Evidence for Continental Drift

Science 10

• Alfred Wegener proposed the Theory of Continental Drift in the early 20th century.

• His theory is based on several observations

Jigsaw Puzzle fit of the Continents

• Wegener proposed that millions of years ago, all the continents were joined together as a supercontinent he called Pangea.

Matching Geological Structures and Rocks

• Mountain ranges that begin on one continent, end at the coast line and then appear to continue on a continent across an ocean.

• are also similarities between rock structures and ages of rocks on continents that are separated by thousands of kilometers of ocean.

Matching fossils

• Similar fossils occur in various locations.

• The fossil of a freshwater reptile called Mesosaurus have only been found in two places; South America and Southwestern Africa.

• Two other land dwelling fossils, Cynognathus and Lystrosaurus have been found throughout the continents in the southern hemispheres, separated by vast oceans.

Paleoglaciation

• Glaciers are masses of ice• When glaciers retreat they leave evidence of their

existence (U-shaped valleys, erratic and deeply scratched rocks)

• Scientists have found evidence of glaciers in now tropical areas

How do the Continents Move?• Earth is broken into large, movable slabs of

rock called tectonic plates.

• The tectonic plates are outlined by the pattern of volcanoes and earthquakes around the planet.

• Volcanoes are openings in Earth’s surface that when active spew out gases, chunks of rocks, and molten rock.

• Earthquakes are sudden, ground-shaking release of built-up energy at or under the Earth’s surface.

Evidence for Continents moving• The Mid-Atlantic Ridge is a long mountain

range running north to south down the length of the Atlantic ocean.

• The youngest part of the Atlantic ocean floor is closest to the Mid-Atlantic Ridge.

• The layer of ocean sediment becomes thicker the farther is away from the Mid-Atlantic Ridge.

• Earth’s magnetic field changes in strength from time to time and over thousands of years it can completely reverse in a process known as magnetic reversal.

• In the past 10 million years, Earth’s magnetic poles have reversed an average of four times per million years.

• Paleomagnetism is the study of magnetic properties of ancient rocks.

• There is a pattern of stripes in the direction that iron containing minerals pointed on the sea floor

• The pattern is called magnetic striping. The pattern was repeated on both sides of the Mid-Atlantic Ridge.

Sea floor spreading

• Magma (molten rock) from beneath the Earth’s surface rises because it is less dense than the material that surrounds it.

• The magma cools and hardens when it breaks through Earth’s surface at a spreading ridge forming new sea floor.

• As convection currents cause more magma to rise, the new magma forces apart the hardened material and like a conveyer belt continuously pushes older rock aside.

• A geologic hot spot is an area where molten rock rises to Earth’s surface in the middle of a tectonic plate.

• Hot spots show that tectonic plates move.

• A Canadian named J. Tuzo Wilson proposed that the continental plates must break up at certain areas, move across Earth’s surface, then rejoin. His theory was later named the Plate Tectonic Theory.