8.3 Earth's Structure, Plate Tectonics, and the Rock Cycle

8.3 Earth's Structure, Plate Tectonics, and the Rock Cycle

Earth science items on the AdUCET reward candidates who can reason from evidence rather than recite lists. This section covers the solid Earth: what is inside it, how we know, how the plates move, and how rocks are made, destroyed and remade. The atmosphere, weather, the water cycle and astronomy are handled in section 8.4.

Inside the Earth: Two Ways to Slice It

Earth is layered twice over. Geologists divide it by chemical composition into crust, mantle and core, and by mechanical behaviour into a rigid outer shell and the weaker layers beneath.

   0 km  +====================================+  CRUST      ) LITHOSPHERE
         |  oceanic 5-10 km | continental     |             ) rigid, broken
  ~100   +------------------ 30-70 km --------+  upper      ) into plates
         |  ASTHENOSPHERE - hot, plastic,     |  mantle
  ~350   |  slowly flowing silicate rock      |
         +------------------------------------+
         |  LOWER MANTLE - solid, very dense  |
  2,900  +====================================+
         |  OUTER CORE - LIQUID iron + nickel |
  5,150  +====================================+
         |  INNER CORE - SOLID iron + nickel  |
  6,371  +====================================+  centre

• The crust is the thin outer skin. Oceanic crust is 5-10 km thick, basaltic and dense (about 3.0 g/cm$^3$); continental crust is 30-70 km thick, granitic and lighter (about 2.7 g/cm$^3$). That density gap decides which plate sinks at a collision.
• The mantle reaches to 2,900 km and holds roughly 84% of Earth's volume. Its uppermost rigid part joins the crust to form the lithosphere, the layer that is fractured into tectonic plates. Below it the asthenosphere is hot enough to flow plastically, letting the plates ride along.
• The outer core is liquid iron and nickel, and its churning generates Earth's magnetic field by geodynamo action.
• The inner core is a solid iron-nickel sphere about 1,220 km in radius, kept solid by immense pressure despite temperatures near 5,400 degrees Celsius.

The seismic evidence

Nobody has drilled past the crust, so the interior is mapped with earthquake waves. P waves (primary, longitudinal, fastest) pass through solids and liquids. S waves (secondary, transverse, slower) pass through solids only. After a large earthquake, seismic stations on the far side of the planet record P waves but detect an S-wave shadow zone where no S waves arrive at all. The only explanation is a liquid layer blocking them — the outer core. P waves also refract sharply at each boundary, and the abrupt jump in speed at 5-70 km depth marks the Mohorovicic discontinuity, the crust-mantle boundary.

Worked example 1 - locating an earthquake from the S-P lag

A station records the P wave 30 s before the S wave. In the local crust P waves travel at 8 km/s and S waves at 4 km/s. How far away is the epicentre?

1. Travel times: $t_P = d/8$ and $t_S = d/4$.
2. Lag: $t_S - t_P = \dfrac{d}{4} - \dfrac{d}{8} = \dfrac{2d - d}{8} = \dfrac{d}{8}$.
3. Set the lag to 30 s: $d/8 = 30$, so $d = 240$ km.
4. Check: $t_P = 240/8 = 30$ s and $t_S = 240/4 = 60$ s, a 30 s gap. Correct.

One station gives only a distance, so a circle of that radius is drawn around it; three stations are needed to triangulate the epicentre.

From Continental Drift to Plate Tectonics

In 1912 Alfred Wegener proposed continental drift: the continents once formed a single supercontinent, Pangaea, which broke apart. His evidence was:

1. The jigsaw fit of the coastlines of South America and Africa.
2. Identical fossils on continents now separated by oceans — the reptile Mesosaurus, the seed fern Glossopteris, and the land reptile Lystrosaurus.
3. Mountain belts and rock sequences that line up across the Atlantic when the continents are reassembled.
4. Glacial scratches and deposits in now-tropical Africa, India, Australia and South America, pointing to a shared polar position.

Geologists rejected the idea for decades because Wegener could not name a force strong enough to shove continents through ocean floor. Plate tectonic theory supplied it in the 1960s: the lithosphere is broken into about a dozen major plates that move over the asthenosphere, driven by mantle convection, ridge push at spreading centres, and above all slab pull, the weight of a cold dense plate dragging the rest of itself down. Seafloor spreading, discovered by Harry Hess, was confirmed by the mirror-image bands of magnetised rock on either side of mid-ocean ridges.

Why the Philippines sits on the Ring of Fire

The Pacific Ring of Fire is the horseshoe of subduction zones circling the Pacific basin, home to roughly three-quarters of the world's active volcanoes and about 90% of its earthquakes. The Philippine archipelago is squeezed between two subduction systems moving in opposite directions: along the Philippine Trench on the east, the Philippine Sea Plate dives westward beneath the islands, while along the Manila Trench on the west, South China Sea lithosphere dives eastward beneath Luzon. The strain between them is taken up by the roughly 1,200 km Philippine Fault Zone, a strike-slip fault running the length of the country, and by local structures such as the West Valley Fault beneath Metro Manila.

