Plate tectonics theory

Learning Objectives Study the evidence supporting the

continental drift theory Discuss how the ideas of continental

drift and seafloor spreading Explain how plate movements are

affected by volcanic activities and earthquakes

Continental Drift Proposed by Alfred Wegener in 1912 Stated that the continents we know

today had once been joined together in a single landmass (Pangaea)

People during his time thought that this idea was radical too radical so Wegener came up with a list of evidences to support his contention

Continental Drift: Evidences The fits of the continents The similarity of fossils in different

continents The similarity of rock type and age

along matching coastlines The continuity of geologic features

from continent to continent The presence of coal seams in

Antarctica Tillites

Continental Drift: Rejection The mechanism presented of Wegener

wasn’t found plausible. The mechanism centered on using the

gravitational pull of the moon as the main reason that caused the motion the continents.

The explanation provided was not accepted.

What could be wrong?

Before we proceed to talking about SEAFLOOR SPREADING, we first talk about PALEOMAGNETISM…

Paleomagnetism Some rocks contain magnetic

minerals known are magnetite. The mineral is made of the elements

iron and oxygen. The magnetite found in the rocks are

capable of recording the magnetic field of earth.

Such record proved that continents did move somewhere in the past… But how?

Deep Discoveries (1950 – 1960) Presence of a belt of underwater

mountains (oceanic ridges) that encircles the globe

Presence of a central valley (rift valley) at the summit of the oceanic ridges

Oceanic ridges (composed of volcanic rocks) give off an abnormally high amount of heat

Earthquakes in the deep sea areas were found to be associated with trenches

The seafloor is just 170 million yrs old

All of the discoveries listed in the previous slide were duly accounted

by a theory proposed by Harry Hess… the

SEAFLOOR SPREADING

PLATE TECTONICS THEORY

The Theory Holds that the outermost rigid layer

of the earth is broken into several segments called plates, and these plates are in motion.

The movement of the plates are described by the three types of plate boundaries; Divergent plate boundary Convergent plate boundary Transform boundary

Divergent Boundary Place where two plates are moving

apart from each other just like the boundary found in the eastern part of Africa

Creates huge cracks or valleys that would later be filled up with water from rain or nearby streams (narrow and elongated lakes are created)

Explains the formation of Lake Malawi, Lake Tanganyika and the Red Sea

Transform Boundary Two neighboring plates are neither

spreading apart nor colliding with each other

The two interacting plates slide past each other i.e. San Andreas Fault

Convergent Boundary Place where two plates move toward

each other Three possible cases could happen

long these boundaries; Two continental plates may collide Two oceanic plates me collide A continental plate and an oceanic plate

may collide

Transform Boundary Two neighboring plates are neither

spreading apart nor colliding with each other

The two interacting plates slide past each other i.e. San Andreas Fault

But what is causing all these movements?

Rock Deformation

Types of Deformation Fracture (break) Fold (bend)

Fracture Common on the uppermost part of

the crust Promoted by conditions of low

pressure and temperature Comes in two types – faults and joints

Faults Breaks along which there is

considerable movement Are of two types:

Dip-slip (involves the movement of blocks of rock mainly in the vertical direction)

Strike-slip (movement is in horizontal direction)

Folding Bending of rocks Happens deep within the crust Rocks deep within become thicker as

they are squeezed, or become thinner when pulled apart

Promoted by high pressures and temperatures