Chapter 1: Plate Tectonics

Section 1.3: Plates move apart.

I. The Big Idea and Key Concepts

A. Big Idea: The movement of tectonic plates causes geologic changes on Earth.

B. Key Concept 1.1: Earth has several layers.

C. Key Concept 1.2: Continents change position over time.

D. Key Concept 1.3: Plates move apart.

E. Key Concept 1.4: Plates converge or scrape past each other.


II. Tectonic plates have different boundaries.

       A. A plate boundary is where the edges of two plates meet

       B. Three types

1. Divergent boundary- occurs where plates move apart; most found in the ocean

2. Convergent boundary- occurs where plates push together

3. Transform boundary- occurs where plates scrape past each other


III. The sea floor spreads apart at divergent boundaries.

       A. Mid-Ocean Ridges and Rift Valleys

              1. Mid-ocean ridges are the longest mountain chains on Earth

              2. Most have rift valleys along their centers

a. In a rift valley molten material rises from the asthenosphere, cold ocean water cools the rock until it becomes solid, then new cracks form in the solid rock 

b. This process repeats constantly, growing the ridge high above the sea floor

3. World’s longest ridge

       a. Mid-Atlantic Ridge

              i. Runs the length of the Atlantic Ocean

              ii. Here, the North and South American plates are moving away from the Eurasian and Aftrican plates

              iii. The ridge extends 6214 mi. from Iceland to near Antarctica.  It is 15 mi. wide and 6 mi. deep (4 mi. deeper than the Grand Canyon

       B. Sea-Floor Rock and Magnetic Reversals

              1. Earth’s magnetic field affects the entire planet

2. Magnetic reversal- the North Pole and the South Pole switch position every so often

3. Each magnetic reversal is recorded in sea-floor rock

a. Magnetic minerals in sea-floor rock (mid-ocean ridges) line up with Earth’s magnetic field

b. When the field reverses, the cooling minerals record the change


IV. Continents split apart at divergent boundaries.

       A. Rift valleys form on land

              1. Hot material rises from deep within the mantle

              2. Heat causes crust to bulge upward

3. The crust cracks when it is stretched, allowing magma to rise through and forming volcanoes

4. As the rift valley grows wider, the continent spreads apart

5. As the rift widens, the valley floor thins and sinks

6. The valley floor falls below sea level, which allows water to enter and lakes or seas to form

B. The Great Rift Valley, Africa

       1. Continuing to get wider

       2. Thousands of meters long and 1800 m (5900 ft.) deep


V. Hot spots can be used to track plate movements.

       A. Hot spots- heated rock rises in plumes (thin columns) from the mantle

       B. Volcanoes develop above the plume

       C. How does it work?

              1. Heat from the plume partly melts some of the rock in the tectonic plate above it

              2. If the plate stays over the hot spot, the rock above it will melt

              3. In time, a volcano will form at the surface of the plate

                      a. The volcano will eventually rise above sea level and form an island

b. As the Pacific Plate moves slowly over a hot spot the Hawaiian Islands are being built

c. Scientists estimate that the North American Plate is moving southwest at about 1 in. per year