Earthquakes and Volcanoes

The focus (or hypocentre) is the point underground where rupture begins; the epicentre is the point on the surface directly above it. Waves arrive in order: P first, S second, then the slow surface waves that do most of the damage.

• Magnitude measures the energy released at the source. It is a single logarithmic number for the whole event: each whole step means about 10 times the ground-shaking amplitude and roughly 32 times the energy.
• Intensity measures how strongly the shaking was felt at a particular place, so one earthquake has many intensity values. The Philippines uses the PHIVOLCS Earthquake Intensity Scale (PEIS), which runs from Intensity I, Scarcely Perceptible, to Intensity X, Completely Devastating, in Roman numerals.

A tsunami is generated when an undersea earthquake, landslide or eruption abruptly displaces the water column. In deep ocean the wave is low but hundreds of kilometres long and travels at jet speed; as it reaches shallow coastal water it slows and piles up into a destructive surge, as in the 1976 Moro Gulf event.

Volcano types follow from magma chemistry. Runny, silica-poor basaltic magma erupts gently and builds broad shield volcanoes; sticky, silica-rich, gas-charged magma traps pressure and erupts explosively, building steep-sided composite volcanoes (stratovolcanoes) from alternating layers of lava and ash. Small cinder cones are piles of ejected fragments. Philippine examples worth knowing: Mayon in Albay, a stratovolcano famous for its near-perfect symmetrical cone and frequent eruptions; Taal in Batangas, a complex volcano sitting inside a caldera lake, which erupted violently in January 2020; and Pinatubo in Zambales, whose 1991 eruption was one of the largest of the twentieth century and buried whole valleys under lahar for years afterwards. PHIVOLCS classifies a volcano as active if it has erupted within historical times.

Minerals, Rocks, and the Rock Cycle

A mineral is naturally occurring, inorganic, solid, has a definite chemical composition and an ordered crystalline structure. Rocks are aggregates of minerals. Minerals are identified by:

• Streak — the colour of the powder on a porcelain plate, far more reliable than surface colour.
• Lustre — metallic or non-metallic (glassy, pearly, earthy).
• Cleavage versus fracture — breaking along flat planes versus breaking irregularly.
• Hardness on the Mohs scale, 1 to 10.

The rock cycle is simply the set of transitions between those three boxes. Any rock can melt to magma and cool as igneous rock; any rock can be weathered to sediment and lithify as sedimentary rock; any rock can be cooked and squeezed into metamorphic rock. There is no fixed order and no starting point.

Worked example 2 - identifying an unknown rock

A dark, fine-grained sample dotted with small gas holes is collected near Mayon.

1. No layering, no fossils, no rounded grains, so it is not sedimentary.
2. No banding and no recrystallised texture, so it is not metamorphic.
3. Crystals too small to see plus gas cavities mean it cooled quickly at the surface: an extrusive igneous rock, dark and therefore basaltic — basalt or scoria.

Surface Processes and Reading Rock Layers

Weathering breaks rock down in place. Physical weathering (root wedging, abrasion, exfoliation, salt crystal growth) changes only the size of the fragments; chemical weathering (dissolution, oxidation, carbonation, hydrolysis) changes the minerals themselves and dominates in the hot, wet Philippine climate, which is why limestone caves and karst towers are so common. Erosion then transports the debris, and deposition drops it where the transporting agent loses energy.

Soil develops where weathered rock mixes with decayed organic matter over long periods, and mature soil shows horizons in order from the top down: O (organic litter), A (dark topsoil), E (leached), B (subsoil where minerals accumulate), C (broken parent rock) and R (bedrock).

Worked example 3 - relative dating of a rock column

An outcrop shows shale at the bottom, limestone above it, an igneous dike cutting through both but stopping at a flat eroded surface, sandstone lying on that surface, and a fault slicing through everything.

1. Superposition: in undisturbed layers the oldest is at the bottom, so shale formed first, then limestone.
2. Cross-cutting relationships: the dike cuts the shale and limestone, so it is younger than both.
3. The dike stops at the eroded surface, so erosion happened next.
4. The sandstone was deposited on that surface, making it younger than the dike.
5. The fault cuts every unit, so it is the youngest event of all.

Order: shale, limestone, dike, erosion, sandstone, fault.

The geologic time scale organises the resulting record. Earth is about 4.6 billion years old, and the Precambrian covers roughly the first 88% of that. The Phanerozoic eon is then split into the Paleozoic era (beginning about 540 million years ago with the Cambrian explosion and ending in the largest mass extinction of all), the Mesozoic era (the age of dinosaurs, ending 66 million years ago), and the Cenozoic era (the age of mammals, including us). Widespread, short-lived index fossils let geologists correlate layers between distant outcrops; radiometric dating using half-lives supplies the actual ages.