Lesson2c – Plate tectonics The Effects of Plate Tectonics

Pacific Basin region

Volcanoes of the Earth

Mountain Ranges of the Earth

What is the evidence so far? Majority of Earthquakes occur at plate boundaries. Particularly in around the Pacific basin. (The Ring of Fire). The majority of volcanoes are located at plate boundaries. Particularly in the Ring of Fire. Mountain ranges are also located at plate boundaries. The continents appear to be on the move. But what is doing the pushing?

Mid-Atlantic Ridge Discovered in the 1950s Stretches the length of the Atlantic Ocean and is part of a world wide ridge system Magma from the mantle reaches the seafloor as lava and creates new material for oceanic plates.

Mid-Atlantic Ridge

Graben results from stretching of crust

Graben on the Moon

Graben On Europa

Where would you expect to find the youngest rocks on the Atlantic seabed?

Age of Rocks from radiometric dating Rock samples near the mid-atlantic ridge are very young Rock ages increase as a function of distance from the ridge. So oceanic crust is added at the ridge and then it moves out away from the ridge with new material taking its place.

Magnetic Anomaly

Magnetic striping is prominent in the Atlantic. Red is strong magnetic field in one direction while blue is strong in the negative direction

Mantle material contains iron rich compounds such as hematite. Iron is ferromagnetic which means that when it is in a liquid form it can align itself with the external magnetic field of the Earth. The same way a compass points north. When the rock solidifies the magnetic field of the iron is locked into place. It is only free to align when it is liquid.

The reversal of the Earth’s magnetic field.

Older Rocks Younger Rocks

What causes the ridge to spread?

Convection currents in the asthenosphere (plastic-like mantle) brings hot material from deep inside the Earth up to the Lithosphere. The current then slows along the bottom of the Lithosphere until it cools and sinks back down. The friction between the moving asthenosphere and the rigid lithospheres causes the spreading seen in the Atlantic. RESULT: Over the course of 180 million years the Atlantic ocean has opened up to the size we see today.

Creation of the Atlantic Ocean

What happens on the other side of the continents?

A subduction zone occurs (remember lithosphere is only about 100 km thick)

The Oceanic crust and lithosphere of the Pacific is forced underneath the continental crust and lithosphere. As the Continent moves there is a huge amount of friction between the oceanic and continental plates.

Mountain Ranges form

Friction causes continent to fold and create mountain ranges When the continental plate over comes friction and breaks free, the result is an earthquake. Heating of the oceanic plate as it is driven deeper into the Earth causes melting of the rock into magma. This magma can come to the surface as a volcano.

Result: Stratovolcanoes

Structure of Stratovolcanoes (also called composite)

The crust of the Earth is rich in silicates. The amount of silica (SiO 2 ) controls the viscosity of the lava. The more SiO 2 the more viscous, or resistant to flow, the lava is. The magma from subduction is, in part, melted crust. Therefore the magma is highly rich in silica. This makes the lava very viscous.

Two different types of volcanoes both built up from lava flow. Volcano A Volcano B Which volcano was created from highly viscous lava?

Two different types of volcanoes both built up from lava flow. Volcano A Volcano B “A” has high viscosity lava, “B” has lava that flows

Result Stratovolcanoes are often explosive. It is difficult for the magma to flow through the volcano. They also have very steep slopes. Near the summit a typical slope is 30 degrees, and near the bottom more like 10 degrees.

Mt. Saint Helens – May 18, 1980

Mt Saint Helens eruption

Mt. Rainer overlooking Tacoma

Observable evidence of plate tectonics Graben and ridge fractures indicate that there may be mantle convection. Mountain ranges typically stretch out near plate boundaries Stratovolcanoes result from magma that contains crustal material. They form along subduction zones and can be identified by their steep slopes.