Geology Video: Rocks, Energy, and Plate Tectonics (Chapter 1-9 Highlights)
Sedimentary Rocks
Definition: Rocks formed from the accumulation and lithification of sediments (fragments of pre-existing rocks, minerals, or organic matter).
Process of Formation (Lithification):
Weathering and Erosion: Breaking down and transport of existing rocks into smaller pieces (sediments).
Deposition: Sediments settle and accumulate in layers, often in basins (e.g., riverbeds, lake bottoms, ocean floors).
Compaction: Overlapping layers of sediment press down on lower layers, reducing pore space and forcing out water.
Cementation: Dissolved minerals (like calcite, silica, or iron oxides) precipitate in the pore spaces between sediment grains, binding them together to form solid rock.
Key Characteristics:
Often display distinct layers or bedding.
May contain fossils, providing a record of past life.
Examples:
Sandstone: Formed from compacted and cemented sand grains.
Limestone: Formed primarily from the accumulation of marine organism shells and skeletons (calcium carbonate).
Shale: Formed from compacted mud (silt and clay).
Real-world Connections: The fossils mentioned in the rock cycle are often found in sedimentary rocks, preserving evidence of ancient life.
Igneous Rocks
Definition: Rocks formed from the cooling and crystallization of molten rock.
Molten Rock Terminology:
Magma: Molten rock beneath the Earth's surface.
Lava: Molten rock on the Earth's surface.
Origin of Name: The word "igneous" has an origin meaning to "turn on fire."
Types of Igneous Rocks:
Plutonic (Intrusive) Rocks:
Cool and crystallize inside the Earth (e.g., within the crustal lithosphere).
Process takes a long time, often millions of years.
Also called intrusive rocks because they intrude through existing rocks.
Example: Granite (forms continental crust).
Volcanic (Extrusive) Rocks:
Cool outside the Earth, erupting from volcanoes.
Also called extrusive rocks.
Example: Basalt (forms oceanic crust).
Context for Discussion: Igneous rocks were previously discussed in the context of oceanic crust (basalt) and continental crust (granite). Basalts are dark, dense rocks making up the oceanic lithosphere, while granites are less dense and lighter, making up the continents, which float higher.
Real-world Examples:
The Sierra Nevada Mountains, including Mount Whitney, are composed of igneous granite rocks.
Hawaiian lava flows and volcanic eruptions (e.g., in Iceland) form basaltic igneous rocks.
All islands in the Atlantic, Pacific, or any ocean are igneous in composition (e.g., Peak of Volcan Indiosaur's Island).
Metamorphic Rocks
Definition: Rocks that have changed from their original form (igneous, sedimentary, or other metamorphic rocks).
Process of Change: Occurs due to intense heat and pressure when buried within the crustal lithosphere.
Key Characteristic: The forces involved are immense, reshaping rocks significantly without fully melting them.
If a rock melts, it becomes igneous.
Examples:
Rocks in Death Valley, once sedimentary, can be metamorphosed.
Gneiss (pronounced "nice"): A metamorphic rock, often formed from granite, where the mineral grains have been flattened and banded due to pressure. The term "organ gneiss" was mentioned, found in specific locations.
Real-world Connections: Homes in Cappadocia, Turkey, were carved into volcanic ash, which can undergo metamorphic changes or be easily carved due to its specific properties.
Bedrock
Definition: A geological term for hard rock, specifically igneous and metamorphic rocks.
Characteristics: Bedrock is generally harder and denser than sedimentary rocks.
Significance: Its strength and density make it important in the context of natural disasters; for example, bedrock areas tend to shake less during earthquakes compared to areas with weaker sedimentary rocks.
The Rock Cycle
Concept: Any rock, given enough geological time, can transform into any other type of rock.
Process: The Earth's active processes, driven by internal heat, provide the vast amounts of time (billions of years) needed for these transformations.
Sedimentary to Metamorphic: Sedimentary rocks (e.g., containing fossils, as discussed previously) can be exposed to pressure and heat, becoming metamorphic.
Metamorphic to Igneous: Metamorphic rocks can melt deep within the Earth to become magma, which then cools and crystallizes to form igneous rock.
Igneous to Sedimentary: Igneous (or metamorphic) rocks can be eroded into sediments, which then compact and cement to form sedimentary rocks.
Driver: Plate tectonics keeps rocks constantly moving and changing positions, facilitating the rock cycle.
Energy: The Ability to Do Work
Definition: Energy is the ability to do work, to move matter, or to cause a change.
Types of Energy (relevant to this course):
Kinetic Energy: The energy of motion.
Anything with mass moving at a certain speed possesses kinetic energy.
