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Hypothesis: A possible natural explanation for a set of data Theory: A scientific idea, supported by an abundance of evidence, that has passed many tests and has failed none

Right after the big bang, the smallest elements formed when enough cooling happened. Hydrogen and Helium formed first, then trace amounts of other elements with atomic numbers of less than 5. This was Big Bang Nucleosynthesis Then, Moderately heavy elements form inside stars by stellar nucleosynthesis, a succession of nuclear reactions during which smaller nuclei fuse together to form larger ones. In other words, stars are element factories!

Stars: Large volumes of hydrogen and helium gas collect into immense, slowly swirling nebulae. The nebulae get denser, then begin to collapse inward due to the force of gravity. As the regions get more compressed, the temperature then begins to rise. An accretion disk is then formed due to the rotation around the axis and increasing gravity. When the center of the accretion disk becomes hot enough, it forms a protostar If enough matter is present, nuclear fusion begins and you have a star. Planets: Planet formation occurs alongside star formation. The leftover gas and dust surrounding a young star form a rotating protoplanetary disk. Dust particles collide and stick together, growing into planetesimals and then protoplanets. Close to the star, rocky planets form where temperatures are high; farther away, icy and gaseous materials accumulate, forming gas giants and ice giants

In terms of relative sizes, the planets fall into two main groups. 1. The terrestrial (rocky) planets—Mercury, Venus, Earth, and Mars—are relatively small and dense. Earth is the largest of this group. 2. The gas giant planets– much larger: Jupiter is by far the biggest planet in the solar system, followed by Saturn, while Uranus and Neptune are slightly smaller but still many times larger than Earth. Distance is very far apart, with the further away from the sun you get the further the distance between planets.

The inner planets and outer planets are differentiated by their composition, size, location, and formation conditions. The inner planets—Mercury, Venus, Earth, and Mars—are also called terrestrial planets. They are relatively small, dense, and made mostly of rock and metal. These planets formed close to the Sun, where temperatures were high, so only materials with high melting points (like metals and silicate minerals) could solidify. As a result, inner planets have solid surfaces, few or no moons, and no ring systems. The outer planets—Jupiter, Saturn, Uranus, and Neptune—are much larger and are known as giant planets. Jupiter and Saturn are gas giants, composed mainly of hydrogen and helium, while Uranus and Neptune are ice giants, containing larger amounts of water, ammonia, and methane. They formed farther from the Sun, where cooler temperatures allowed ices and gases to accumulate. Outer planets have thick atmospheres, many moons, and ring systems.

After the Sun formed about 4.6 billion years ago, a rotating protoplanetary disk of gas and dust remained around it. Within this disk, solid particles collided and stuck together, forming planetesimals, which gradually grew into protoplanets. In the inner solar system, where temperatures were high, rocky and metallic materials dominated, leading to the formation of Earth and the other terrestrial planets. Farther from the Sun, colder conditions allowed gases and ices to accumulate, forming the giant planets. As Earth grew, it experienced intense heating from impacts, radioactive decay, and compression. This caused the planet to partially melt and differentiate, forming a dense metallic core, a rocky mantle, and a crust. The Moon formed shortly after Earth through the giant impact hypothesis. A Mars-sized body (often called Theia) collided with the early Earth. The impact ejected debris into orbit around Earth, which eventually coalesced to form the Moon.

Earth has a liquid outer core made mostly of molten iron and nickel. Heat escaping from the core causes this molten metal to move in swirling, convective currents. As Earth rotates, these moving, electrically conductive fluids generate electric currents, which in turn produce a global magnetic field surrounding the planet. This magnetic field is crucial for life on Earth because it acts as a protective shield. It deflects much of the solar wind that would otherwise strip away Earth’s atmosphere and hit the surface with harmful radiation.

In the context of Earth’s formation, differentiation is the process by which Earth separated into distinct internal layers based on density and composition. When the early Earth was partially molten, dense materials like iron and nickel sank toward the center to form the core, while less-dense silicate materials rose to form the mantle and crust. This process created the layered structure of Earth we observe today.

Density is the amount of mass contained in a given volume of a substance, commonly expressed as mass per unit volume. Earth’s structural layers are organized by density due to gravitational settling during differentiation. The densest materials—mainly iron and nickel—sank to the center to form the core. Surrounding the core is the mantle, composed of less-dense silicate rocks. The crust is the least dense layer and floats on top of the mantle.

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Pressure! Both the inner and outer core are extremely hot—hot enough that iron would normally be liquid. However, the pressure at Earth’s center is immense, and this very high pressure forces the iron atoms in the inner core into a solid structure, preventing them from flowing. In contrast, the pressure in the outer core is lower, so despite similarly high temperatures, the iron and nickel there remain liquid.

We know the thicknesses and compositions of Earth’s layers mainly through indirect evidence, since we cannot drill very far into the planet. The key evidence comes from seismic waves generated by earthquakes. As these waves travel through Earth, they change speed, bend, or stop depending on the material they pass through. P-waves travel through solids and liquids, while S-waves can only move through solids. The disappearance of S-waves at a certain depth shows that the outer core is liquid and sudden changes in wave speed reveal boundaries between the crust, mantle, outer core, and inner core. By measuring how long waves take to reach seismometers around the world, scientists calculate the depth and thickness of each layer.

Alfred Wegener’s continental drift hypothesis proposed that the continents were once joined together in a single supercontinent, Pangaea, and have since slowly drifted apart to their present positions. Evidence: First, the jigsaw-like fit of the continents, especially South America and Africa, suggested they were once connected. Second, matching fossils of plants and animals were found on continents now separated by oceans, implying those lands were once joined. Third, similar rock types, ages, and mountain belts appear on different continents, such as matching Appalachian and Caledonian mountain ranges. Finally, paleoclimate evidence—including glacial deposits in now-tropical regions and coal deposits in now-cold areas—indicated that continents had changed position relative to Earth’s climate zones.

