The Earth

The Earth • Insides • Plate Tectonics • How the Philippines Came to Be

Parts

The Earth’s Surface

• Has the Earth’s surface always been this way? • Is it just a coincidence that Africa and the Americas seem to have split from each other?

Continental Drift Theory • First developed by Alfred Wegener (1912) • Shapes of continents suggest they were once joined • Rocks and fossils on continents on opposite sides of oceans are identical and of same age • Parts of Africa and India were once buried in ice sheets; North America and Northern Europe hold fossils of tropical organisms • More evidence in 1960’s: – Polar wandering, reversal of magnetic poles – Sea floor spreading

Evidence for Continental Drift Similar fossils occur in separate continents

Geologic histories of regions in separate continents best explained by their being once adjacent to each other

Continental Drift • Theory was initially dismissed due to the lack of a mechanism to cause massive continents to move

Plate Tectonics: Main premises • Earth’s crust made up of plates (~20) – Two kinds: oceanic plates (denser and thinner), continental plates

• Plates move as if on a conveyor belt (the mantle), – Speed: 10 cm/yr (10,000 kms in 100 million years)

• Mantle flows like a viscous liquid – Energy comes from radioactive decay – Unequal heating leads to convection currents – Plates float on and move with flowing mantle

• Many geologic processes take place near plate edges – Volcanoes, sea-floor spreading, earthquakes, mountain ranges

Mechanism for Continental Drift: Convection Currents in the Mantle

COOL

T O H

• Parts of the Mantle are hotter than others • Hot spots expand  decrease density  become buoyant  rise like a balloon • Adjacent areas move in to fill spaces left behind by rising fluid • Cool spots sink, creating a loop

The Plates

Geologically active areas are at plate boundaries

Convergent: Subduction zones; destroys crustal material

Divergent: Ocean ridges; creates new crustal material

Types of Plate Boundaries Conservative: Near faults; translational, no loss nor creation of crustal material

Volcanoes • Form along a line (arc) near subduction zones • Water in subducted oceanic plate helps form magma • Associated with trenches, deepest parts of the ocean • May also form over hot spots in the mantle, or where two oceanic plates converge

Sea Floor Spreading

• Occur at mid-oceanic ridges where mantle is moving apart, creating openings for magma to be released and form new crust • Rocks are older the farther away from ridge

Earthquakes

• Form near plate edges, or where plates are weakest (faults) or subject to stresses • Occur when rocks suddenly give way to pressure

Mountain Ranges • Form where two continental plates push against each other

The Philippines

• Country lies on a narrow oceanic microplate between Eurasian Plate and Philippine Plate • Philippine Plate is oceanic, moving northward into Eurasian plate

• Part of Pacific Rim of Fire • Near subduction zones, trenches and faults; highly active

The Philippines • Plate tectonics explain many of its features • Rich in limestone due to oceanic origin • Highly volcanic; most of its mountains and lakes are extinct volcanoes • Rich in mineral resources • Earthquake-prone • Too young to contain dinosaur fossils • Rich in diversity – biological and linguistic

• In a few tens of millions of years, islands will disappear