Plate Tectonics and Plate Boundaries Earth Science Standards

Plate Tectonics and Plate Boundaries Earth Science Standards 3. a - Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate tectonics. 3. b - Students know the principal structures that form at the three different kinds of plate boundaries.

Wegener’s Evidence • Matching Coastlines • Rock Evidence • Fossil Evidence • Climate Change Evidence

Hess’s Evidence

Two Types of Crust 1. Oceanic Crust • Mainly Basalt (more dense rock – sinks under continental crust) • Thickness: 3 -5 miles (thinnest at the ocean ridges) • Mineral composition: low in silicates; darkcolored rock (high in magnesium and iron)

2. Continental Crust • Mainly Granite (less dense rock – floats on top of the mantle) • Thickness: 18 -30 miles • Mineral composition: lots of silicates; lightcolored rock (quartz and feldspar)

Earth’s Plates • The lithosphere (crust and solid upper mantle) of Earth is broken into fragments called plates • Vary from 62 -125 mi in thickness • Plates move on top of the hot asthenosphere

• There about 8 major plates and nine minor ones. • The major plates include both continents and oceans. • The number of plates and their sizes, shapes, and motions have changed throughout Earth's history. . . and continue to do so today. • Different plates move at different speeds. What plate are we riding on?

Why do continents rise above the ocean floor? • Because continental crust is less dense and thicker than oceanic crust, it "floats" higher on the underlying mantle rock. • Oceanic crust is made of dense basalt; continental crust consists of less dense granite. Notice how high the continental crust rises above the oceanic crust in this cross section of the North American Plate.

Plate Boundaries • Plate Boundaries – places where 2 broken plates meet • Most earthquakes, volcanoes, and mountains occur at plate boundaries Plotting earthquakes and volcanoes help scientists define plate boundaries!

3 Types of Plate Boundaries 1. Divergent – Separation 2. Convergent - Collision 3. Transform – Sliding Past

1. Divergent • Plates move apart • Mostly on seafloor but can occur on land • Occurs when 2 oceanic plates separate or when 2 continental plates separate • Hot, rising material causes the plates to separate (convection currents)

• On seafloor: • Oceanic crust separates • Creates ocean ridges and rift volcanism • On land: • Continental Crust separates • Creates Rift Valleys and rift volcanism • Ex. African Rift Valley and Iceland

2. Convergent • Plates move towards each other • Cooler, sinking material pulls plates together (convection currents) • 3 types of convergent boundaries (classified by the type of crust involved) a) Oceanic-oceanic b) Oceanic-Continental c) Continental

a) Oceanic-Oceanic Convergence • 2 ocean plates collide Ex. - JAPAN • Oceanic plate subducts underneath oceanic plate & a trench forms • Subducting plate melts forming magma • Magma rises to surface forming a chain of volcanic islands (island arc) Subduction – one plate goes underneath another

Why is Japan known for its earthquakes and tsunamis?

b) Oceanic-Continental Convergence • An ocean plate and a continental plate collide • Oceanic plate subducts underneath continental plate which forms a trench at coastlines • Subducting plate melts forming magma • Magma rises to surface forming a volcanic mountain range on the continents t s u r C n a e c O n a h t r e s n e d continental crust! Ex. - CASCADES

Subduction ALWAYS forms Volcanoes!

c) Continental-Continental • Continental crust collides with continental crust • Crust same density – NO SUBDUCTION • Folded Mountains form as rock gets crumpled Ex. – Sierra Nevadas & Himalayas

Evidence of Moving Plates Old Rocks Young Rocks

3. Transform • Strike-Slip • 2 plates slide past each other • Crust deformed or fractured as giant slabs of crust grind past each other • Causes earthquakes • Ex. San Andreas

Ridge Push and Slab Pull • Ridge Push – heat rising pushes crust at a ridge toward a trench. • Slab Pull – cooler material sinking pulls oceanic crust down into a subduction zone (trench)