3-1 Notes Drifting Continents
continents – 7 large landmasses (Africa, Antarctica, Asia, Australia, Europe,
South America, North America)
boundary – point or line where one region ends and another begins
In 1910 Wegener hypothesized continental drift which stated:
1. all continents were once joined together in a single landmass called
Pangaea which means “all land”
2. the landmass broke apart and these pieces are drifting
Evidence that the continental drift happened:
1. land features
a)continent boundaries fit together
ex
Africa and South America
b) mountain ranges on different continents line up
c) coal fields on different continents match up
2. fossils – preserved remains or traces of an organism that lived in the past
Ex
Ex
glossopteris (fernlike plant) seeds could not blow that far
mesosaurus (dinosaur) could not swim across an ocean
3. climate- a landmass moving toward the equator gets warmer
a landmass moving toward a pole region gets colder
3-2 Notes Sea-Floor Spreading
SONAR – device that uses sound waves to measure the distance to an object
mid-ocean ridges – underwater mountain chain where new ocean floor is produced
sea-floor spreading – process by which molten (hot liquid) material adds new oceanic crust to
the ocean floor which takes millions of years
evidence of sea-floor spreading – 1) new molten material inside the Earth rises, erupts,
cools, and hardens to form a solid strip of rock
2) magnetic mineral inside the rock line up in the direction
of Earth’s magnetic poles creating magnetic stripes, mirror images on both sides of the
ridge showing that crust on two sides of the ridge spread from the ridge at the same
time and at the same rate
3) as more crust is added to the ocean floor, the older strips
of rock move outward from either side of the ridge
deep-ocean trenches – where the ocean floor plunges into a deep underwater canyon
-Mariana Trench – deepest part of oceanic crust under the Pacific Ocean
subduction - ocean floor sinks back into the mantle at the trenches
-subduction and sea-floor spreading can change the size and shape of the oceans.
Ex
Atlantic Ocean is expanding because its trenches are very short
Ex
Pacific Ocean is shrinking because of large trenches
-during subduction, gravity pulls denser plate edges downward into the mantle
3-3 Theory of Plate Tectonics
tectonic plate – section of the lithosphere that slowly moves over the asthenosphere, carrying
pieces of continental and oceanic crust
Theory of Plate Tectonics – states that Earth’s plates are in slow constant motion, driven by
convection currents in the mantle
-explains the formation, movement, and subduction of Earth’s plates
-as plates move, they change Earth’s surface, producing earthquakes,
volcanoes, mountain ranges, and deep trenches.
Plates meet at boundaries:
1. divergent boundaries - (constructive boundary) creating new oceanic floor
– move apart from each other by sea-floor spreading at mid-ocean ridges
-rift valley – deep valley that forms where 2 plates move apart on land
ex
Iceland, East Africa rift system
2. convergent boundaries - (destructive boundary) destroying old oceanic floor
-move together or collide at trenches causing subduction
3. transform boundaries - crust is neither created nor destroyed
- fault (place where two plates slip past each other in opposite directions)
- earthquakes often occur
ex
San Andreas fault
collision of 2 oceanic plates cause the older more dense plate to subduct
-often volcanoes cause island arcs ex Japan, Aleutian Islands
collision of 2 continental plates squeeze the crust into high mountain ranges ex Himalayas
collision of a continental plate and an oceanic plate cause the more dense oceanic crust to
subduct and the less dense continental crust to push up forming mountains
ex
Andes and volcanoes
ex
Mount St. Helen