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.

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; dark- colored rock (high in magnesium and iron)

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

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

There are 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.

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 – places where 2 broken plates meet Most earthquakes, volcanoes, and mountains occur at plate boundaries

1.Divergent – Separation 2.Convergent - Collision 3.Transform – Sliding Past

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

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- Continental

2 ocean plates collide 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)

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

Continental crust collides with continental crust Crust same density – NO SUBDUCTION Folded Mountains form as rock gets crumpled

Old Rocks Young Rocks

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