Example: An object from outer space, a magma chamber wanting to erupt, hands rubbing together to create warmth.
Heat (Thermal) Energy: A specific type of kinetic energy involving the transfer of energy from hot to cold.
Always proceeds from hotter to colder regions; never the reverse (due to thermodynamics).
Example: Magma inside the Earth, being hot, has heat energy and thus kinetic energy, leading it to rise and eventually erupt.
Convection:
Definition: A mechanism for transferring heat and energy, characterized by the movement of fluid or viscous material in cycles due to density differences.
Process:
Hot material (liquid, gas, or viscous solid) expands in volume, becoming less dense.
Due to lower density, it rises.
As it rises, it cools and becomes denser.
Due to higher density, it sinks back down.
This creates a continuous circulation, a convection cell.
Examples:
Boiling water in a pot: Hot water at the bottom rises, cools at the top, and sinks.
Earth's mantle: Hottest mantle near the core expands, rises (e.g., at mid-ocean ridges), cools, and sinks back down due to gravity (e.g., at subduction zones). This process takes hundreds of millions of years (350 million years was mentioned).
Significance: Convection in the Earth's mantle is the primary driver of plate tectonics, and it also plays a crucial role in weather phenomena.
Sources of Earth's Heat Energy
Internal Heat Energy (Geothermal Energy): Heat from within the Earth.
Primordial Heat: Heat remaining from the Earth's formation 4.5 billion years ago (from space impacts and gravitational crushing). This heat is still escaping and will continue to do so for billions more years.
Radioactivity: Decay of unstable isotopes (radioisotopes) of elements found in the mantle.
Examples: Uranium, thorium, potassium, rubidium.
As these elements decay to stable forms, they release enormous amounts of energy.
This process is similar to that used in nuclear power plants.
Application: Radioactivity is used by geochronologists to compute the ages of rocks (e.g., a rock body being 30 million years old or oceanic crust up to 200 million years old). This is not carbon dating; it uses these specific isotopes, especially abundant in igneous and metamorphic rocks.
External Heat Energy (Solar Radiation): Energy from the Sun.
Distinction: Unrelated to geothermal energy.
Role: Drives all weather phenomena, including rain, drought, severe weather (hurricanes, tornadoes, thunderstorms), by interacting with the oceans and atmosphere.
Plate Tectonics: A Fundamental Unit
Introduction: The scientific theory explaining the large-scale motion of the Earth's lithosphere.
Key Layers:
Lithosphere: The rigid outermost layer of the Earth, composed of the crust and the uppermost part of the mantle. It is broken into tectonic plates.
Continental lithosphere can be up to 90 km (roughly 60 miles) deep.
Oceanic lithosphere can be about 8 km (roughly 5 miles) thick.
Asthenosphere: The weak, ductile (viscous solid) layer of the upper mantle directly beneath the lithosphere. Its plasticity allows the lithospheric plates to move over it.
Core Concept: The lithosphere is broken into plates that move like "bumper cars" over the asthenosphere.
Driving Mechanism: Convection currents within the Earth's mantle.
Evidence for Plate Tectonics (Lines of Evidence):
Continental Drift:
Proponent: Alfred Wegener, a German climatologist, proposed the idea in 1915.
Hypothesis: Continents were once joined in a supercontinent called Pangaea (meaning "all land") about 200 million years ago.
Evidence presented by Wegener:
Continental Fit: The apparent jigsaw-puzzle fit of continents, especially across the Atlantic.
Fossil Evidence: Distribution of identical fossils across continents separated by oceans.
Glacial Evidence: Evidence of ancient glaciation (e.g., glacial striations on rocks) in modern-day equatorial regions (Africa, India, Australia) which would only make sense if these continents were once located closer to the South Pole (around 300 million years ago during a major ice age).
Initial Reception: Wegener's idea was openly mocked by scientists at the time because he lacked a plausible mechanism to explain how continents could move.
Vindication: His ideas were vindicated later in the 1930s and onwards as more evidence emerged (though he died before seeing this).
Supercontinent Cycle: Continents continuously come together, form a supercontinent, and then rift apart again in a cycle lasting hundreds of millions of years.
Mid-Ocean Ridges and Seafloor Spreading:
Discovery: Mid-twentieth century discovery of extensive mountain ranges in the middle of all oceans.
Composition: Drilling revealed these submarine mountain ranges are made of volcanic basaltic rock.
Seafloor Spreading: The process where new oceanic crust is generated at these mid-ocean ridges (spreading centers) as magma rises, cools, and pushes existing crust away. This is a primary site of igneous rock formation.
