science

CRUST - the outermost layer

MANTLE - thickest layer

OUTER CORE - A liquid layer of molten iron and nickel beneath the Earth's mantle, responsible for generating the planet's magnetic field. INNER CORE - The Earth's solid. Super-hot center, primarily composed of iron and nickel.

Earth’s Lithosphere - composed of the crust (outermost layer of the earth) and the upper mantle.

Crust is the outermost and the thinnest layer of the earth. In this layer, it is where living organisms can survive. The average density is 2.8g/cm^3 and the thickness ranges from 5 to 50 km.

There are two types of crust; continental and oceanic crust. The continental crust varies between 6 and 47 miles in thickness depending on its location. It is much older than oceanic crust and rocks found on continental crust are often the oldest in the world. Thicker than oceanic, but less dense than oceanic. The oceanic crust is found under the oceans or bodies of water. It is about four miles thick and is the youngest type of rock. Thinner but denser than continental.

Differences between Continental and Oceanic crust:

Oceanic crust is denser than continental crust. With this, the oceanic crust floats higher than the mantle. When continental crust and oceanic crust collide, oceanic crust subducts because of its higher density.
The continental crust is older than the oceanic crust.
Oceanic crust is made mostly of basalt while continental crust is made mostly of granite.

MAPS OF PLATE BOUNDARIES

Tectonics - plates move very slowly but constantly.

Plate tectonics - The theory that Earth’s crust is made up of plates that interact in various ways. Thus, producing earthquakes, mountains, volcanoes, and other geologic features.

Plate boundaries are the lines at the edges of the different pieces of the lithosphere. Lithospheric plates are moving due to the convection current in the Earth's interior. The lithosphere is made up of the crust and the upper part of the mantle. There are two types of crusts: the continental crust which is thicker but less dense, and the oceanic crust, which is thinner and denser.

According to the Plate Tectonic Theory, the Earth's lithosphere consists of the crust and upper mantle that move slowly and constantly over time. This movement causes the formation of plate boundaries namely: divergent, convergent, and transform fault boundaries.

(a) **Divergent boundaries** refer to plates that separate and move apart in opposite directions forming new lithosphere - the young seafloor. This either occurs at mid-ocean ridges (seafloor spreading) or at rifted continental margins (rift valley).

(b) **Convergent boundaries** are formed when two plates move toward each other. The oceanic plate bends downward at the subduction zone. This occurs in two oceanic plates: convergent boundary and continental plate-oceanic plate convergent boundary. Oceanic plate sinks because it is denser than the continental plate. In the case of convergence of two oceanic plates, the older plate sinks. Whereas in the convergence of two continental plates, they collide and buckle up forming mountain ranges. No subduction occurs in this type of convergence.

(c) **Transform fault boundaries** are plates sliding past or slipping past each other.

Scientific Basis for Identifying Major Plates:

A. Earthquake Epicenter Distribution: Concentrated in specific areas, often near continent edges or in oceans.

B. Active Volcano Distribution: Concentrated along continent edges, with 70% underwater. The Pacific Ring of Fire is a prime example.

C. Mountain Range Distribution: Often aligned with earthquake and volcanic activity zones and formed by plate movements and other geological processes.

Mountain ranges are typically found in areas with high seismic and volcanic activity.

Seafloor Spreading: Oceanic plates diverge at mid-ridges, where magma flows and cools to form new seafloor. This process creates new crust, while subduction destroys old crust as it descends into the mantle.

Convection currents drive tectonic activities: • Slow, stirring motion of Earth's mantle • Carries heat from lower mantle and core to lithosphere • Recycles material back to mantle

Convection currents drive plate movement. As oceanic crust moves from the ridge, it becomes denser and sinks.

Magma release at the ridge pushes oceanic crust toward the subduction zone trench, known as ridge push.

In 1965, Canadian geophysicist J. Tuzo Wilson proposed the Theory of Plate Tectonics, unifying Continental Drift and Seafloor Spreading. He posited that Earth's lithosphere consists of large, rigid plates floating on the asthenosphere, where extreme heat and pressure cause rocks to behave like viscous liquid. Wilson identified three boundary types: mid-ocean ridges (crust creation), trenches (plate subduction), and transform faults (plate slippage). This theory explains Earth's fundamental shaping processes.

Earlier, Alfred Wegener's Continental Drift theory suggested Earth once had a single continent that drifted apart, based on fossil evidence across continents. He proposed continents floated on the mantle, driven by heat-induced currents.

