minerals and fossils found on separate continents.

3. Tropical plant fossils found in Antarctica.

VI. Plate Tectonics Theory: states that Earth’s crust is divided into large plates riding on the asthenosphere.

*plates can consist of continental crust, oceanic crust or both.

Ref. Table pg. 5

Arrows indicate direction of movement.

A. Mantle Convection Currents: are believed to be the driving force behind plate movement.

1. Hot, less dense magma rises at mid ocean ridges and pushes plates apart.

B. Evidence of Plate Tectonics:

1. Seafloor Spreading: new ocean crust is formed by rising magma at mid-ocean ridges.

*the further from the ridge the older the rock!

2. Magnetic Reversals: Earth’s magnetic poles reverse over time creating a pattern in newly formed ocean crust.

*magnetic minerals in the new rock align themselves with Earth’s magnetic field.

*identical magnetic patterns are found on both sides of the ocean ridge.

C. Plate Boundaries: the interface between plates where earthquakes and volcanoes occur.

1. Convergent Boundary: two plates collide with each other.

*subduction occurs if dense ocean crust collides with continental crust, creating deep ocean trenches.

2. Divergent Boundary: where two plates are moving apart. Occurs at mid-ocean ridges.

3. Transform Boundary: two plates slide past each other. Ex. San Andreas fault

4. Hot Spots: plumes of hot magma from the mantle producing volcanoes.

*as a plate moves over the hot spot, chains of volcanic mountains form. Ex. The Hawaiian islands.