Pangaea was a supercontinent that existed about 300 million years ago, when nearly all of Earth’s continental landmasses were joined together into one large body surrounded by a single global ocean. Pangaea no longer exists because of plate tectonics.

Magnetic anomalies and sediment thickness patterns on the seafloor provide strong, independent evidence for seafloor spreading. When magma rises at mid-ocean ridges and cools to form new basaltic crust, iron-bearing minerals in the rock align with Earth’s magnetic field at that time. Because Earth’s magnetic field has reversed many times, the seafloor records a pattern of alternating magnetic polarities. These magnetic “stripes” are symmetrical on either side of mid-ocean ridges and get progressively older with distance from the ridge. This mirror-image pattern shows that new crust is created at the ridge and pushed outward as more magma emerges. Sediment thickness also supports seafloor spreading. Sediments accumulate slowly over time, so young oceanic crust near mid-ocean ridges has very thin or no sediment, while crust farther away has thicker sediment layers. The systematic increase in sediment thickness with distance from the ridge indicates that the seafloor becomes older as it moves away from the spreading center.

When igneous rocks form or sediments settle, magnetic minerals within them align with Earth’s magnetic field. Once the rock hardens, this magnetic orientation is locked in, preserving the direction and latitude of the magnetic field at the time the rock formed. Because Earth’s magnetic field is roughly aligned with its rotational axis, paleomagnetic data can show where a continent was located relative to the poles in the past. Studies found that rocks of different ages on the same continent record different apparent positions of the magnetic poles, called apparent polar wander paths. When these paths are compared between continents, they do not match unless the continents are reassembled into their former positions (such as in Pangaea). This demonstrates that the poles did not move significantly; rather, the continents themselves moved.

Continental drift, proposed by Alfred Wegener in the early 1900s, suggested that continents were once joined together (Pangaea) and later drifted apart. Wegener supported this idea with evidence such as the fit of continents, matching fossils, similar rock formations, and paleoclimate indicators. However, his hypothesis lacked a convincing mechanism for how continents could move through solid oceanic crust, which led many scientists to reject it at the time. Plate tectonics, developed in the mid-20th century, expanded and replaced continental drift. It explains that Earth’s lithosphere is broken into rigid plates that include both continental and oceanic crust, and these plates move over the softer asthenosphere. Plate tectonics is supported by new evidence that was unavailable to Wegener, including seafloor spreading, symmetrical magnetic anomalies, age patterns of oceanic crust, global earthquake and volcano distributions, and direct measurements of plate motion using GPS.

Seafloor spreading is the process by which new oceanic crust is created at mid-ocean ridges and then moves outward across the ocean floor. At divergent plate boundaries, hot mantle material rises and partially melts as pressure decreases. This magma erupts or intrudes at the ridge, cools, and solidifies to form new basaltic crust. As more magma rises and hardens, it pushes the older crust away from the ridge on both sides. Over time, this causes the ocean floor to spread apart, making the seafloor progressively older with increasing distance from the ridge.

Plate tectonics describes how Earth’s rigid outer shell—the lithosphere—is broken into plates that move slowly over the softer, ductile asthenosphere beneath them. This motion is driven by heat from Earth’s interior and gravity-related forces. New oceanic crust is created at divergent plate boundaries, mainly along mid-ocean ridges. Here, plates pull apart, hot mantle material rises, and pressure decreases, causing partial melting. The magma cools to form basaltic oceanic crust. As more magma rises, the newly formed crust is pushed sideways, carrying the plates away from the ridge. Oceanic crust does not accumulate indefinitely because it is also destroyed at convergent boundaries. As oceanic crust moves away from the ridge, it cools, becomes denser, and eventually subducts beneath a less-dense plate, sinking back into the mantle.

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Crust – Earth’s outermost compositional layer. It is thin and made of relatively low-density rock. There are two types: continental crust (thicker, less dense, granitic) and oceanic crust (thinner, denser, basaltic). Mantle – The thick layer beneath the crust and above the core, defined by composition. It is made mostly of solid silicate rock rich in magnesium and iron. The mantle extends to about 2,900 km depth and makes up most of Earth’s volume. Lithosphere – A mechanical (physical) layer, not a compositional one. It includes the crust plus the uppermost mantle and behaves as a rigid, brittle shell. The lithosphere is broken into tectonic plates. Asthenosphere – A mechanical layer beneath the lithosphere within the upper mantle. It is hot and weak, and although solid, it flows slowly over geologic time. This ductile behavior allows the rigid lithospheric plates to move on top of it.

Divergent: Moving apart Transform: Sliding past Convergent: Moving together

FEATURES: Mid-ocean ridges: Underwater mountain chains where new oceanic crust is created (e.g., the Mid-Atlantic Ridge). Rift valleys: Linear valleys along the ridge where the crust is stretching and thinning. Volcanoes: Magma rises through the thinning crust, forming volcanic activity along the ridge. Shallow earthquakes: Small to moderate earthquakes occur due to tensional stresses. PROCESSES: Mantle upwelling: Hot mantle rises toward the surface as plates separate. Partial melting: Decreased pressure in the rising mantle causes it to melt and produce basaltic magma. New crust formation: Magma erupts or intrudes at the surface, solidifies, and pushes older crust outward. Seafloor spreading: Continuous creation of new crust pushes plates away from the ridge. PLATE MOTION: Away from each other