Download (continental drift)!!!

Seafloor Spreading
Seafloor Spreading
Crust – Earth’s Outer Layer
 There are two kinds of crust on the Earth
 Continental Crust
 Makes up the continents
 Oceanic Crust
 Is found under the oceans
 Is denser than continental
Seafloor Spreading
Seafloor Spreading
Mid-Ocean Ridges
 Are openings in the Earth’s crust
 Part of underwater mountain ranges on the ocean
bottom (longest on Earth)
 Above the surface they are islands (ex. Iceland)
 Deep cracks in the center allow magma to come out,
harden, and form new sea floor
 As new magma comes up, it pushes the sea floor
further apart
Seafloor Spreading
Seafloor Spreading
It is this spreading of the sea
floor that drives the
movements of the continents
(continental drift)!!!
Seafloor Spreading
Seafloor Spreading
Mid-Atlantic Ridge
(and others)
Seafloor Spreading
Rift Valleys
 The flat area between two ridges, formed from crust
spreading apart
 Lots of earthquakes and volcanoes occur here
 Magma comes out of the ocean floor here
Seafloor Spreading
Seafloor Spreading
Ocean Trenches
• Very deep area where ocean crust meets
continental crust
• Subduction Zone: place where old oceanic
crust is forced under continental crust, back
into the mantle at an ocean trench
Seafloor Spreading
Seafloor Spreading
Age of the Seafloor
 Young ocean crust = middle of ocean floor
 Magma coming out of rift cools, forms new rock
 Old ocean crust = far away from ridge
 Rocks are older because they were pushed away from the
ridge
Seafloor Spreading
Disappearing Ocean Crust
 Oldest ocean floor rocks are only 160 million years old
(earth is 4.6 billion years old)
Seafloor Spreading
Complete the following…
Define: Sea Floor Spreading
List evidence that supports it: