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Before the Space Age, there were three main ways to prove that the Earth is round:

1 By watching ships sailing towards the horizon. A ship's hull follows the curve of the Earth and drops out of view first. Only later do the sails or masts disappear from sight. If Earth was flat, a ship should simply get smaller and smaller until it is no longer visible.

2 People traveling south see different constellations rise higher above the horizon. At the same time, the familiar northern constellations disappear below the horizon. If Earth was flat, travelers would always see the same star patterns.

3 During a lunar eclipse, the Moon passes through Earth's shadow. The edge of the shadow seen on the Moon is always curved, no matter how high the Moon is over the horizon. Only a sphere casts a circular shadow in every direction.

If Earth is spherical, how big is it? Once again, the ancient Greeks found an answer. It was clear from the size of its shadow during a lunar eclipse that our planet is bigger than the Moon. The actual size was first calculated by Eratosthenes (276–195 BCE).

Figure 3.21 Simple geometry was used by Eratosthenes to calculate the Earth's size. The first step was to calculate the distance between Syene and Alexandria. He then measured the length of the shadows at these two places at midday on June 21, and calculated the angle of the Sun (θ) above the horizon at Alexandria (a little over 7°). Since there are 360° in a circle, he was then able to calculate Earth's circumference.

(Peter Bond)

The first step was to place sticks in the ground at two places, 800 km apart. On the day of the summer solstice (June 21), he obtained measurements of the length of their shadows at noon. At one location, Syene (modern Aswan), the Sun was overhead and cast no shadow. At Alexandria, the Sun was about 7° away from the zenith, so there was a noticeable shadow. A simple calculation using the length of the shadows and the distance between the two sticks enabled him to work out Earth's circumference and diameter. (If a difference of a little over 7° is 800 km, then a full circle of 360° must be about 40,000 km. The diameter is equal to the circumference divided by 3.14, the value of π [“pi”]. This gives a diameter of about 12,700 km – very close to the actual figure.)

Later astronomers attempted to repeat his observations and obtained smaller figures. The belief in this “shrunken” Earth prevailed for centuries, and resulted in Christopher Columbus believing that the westward journey from Europe to Asia would be much shorter than it really is. His expedition was only saved by the unexpected discovery of the Americas.

Not all plate boundaries involve the formation or destruction of crust. In some places, known as transform faults, two plates slide horizontally past each other. The most famous example is the San Andreas fault system in western California.

The San Andreas fault zone, which is about 1,300 km long and sometimes tens of kilometers wide, separates the North American Plate from the Pacific Plate. These have been grinding horizontally past each other for 10 million years, at an average rate of about 5 cm per year. Land on the western side of the fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to the land on the eastern side (on the North American Plate) – so Los Angeles and San Francisco are slowly moving away from each other. Sudden releases of built‐up stress cause powerful earthquakes in this region.

By reversing current movements and studying the different magnetization of minerals in the rock, geologists can trace back the positions of the continents through time. On several occasions in the past, the present land masses have come together to form a supercontinent.

Approximately 550 million years ago, Africa, South America, Australia, Antarctica, and India were joined to form one gigantic land mass known as Gondwana. Over the next 300 million years, Gondwana was further enlarged by the addition of Europe and North America, so that all of the world's major land masses were eventually combined in a supercontinent known as Pangaea.

Figure 3.22 Earth's surface exhibits a difference in height of about 20 km from the highest mountain to the deepest ocean trench. The highest land (apart from the ice sheets of Greenland and Antarctica) is found in ranges of fold mountains – shown in dark red. Most of the continental lowlands (green) are drained by large river systems and covered by sedimentary deposits. The ocean floors exhibit major ranges of volcanic mountains, isolated volcanic islands, and deep, narrow trenches.

(NASA)

Pangaea began to break into two large masses, Gondwana and Laurasia, around 170 million years ago (Figure 3.25). Eventually, crustal rifting enabled the South Atlantic Ocean to open up as South America began to drift slowly westward away from Africa. Then India began to move northward, eventually colliding with Eurasia to form the Himalayan mountains and the Tibetan plateau. Other young fold mountains are growing around the Mediterranean Sea, where Africa is plowing into Europe, and along the west coast of the Americas, where oceanic plates are diving beneath the continents.

The driving forces behind the plate movements are not completely understood. It was formerly believed that the plates are like rafts, passively drifting on top of the moving asthenosphere. However, more recent studies suggest that they contribute to their own movement.

One hypothesis suggests that the weight of a cold slab of crust descending into the asthenosphere pulls the entire plate along. This is supported by normal faulting in the ocean crust, evidence that these plates experience tensional stresses. There is also a suggestion that gravity causes plates to slowly slide downhill, away from the high mid‐ocean ridges. However, during initial rifting, plates begin to move without the help of such an elevated ridge.

It seems that convection cells also exist in the solid mantle. In this scenario, warm, rising rock from the mantle reaches the lithosphere and then spreads out laterally. The drag applied to the lithosphere causes it to move. The size of these convection cells is uncertain. It may be that they are confined to the asthenosphere and upper mantle, down to a depth of 700 km.

Alternatively, the convection cells could extend much deeper, reaching all the way down to the core boundary. There is considerable evidence for hotspots in the mantle beneath both the ocean floors and the continents. These are where thermal plumes, vertical columns of upwelling mantle 100–250 km in diameter, lift the overlying lithosphere and spread laterally at divergent plate margins, e.g. central Iceland.

Plates probably move as a result of a combination of these mechanisms. Plate separation may be initiated by mantle convection or a plume, but the subsequent formation of a topographically high spreading ridge may then drive the plates apart.

Earth is the only planet in the Solar System where such complex plate tectonics occur at the present time. On the Moon, Mercury, Venus, and Mars the rigid lithosphere forms a single layer, rather than separate slabs whose motions are driven by convection in the mantle.