Lesson 14:

Plate Tectonics II

Geological Oceanography

Last class we learned about Earth’s plates

The Earth’s surface is a mosaic of rigid plates all moving with respect to each other

These movements result in many of the structural features we see on Earth, like mountains, trenches and ocean basins

Plate movement also shapes continents, for example leading to formation of mountains like the Himalayas

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Today we’ll learn more about plate movement or plate tectonics

As plates moves, they always interact with other plates The ways that plates interact along their boundaries

shape geological features of Earth’s surface and the terrain

The terrain of the seafloor includes seafloor (abyssal) hills, mountain chains (ridges, islands and seamounts), trenches and great platforms, all formed by tectonic processes

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Map of the ocean floor

This map showsthat the ocean floor has underwater mountain chains (shown by the tan lines)

Photo: NOAA-OE/WHOI4

There are three types of plate boundaries

1. Divergent Boundary: Two plates move apart, called Spreading Centers

2. Convergent Boundary: Two plates collide, called Subduction Zones

3. Conservative Boundary: Two plates slide past one another, called Transform Faults

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Divergent boundary

Two plates move away from each other, pulling crust apart As crust pulls apart, magma (molten rock) rises up to form new

crust and seafloor, a process called seafloor spreading The spreading center is raised because of the heat of the

magma and sometimes has a central rift valley, as in the mid-Atlantic Ridge

Most spreading centers are found within the new seafloor they are forming

Plate 1 Plate 2

Spreading center

Magma6

Divergent boundaries and mid-ocean ridges

Mid-ocean ridges are continuous mountain chains found in all the world’s ocean basins

Mid-ocean ridges occur along divergent boundaries (spreading centers)

The entire mid-ocean ridge system is the longest continuous mountain chain on earth!

– (Over 40,000 miles long and found primarily below 2000 m)

Examples of mid-ocean ridges: – Mid-Atlantic Ridge: runs down center of Atlantic Ocean basin– East Pacific Rise: runs down Pacific Ocean basin, parallel to

South and Central America

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Close-up of ocean ridge

This image shows a mid-ocean ridge mountain chain called the East Pacific RisePhoto: NOAA-OE/WHOI

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Convergent boundary

Two plates collide with each other– Oceanic-Oceanic plates: Can form trenches or

island arcs like the Aleutian Islands– Oceanic-continental plates: Oceanic plate gets

subducted, pushed beneath, the continental plate Forms oceanic trenches and volcanic mountains on land,

like the Cascade Mountains and the Andes Mountains

– Continental-continental plates: Form mountains as crust folds together, like the Himalayas

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This map shows the location of the Aleutian Islands, which were created as a resulted of oceanic-oceanic plate convergence

Photo: NOAA

A recent NOAA voyage to the Aleutian Island area uncovered rarely seen Subarctic oceanic life, like this snailfish

Aleutian Islands: A result of plate tectonics

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Conservative boundary

Two plates slide past one another in opposite directions, parallel to, and along a transform boundary, also known as a transform fault

This results in earthquakes, like those in California along the San Andreas Fault (the boundary between the Pacific and North American plates)

Plate 1

Plate 2

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Viewing the global terrain

The plate movement you learned about results in the geological terrain that makes up our planet– Examples include mountains, trenches, valleys

and volcanoes

We’ll get a global view of these features during the student activity, when we take a trip to space

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