In 1962, Harry Hess proposed Seafloor Spreading, suggesting that seafloor pushes continents apart at rifting lithospheric plates. Rising magma fills crust cracks, solidifying into basalt and igneous rock, creating a magma-driven conveyor belt of new seafloor.

Plate movement is attributed to a combination of lithosphere dragging, ridge push, sliding, and slab pull, collectively driving plate tectonics.

First Evidence – The Continental Jigsaw Puzzle: The shapes of South America and Africa, North America and Europe, and the tips of Southern Africa, Australia, India, and Antarctica fit together. Alfred Wegener noticed that Earth's large landmasses almost fit together like a jigsaw puzzle. Second Evidence – Evidence from Fossils: Fossils are preserved remains of plants and animals from the past geological age. Fossil remains give us a peek into the past, they help us to understand how prehistoric plants and animals behaved.

Mesosaurus was a half-meter-long freshwater reptile with limbs for swimming and walking on land. Its anatomy indicates that it couldn't swim long distances or survive in seas, being a freshwater creature. The presence of Mesosaurus fossils in both South America and Africa suggests these continents were once connected.

Third Evidence – Evidence from Rocks: Beyond the apparent fit of continental shapes and fossil evidence, Wegener discovered that rocks on both sides of the Atlantic Ocean are of the same type and age. The Appalachian Mountains in the eastern United States and Canada are comparable to mountains in eastern Greenland, Ireland, Great Britain, and Norway.

Wegener concluded that these mountains were once joined, forming a single mountain range. They later separated due to continental drift.

Fourth Evidence – Evidence from Coal Deposits: Bituminous coal, also called "soft coal," is the most abundant type of coal. It is usually black or dark brown in color. Bituminous coal forms from the remains of swamp plants buried beneath the Earth. Due to heat and pressure, these plant remains transformed into coal over time. Rich deposits of this type of coal are found in South America, Africa, the Indian subcontinent, Southeast Asia, and even in cold regions like Antarctica.

The presence of large coal deposits in Antarctica implies that this region once supported abundant life. Long ago, Antarctica experienced a warm climate—likely due to its location closer to the equator.

The Existence of Plant and Animal Fossils as Evidence Supporting Plate Movement - Fossils are preserved remains of plants and animals from past geological ages. These remains offer a glimpse into the past, helping us understand how prehistoric plants and animals lived.

Fossilized leaves of the extinct plant Glossopteris have been found in Southern Africa, Australia, India, and Antarctica. Today, these continents are separated by wide oceans. The plant's remains reveal that it had large seeds, making it impossible for them to have been dispersed by wind or ocean currents. This evidence suggests that these regions were once connected.

Seafloor Spreading - Support to Continental Drift Theory: The Seafloor Spreading Theory posits that an oceanic ridge system exists in all the world's oceans. The Mid-Atlantic Ridge extends from the North Pole to the South Pole, spanning half the Atlantic Ocean. Other interconnected ridges include the Southeast Indian Ridge, Pacific-Atlantic Ridge, East Pacific Rise, and Chile Ridge. Ocean ridges form from magma rising at divergent boundaries.

Scientists have made additional findings supporting the Seafloor Spreading Theory:

Rocks at the mid-ocean ridge are younger than those farther away. Thinner sediments are found near the ridge, and ocean floor rocks are younger compared to continental rocks. The concept of seafloor spreading bolstered the continental drift theory, leading to the development of plate tectonics theory.

Magnetic Reversal - Ocean floor rocks display a striped pattern corresponding to periods of geomagnetic reversal. A magnetic reversal occurs when a planet's magnetic field changes, swapping the positions of magnetic north and south.

When magma cools, magnetic minerals—particularly those containing iron—align with Earth's magnetic field. Throughout Earth's history, its magnetic field has reversed polarity at intervals ranging from hundreds of thousands to millions of years.

The magnetic stripes on both sides of the divergent boundary are equal in size and polarity. To count the number of magnetic reversals, one simply needs to tally the white areas (representing reversed magnetic polarity).

The major evidence of plate-tectonic theory is (1) the shapes of the displaced together (2) the matching of rock in areas that were once adjacent, like West Africa and South America (3) similar fossil evidence found in separate continents and (4) evidence from rocks that are similar in structure and age.

Harry Hess in Seafloor Spreading Theory found that there is (4) stripping of equal distances from the mid-ocean ridge and (5) reversals of Earth’s magnetic field are recorded by rocks in strips parallel to ridges. Further studies have shown that the (6) rocks making up the oceanic crust get older as their distance from the mid-ocean ridge increases.

Alfred Wegener had lived before Harry Hess and their independent sources of evidence put together built a strong explanation for the plate tectonics theory.