Lecture 7: Plate Tectonics Paradigm, and Earth Materials

What is permanentism (fixism)?

The idea that continents and ocean basins are fixed in place and do not move.

What explanations did permanentism propose for oceans?

• Land bridges that later sank

• Vertical subsidence

• Flooding of low areas

How did permanentism explain mountain folding?

Earth shrinking over time → crust wrinkles like a drying apple

What did Alfred Wegener propose?

Continental drift — continents were once joined (Pangaea) and later moved apart

Key evidence Wegener used?

• Fossils on separate continents

• Paleoclimate (glaciers, coal, deserts)

• Continental fit

Why were fossils strong evidence?

Same species found on continents separated by oceans (organisms couldn’t cross oceans)

Paleoclimate evidence includes what?

• Glacial deposits → cold climates

• Coal → tropical climates

• Salt → dry climates

Why did scientists reject Wegener initially?

• No mechanism for movement

• Mantle seemed too rigid

• Alternative explanations (land bridges, climate change)

What is Earth’s magnetic field?

A dipole field (like a bar magnet with north and south poles).

What is inclination angle?

Angle of magnetic field relative to Earth’s surface

• 0° at equator

• +90° at North Pole

• –90° at South Pole

How do igneous rocks record magnetism?

As magma cools, iron minerals align with Earth’s magnetic field and “lock in” direction.

What is apparent polar wander (APW)?

The apparent movement of the magnetic pole over time based on rock records.

Key evidence from APW paths?

Different continents show different paths → continents must have moved.

Why is APW strong evidence for plate tectonics?

Because poles cannot move differently for each continent → continents moved instead.

What is bathymetry?

Study of ocean floor depth/topography.

Major seafloor features discovered?

• Mid-ocean ridges

• Trenches

• Continental shelves

What is seafloor spreading?

New crust forms at mid-ocean ridges and moves outward.

Where is old crust destroyed?

At trenches (subduction zones).

Evidence for seafloor spreading?

• Youngest rocks at ridges

• Older rocks farther away

• Symmetrical magnetic stripes

What is a geomagnetic polarity chron?

A time period with no magnetic reversal.

What are magnetic stripes?

Alternating bands of normal/reversed polarity on ocean floor.

Why are magnetic stripes symmetrical?

Because crust forms equally on both sides of mid-ocean ridge.

Who proposed seafloor spreading?

Harry Hess

Who mapped the ocean floor?

Marie Tharp & Bruce Heezen

Who explained transform faults and hotspots?

Tuzo Wilson

What are the 3 plate boundaries?

• Divergent

• Convergent

• Transform

What is a hotspot?

A stationary mantle plume that creates volcano chains as plates move over it.

What is a seamount?

Underwater volcanic mountain

What is a guyot?

Flat-topped seamount (eroded + submerged)

What is an atoll?

Ring-shaped coral reef formed after volcanic island sinks

What is the basic unit of silicates?

Silicate tetrahedron (1 Si + 4 O)

What is polymerization?

Sharing of oxygen atoms between tetrahedra

Silicate structures + ratios?

• Isolated → 1:4 (Olivine)

• Single chain → 1:3 (Pyroxene)

• Double chain → 4:11 (Amphibole)

• Sheet → 2:5 (Mica)

• Framework → 1:2 (Quartz/Feldspar)

Trend in polymerization?

More sharing → higher Si:O ratio

Pyroxene cleavage?

~90° (2 planes)

Amphibole cleavage?

~60° and 120°

Mica cleavage?

One perfect plane (sheets)

Which minerals crystallize first?

Less polymerized (e.g., olivine)

Which minerals melt first?

More polymerized (generally)

Important exception in melting?

Quartz → melts at high temperature despite high polymerization

What happens under high pressure?

Rocks become denser

Which structures survive high pressure?

Less polymerized (denser)

What controls magma viscosity?

Silica content

Felsic vs mafic magma viscosity?

• Felsic → high silica → HIGH viscosity

• Mafic → low silica → LOW viscosity

What is silica?

SiO₂

Examples of non-silicates?

• Calcite

• Dolomite

• Halite

What are metallic minerals?

Minerals with metallic luster (due to free electrons)

What are ores?

Economically valuable metallic minerals

Why does water recede before a tsunami sometimes?

The trough (low point) of the wave arrives first