Physical Geography: Landforms

Overview

• Geologic Time• Movements of the Continents• Earth Materials• Tectonic Forces• Weathering and Erosion Processes• Erosional Agents and Deposition

Geologic TimePretend the age of the earth (4.6+ billion years) is

compressed into one calendar year.

January 1 - Earth and planets formedEarly March - liquid water stands in pools.Late March - earliest lifeJuly - oxygen is important part of atmosphereOctober 25 – multi-cellular organismsLate November - plants and animals abundantDecember 15 to 25 - dinosaurs arise and disappear11:20 pm, December 31 - Humans appearOne second before midnight - Automobile invented

What is ‘tectonics’?• From Greek ‘tektonikus’

meaning building or construction

• Plate tectonics refers to the process of earth crust formation, movement, and destruction.

What is a ‘Plate?’

• Lithospheric plate: crust + upper mantle• Aesthenosphere: plastic mantle

History of Plate Tectonics

• ‘Fit’ of coastlines recognized early– Sir Francis Bacon (1600s)

• No mechanism for motion

• 1915 Alfred Wegener proposes theory of continental drift.

• Supercontinent Pangaea (‘all-earth’) [225mya].• Fragmentation and drift to current positions.

Plate Movement History

• Wegner’s evidence– Fit of continents– Fossil plants, animals, rock types /

geology • match on opposite shores• deposits inconsistent with current geography

Striking Match of Geologic Regions

Striking Match of Biological Regions

History of Plate Tectonics• Problem with continental drift?

– No sound mechanism for the ‘drift’!– Wegner hypothesizes spin of earth or

tides…..

History of Plate Tectonics• New theory for motion: Arthur Holmes

(1930s)– thermal convective cells in the upper

mantle (aesthenosphere) – theory is largely ignored

History of Plate Tectonics

• In the 1960s, Harry Hess and Robert Deitz (geophysicists) propose sea floor spreading along mid-oceanic ridges for plate motion.

Sea Floor Spreading

Plate Tectonics Theory• Continental Drift + Sea Floor

Spreading + new data Theory of Plate Tectonics

Plate Tectonics Theory

• Plate boundaries: main location for Earth’s volcanic and earthquake activity. This is main place where mountains are created.

• Type of plate boundary determines activity.• 3 types

– diverging (spreading)– converging (colliding)– transform (sliding past each other)

Geography of the Plates

• 7 major plates; several minor plates• Small plates / boundaries still unknown

Plate Margins: how do we know?• Marked by volcanic and tectonic

activity

Convergent Plate Boundaries

• Activity: – subduction; shallow to deep earthquakes;

volcanism (continental)

• Features: – ocean trench; explosive volcanic mtns on

continental margin

Divergent Plate Boundaries

• Landscape features: – land: rift valleys,

volcanic mountains, thinning crust

– ocean/sea: rift valleys, mountain ranges

Divergent Plate Boundaries

• Examples:– Atlantic Mid-Oceanic

Ridge– Red Sea– Rift valleys of

eastern Africa

The Rock Cycle

Earth Materials

• Formation of Earth• Three major rock types

– Igneous– Sedimentary– Metamorphic

Formation of the Earth’s Interior

• @5 bya, plantesimals (meterorites,icy comets) collide heat released (Kinetic energy to thermal energy)

• Entire planet melts (still cooling today)• Gravity sorts

materials by density– Fe in center– Si and O compounds

towards surface

• General trends: temperature, density • Horizon composition, behavior

The Earth’s Interior

Distance: 6730 km (3963 miles)

Igneous Rocks

• Igneous (ignus = fire)• Formed from the cooling of molten rock

(magma/lava), a process called crystallization.– Slow cooling larger crystals > dense rock– Rapid cooling small crystals > lighter rock

• Two classes of igneous rocks– intrusive: formed inside the Earth– extrusive: formed at Earth’s surface

Igneous Extrusive

Landscapes

Volcanic neck and dike: Shiprock, NM

Volcanic cones, obsidian flow: Mono Craters, CA

Volcanic Crater and Cinder Cone: Indonesia

Igneous Extrusive Rocks• Cools rapidly - exposed to surface• No visible crystals• Examples - rhyolite - andesite -

basalt

Some unique volcanic rock types

Pumice (vesicular)

- sometimes so light it floats!Obsidian

– glassy, ‘curved’ fracturing– used for arrowheads by Native

Americans

Igneous Intrusive Rocks• Cools slowly (thousands of years)• Visible crystals• Examples - granite

Typical Igneous Intrusions

Exposed Batholiths

Sierra Nevada, CA

Sedimentary Rocks

Compaction Cementing

Sedimentary Rocks

Formation

Relative Abundance by Type

Sandstone (larger grains)Shale (fine grains)

Limestone (CaCO3)

Where do Sedimentary Rocks Form?

Terrestrial environments (non-marine) Rivers and floodplains

(fluvial environment) Lakes Deserts (aeolian

environment)

Marine environments

Continental shelf Continental slope

and rise (deep sea fans)

Abyssal plain Beach and

barrier islands

Metamorphic Rocks or That’s very Gneiss,

but I don’t give a Schist!

Gneiss (broad foliation)

Schist (narrow foliation)

Which Type?

Which Type?

Sedimentary - limestone and shale

What type?

What type? Metamorphic - Amitsoq Gneiss, Greenland, 3.8 billion old

What type?

What type? Sedimentary - Sandstone in Utah

What type?

What type? Extrusive Igneous -

Reunion Island, Indian Ocean

What type?

What type? Folded Sedimentary -

‘Sheep Fold’, Wyoming

What type?

What type? Sedimentary - Vasquez

Rocks, Southern California

What type?

Morro Rock, CA

What type? Intrusive Igneous

Morro Rock, CADevil’s Tower, Wyoming

The Rock Cycle