Section K.1 & K.2- Tectonic Plates and Boundaries

Wegner’s theory of continental drift

proposes that continents were originally connected as 1-2 large land masses that have broken up and drifted apart over several million years

What were the matchings that Wegener’s theory was bases on (4)

• rocks and fossils

• mountain belts

• glacial evidence

• coastlines of S. America and Africa

How many years ago was Supercontinent Pangaea?

~275 million years ago

Why was Wegener’s theory rejected by most scientists at the time, 1920s? (2)

1. earth’s crust was believed to be too rigid to permit such large-scale motions

2. Wegener did not offer a suitable mechanism that could displace such large masses for a long journey

Evidence for plate tectonic theory (3)

• detailed map of ocean floor

• plate boundaries

• seafloor spreading

  • paleomagnetism

  • ocean core sampling

What is bathymetric mapping providing?

evidence that led to the development of theory of plate tectonics

In 1950-60, what did the deep sea soundings via sonar reveal? (3)

• mid-oceanic ridges

• seamounts

• deep-sea trenches

Plate boundaries: world wide seismographic network by _

1960s

Seismographic network are used to measure:

earthquake waves

What did Harry Hess propose in 1960 and what did it develop into?

the idea of seafloor spreading, developed into broader theory of plate tectonics

What 2 evidence was the theory supported by?

• paleomagnetism

• ocean core sampling (age of ocean floors)

What is paleomagnetism? (3)

• iron in cooled magma orients itself with magnetic poles of earth

• provides a record of past magnetic fields

• magnetic field has changed oreintation at least 170 times

What does ocean core sampling do? (2)

• youngest crust + thinnest accumulation of sediments at ridges

• oldest crust + thickest accumulation of sediments near continents

What are plate tectonic moved by?

convection loops, driven by earth’s internal heat

What do tectonic plates “float” on?

the asthenosphere

What is mantle (magma) plume?

mostly stationary column of hot rock that extends from deep in mantle up to base of lithosphere

What does mantle plume do after rising to the crust?

it spreads horizontally and cools, moving segments of the crust

Seafloor spreading (2)

• oceanic ridges formed by currents of magma rising from mantle

• creates new crust on ridges

Subduction -melted and recycled-

older crust descends into earth

Types of plate boundaries (3)

• divergent

• convergence

• transform

Types of plate divergence (2)

• seafloor spreading (midocean ridges)

• rifting (rift valleys)

What does seafloor spreading create?

mid oceanic ridges

How are mid-oceanic ridges formed?

• two plates move away from each other on the ocean floor due to magma upwelling from the asthenosphere

• the extrusion of magma creates ridges-like features on the seafloor

What is magma upwelling?

mantle plume

Constructive boundary

rock is created

How does rifting occur?

two continental plates move away from each other due to mantle upwelling from asthenosphere

What might rifting cause? (and example)

• a gradual split in the landmass

  • East African Rift Valley

What does rifting produce?

distinct valley landscapes bordered by steep canyon walls

Lowlands can fill with water, forming:

lakes

What does rifting do? (3)

• elevates region

• causes earthquakes, volcanic eruptions

• forms long mountain ranges separated by broad valleys

Plate transform on seafloor _ in mid-ocean ridges

offsets

Plate transform on land:

San Andreas Fault System

Transform plate margins, boundaries where plates are sliding horizontally past each other

plane of motion is along a nearly vertical break (or fault) that extends through much of the lithosphere

Transform plate margins, the San Andreas Fault System (2)

• transform plate boundary (continental plates)

• N. American and Pacific Plate

In Mid-Atlantic ridge, seafloor spreading creates:

transform faults

What happened in the San Francisco earthquake in 1906?

700 people died and $6 billion in damage

What happened in Loma Prieta earthquake in 1989?

62 people died and $6 billion in damage

What is the chance of a large earthquake in Southern California (>6.7) in the next 30 years?

72%

Three types of plate convergence

• oceanic crust to continental crust

• oceanic crust to oceanic crust

• continental crust to continental crust

Ocean to continent is when:

denser oceanic plate subducts beneath buoyant continental plate

Ocean-to-continent is a destructive boundary because:

rock is destroyed (subduction)

Where do subduction trenches form?

next to continents

What is “slab pull”?

the subducting plate pulls the rest down, causing movement

Composite volcanoes (2)

• magma rises, producing extrusive and intrusive igneous rocks

• continental volcanic arc

Example of composite volcanoes

Andes Mountains (S. America) or Cascades (N. America)

Ocean to continent have both shallow and deep earthquakes (due to subduction) that can be as deep as:

600km below the surface

Ocean-to-ocean (2)

• both plates have similar densities, eventually the denser plate is subducted

• destructive boundary (subduction)

Features of ocean to ocean (3)

• deep ocean trench

• deep and shallow earthquakes

• volcanic island arcs

Examples of volcanic island arcs (3)

• Aleutian Islands (near Alaska)

• Japan

• Mariana Islands (western N. Pacific)

Continent-to-continent (2)

• two converging continental plates of similar density

• conservative boundary: rock is neither created nor destroyed

Orogeny (2) -mountains-

• a distinct period of mountain building

• compression (plate convergence) can create mountains

Terrane

a piece of lithosphere that is ‘added’ to a continental plate