EESC 101 Lecture 5: Divergent Boundaries

Exam 1

Divergent Plate Boundaries

• two tectonic plates move apart, allowing magma to rise from the Earth's mantle and form new oceanic crust

• could be considered largest volcano on the planet

Mid-Ocean Ridge Structure

• ridge (2-2.5km above ocean floor)

  • abyssal sea floor (~4km below surface)

• black smokers found very close to ridge axis

• normal faults

Fault

a fracture within a tectonic plate on which one body of rock slides past another

Normal Fault

with gravity

Reverse Fault

against gravity, high angle

Thrust Fault

aqgainst gravity, low angle

Transform Fault

sideways motion

Earthquakes

• caused by sudden motion of plates

• tend to be shallow at ridges (<70km depths)

• creates by both normal and transform faults

Mid Ocean Ridge Layers

• only the upper 1-3km of oceanic crust (7-10km thick) is from erupting magma

• Juan de Fuca plate extrudes ~6m/century

Dike

a vertical shaft of rock that intrudes through a pre-existing weakness

• rock is younger than that of which it intruded

Sill

a horizontal sheet of rock that intrudes through a pre-existing weakness

Magma

semi-molten rock beneath the Earth’s surface

Lava

semi-molten rock above the Earth’s surface

• magma becomes lava upon reaching the surface

Pillow Basalts

• erupting magma cools immediately on contact with sea water

• outer edge solidifies, but inner portion is still hot

• additional magma intrusions burst through and create a new pillow

  • repeat many times over

Mid-Ocean Ridge Rocks: Gabbro

mafic rock with <50% SiO2

Mid-Ocean Ridge Rocks: Basalt

MORB, extrusive equivalent of gabbro

• has smaller crystal size than gabbro

Mid-Ocean Ridge Rocks: Vesicular Basalt

near surface eruption

• trapped air bubbles from holes during solidification

Passive Margin Basin

• crust thickens and becomes more dense away from ridge axis

• properties of isostasy means that thicker crust sill sink lower into mantle

• creates a basin along continental margins that can be filled by sediments

Journey to becoming a Mid Ocean Ridge

1. Plates Pull Apart (Divergence):

   • at a divergent plate boundary, two tectonic plates move away from each other creating a gap or rift in the oceanic crust

2. Magma Rises (Mantle Upwelling):

   • as the plates separate, hot mantle material rises to fill the gap

   • when it reaches near the surface, the drop in pressure causes it to partially melt, forming magma

3. New Crust Forms (Ridge Building):

   • magma erupts on the seafloor and cools, creating new oceanic crust

   • repeated eruptions and spreading build a long, continuous underwater mountain chain — the mid-ocean ridge

How do Divergent Boundaries Initiate?

• start within continental blocks

• tend to form along older faults/weaknesses

• driven by a combination of several processes:

  1. slab pull

  2. lithosphere bulging (from mantle plumes)

  3. differences in density and mass of crustal blocks

Rift

a crack in Earth’s crust where it’s pulling apart

Continental Rift

a place where a continent is splitting apart, forming valleys and volcanoes

• characterized by normal faults, shallow earthquakes (<30km depth), and often, but not always, volcanism

How do Continental Rifts Start and End?

Start at a ridge–ridge–ridge triple junction → usually only 2 ridges continue to form oceans.

Triple Junction

• doming creates a 3-way fracture

• 2 of 3 proto-ridges will normally become oceans

• failed arm is an aulacogen, a weak zone often reactivated later

Rifting Stages

1. Stretch: Tectonic forces (ridge push + slab pull) pull the continent apart

2. Crack: Faults form and magma (dikes) intrudes along them

3. Ocean forms: Crust thins completely → new oceanic crust/lavas accumulate.

Narrow Rift

EX: East African Rift

• normal faults of similar size

• faults and earthquakes very localized

• formed in very old tough crust

Wide Rift

EX: Aegean Sea, Basin and Range Province

• normal faults of many different sizes

• faults and earthquakes more dispersed

• formed in younger weaker crust

East African Rift

• Type: narrow, active continental rift / proto-ocean

• Age & Crust: ~30 Ma; cuts through very old, strong crust → called an aulacogen.

• Volcanoes: many active volcanoes (Mt. Kilimanjaro).

• Significance: likely area where humans evolved.

Basin and Range Province - Why and How?

• Cause: thick crust stretched by ridge push + slab pull

• Process: crust thins → normal faults → horsts (mountains) & grabens (basins)

• History: previously compressed in Mesozoic (100-60Ma); extreme extension possible because crust was thick

• Result: elongated mountains & valleys, some volcanism, stretched 500–800 km

• rift never became a full ocean basin

Aulacogen

• Definition: a failed rift arm that didn’t become an ocean

• Formation: forms at a triple junction where one arm “fails” while the other two continue spreading

• Features: weak crust zone → may collect sediments or have volcanism later

• Key idea: “failed rift = aulacogen”

Volcanism at Rifts

• Early stage: mantle melts mix with continental crust → more silica and felsic minerals (between MORB & continental crust)

• Mature stage: rift volcanism becomes more like MORB (basaltic, mantle-derived)

Continental Flood Basalts

• Composition: MORB-like basalt erupted on land

• Cause: massive mantle plumes, sometimes linked to rifting

• Category: continental type of Large Igneous Province (LIP).

• Size: extremely large – >2 km thick, cover >500,000 km²

• Impact: release huge CO₂ & sulfur, possibly triggering mass extinctions

Deccan Traps of India

• Type: Continental Flood Basalts (CFB)

• Age: ~66 million years ago (end-Cretaceous)

• Cause: massive mantle plume, possibly linked to rifting

• Size: one of the largest volcanic provinces on Earth – >2 km thick, ~500,000 km².

• Impact: released enormous CO₂ & sulfur → may have contributed to the dinosaur mass extinction