Earth's Mantle and Convection Currents

Earth's Mantle

• The mantle is Earth's largest layer, extending from the Mohorovicic discontinuity (35 km) to the Gutenberg discontinuity (2980 km).
• It's almost entirely solid rock, but behaves like a viscous liquid over geological timescales due to convection currents.
• Convection currents are driven by Earth's geothermal gradient (leftover heat from formation, radioactive decay).
• Mantle material rises, reaching the asthenosphere (ductile upper mantle).
• As it moves near the lithosphere, it spreads, decompresses, and some melts.
• Melted material can form volcanoes at mid-ocean ridges, revealing mantle composition (mostly peridotite).
• Convection can also uplift mantle rock to the surface (obduction).
• Seismic waves' velocity change based on the material they pass through, helping geophysicists understand conditions within the earth.

Convection Currents

• Convection is a method of heat transfer in fluids (liquids or gases).
• Uneven heating creates density differences that drive currents.
• Heated fluid expands, becomes less dense, and rises.
• Cooler, denser fluid sinks to replace it.
• Examples include sea breezes (land vs. ocean) and a campfire.
• Heat transfer from the fire is also radiant.

Index Fossils and Relative Dating

• Fossils are the remains or traces of ancient organisms.
• Some fossils can't determine rock age or correlation to other layers; index fossils can.
• Index fossils are organisms that lived for a short time, are common, wide-spread, and easily identified.
• Examples include ammonites, trilobites, and graptolites.
• Relative dating determines the order of geological events without numerical dates.
• The principle of fossil succession states specific fossils exist in a limited time frame.
• Combined with the principle of superposition, it helps narrow down the age of rock layers.
• Superposition dictates that older layers are lower in an undisturbed rock sequence.

Topographic Maps

• Topographic maps show elevation using contour lines.
• Contour lines connect points of equal elevation.
• Index contours have elevations marked.
• Contour interval shows elevation change between lines (constant).
• Contour lines don't cross and form circles around hills.
• Hachures indicate depressions.
• Understanding topographic maps is fundamental to geologic maps, which also show rock type and age.
• Geologic maps use colors, and a key to interpret rock type.

Continental Drift

• Alfred Wegener proposed continental drift, suggesting continents have moved.
• Evidence included fitting continental shapes, similar fossils, and landforms.
• Harry Hess later proved the theory after noticing mid-ocean ridges and the seafloor spreading.

Tectonic Plates

• Earth's lithosphere is broken into 7 major plates (and 8 minor).
• Plates are named after continents/oceans above.
• Plates (largest to smallest): Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, South American), and others.
• Plate movement (towards, away, or past each other) causes geological features – mountains, valleys, plateaus – also due to heat transfer from the core..
• A mid-ocean ridge is a ridge in the Atlantic Ocean.