Subduction Zones (Deep-sea Trenches): Counterpart to mid-ocean ridges, these are deep valleys where oceanic lithosphere is eventually pulled back down into the Earth's mantle.
Convection Link: Mid-ocean ridges are locations where hot, less dense mantle material rises, while subduction zones are where cold, denser oceanic crust descends due to gravity.
Continental Rifting: The process of new oceans forming involves the asthenosphere rising, breaking up continental lithosphere, and eventually forming a linear sea and then a full ocean.
Examples: The Red Sea is a young ocean basin formed by this process; the East African Rift Valley is an ongoing example of present-day continental rifting.
The Baja California peninsula is separating from mainland Mexico due to the East Pacific Rise, which is continuing to rift through the Gulf of California. San Diego is west of the San Andreas Fault, on the Pacific Plate, moving northwest.
Ages of the Ocean Floor:
Observation 1 (Age Distribution): The youngest seafloor is found directly along the mid-ocean ridges (age 0 million years), while the oldest seafloor (up to 200 million years) is located farthest away from the ridges, typically near continental margins or subduction zones.
Observation 2 (Symmetry): The ages of the seafloor exhibit a symmetrical pattern on either side of a mid-ocean ridge. Rocks equidistant from the ridge axis on opposite sides have the same age.
Magnetic Striping: This symmetrical pattern is due to the oceanic crust acquiring the Earth's magnetic field as it forms and cools. The Earth's magnetic field periodically reverses, creating a distinctive pattern of normal and reversed polarity stripes parallel to the ridge.
Plate Boundaries:
Definition: Locations where two or more tectonic plates meet.
Significance: Most geological activity, including earthquakes, volcanoes, tsunamis, and landslides, occurs at plate boundaries.
Types of Plate Boundaries:
Divergent Boundaries: Plates move apart (rift).
Characteristics: Creation of new crust (e.g., mid-ocean ridges), volcanic activity, shallow earthquakes.
Mechanism: Magma rises to fill the gap created by separating plates.
Convergent Boundaries: Plates move together (converge).
Characteristics: Destruction of crust (subduction zones), mountain building, intense earthquakes (often the strongest), volcanic activity.
Types of Convergence:
Oceanic-Oceanic or Oceanic-Continental: One plate (the denser one, usually oceanic) subducts (goes down) beneath the other, forming a subduction zone (marked by a deep-sea trench). Volcanic arcs form on the overriding plate.
Continental-Continental: Neither plate subducts significantly; instead, the continents collide, crumple, and uplift, forming large mountain ranges (e.g., the Himalayas).
Transform Boundaries: Plates slide * horizontally past* each other in opposite directions.
Characteristics: No significant creation or destruction of crust, but intense, shallow earthquakes. Often connect segments of mid-ocean ridges.
Example: The San Andreas Fault in California.
Forces for Plate Motion
Gravity: The fundamental force driving plate tectonics.
Slab Pull: The most significant force. Occurs at subduction zones where a cold, dense oceanic plate (slab) is pulled downwards into the mantle by gravity due to its own weight. This pulls the rest of the plate along.
Ridge Push: A secondary force. Occurs at mid-ocean ridges where new, hot, buoyant oceanic crust forms and is elevated. Gravity causes this elevated crust to slide away from the ridge crest, pushing the plate in front of it.
Mantle Convection: The ultimate driver, causing hot mantle material to rise and cold, dense material to sink, creating a conveyor belt-like motion that pulls and pushes the lithospheric plates.
Hotspots
Definition: Volcanic regions on Earth that are not located at plate boundaries.
Mechanism: Caused by plumes of hot mantle material (mantle plumes) rising from deep within the Earth's mantle (potentially from the core-mantle boundary). These plumes remain relatively stationary while the lithospheric plate moves over them.
Formation of Island Chains: As the overlying tectonic plate moves, the stationary mantle plume creates a succession of volcanoes. The active volcano is typically directly above the hotspot. As the plate continues to move, the volcano moves away from the hotspot, becomes inactive, and a new volcano forms over the active mantle plume. This process results in a chain of volcanoes or seamounts that show a progressive increase in age with distance from the currently active hotspot.
Characteristics of Hotspot Volcanism: Usually characterized by effusive (non-explosive) eruptions of basaltic lava, in contrast to the more viscous and explosive volcanism often found at subduction zones.
Example: The Hawaiian island chain is a classic example, where the oldest inactive volcanoes are to the northwest (e.g., Kure Atoll), and the youngest, active volcanoes are to the southeast (e.g., the Big Island of Hawaii, with Kilauea and Mauna Loa) because the Pacific Plate is moving in a northwest direction over the stationary Hawaiian hotspot.