## You'll need

• 4 flat cream-filled biscuits such as Oreos (more if you’re hungry!)

## What to do

1. Pick up a biscuit, and try to slide the top biscuit layer over the cream layer. Pay attention to how easy or hard it is to slide.
2. Take another biscuit and carefully crack the top biscuit layer in half, while keeping the bottom biscuit layer intact. Slide the 2 broken pieces outwards, to make a gap in the middle between them. Is it easier or harder to slide compared with step 1?
3. Take another biscuit and crack the top biscuit layer in half. Try to slide one of the broken pieces underneath the other. Is it easier or harder to slide compared with the other steps?
4. Take another biscuit and crack the top biscuit layer. Slide the broken pieces so that they rub against each other. Is it easier or harder to slide compared with the other steps?

Which biscuit was the most difficult to slide?

Which caused the most damage to the biscuit?

Can you think of a plate boundary that acts like each of the biscuit actions? Hint: look at a map of different types of tectonic plates.

What sort of events can happen when tectonic plates move in these different ways?

## What's happening

The top layer of the biscuit represents Earth’s crust (known as the lithosphere) and tectonic plates. The creamy layer represents the soft molten layer underneath Earth’s crust (part of the mantle known as the asthenosphere). The bottom layer of biscuit represents the lower mantle, which is hard. Tectonic plates slide over the mantle, similar to how the top layer of biscuit slides over the creamy layer.

Tectonic plates can move away from each other, just as the biscuit pieces slid away from each other in step 2. Where this occurs in Earth’s crust, it is known as a divergent plate boundary. These occur in mid-ocean rifts, where the soft molten layer emerges and cools to form new rocks.

Tectonic plates can move underneath each other, just as the biscuit pieces in step 3. Where this occurs in Earth’s crust, it is known as a convergent plate boundary. When plates converge, they can push up the crust to make mountains, or cause earthquakes or volcanoes.

Tectonic plates can also slide against each other, just as the biscuit pieces in step 4. Where this happens in Earth’s crust, it is known as a transform plate boundary. When plates grind against each other, they can cause earth tremors and earthquakes.

## Did you know

Marine fossils have been found on the summit of Mt Everest! About 50 million years ago, 2 tectonic plates converged when the Indian Plate began crashing under the larger Eurasian Plate. This caused Earth’s crust in the Eurasian Plate to crumple up and create the highest mountain range in the world – the Himalayas. The Indian Plate is still crashing into the Himalayas, causing them to grow even taller – at a rate of 5 mm each year. The convergent plate boundary also causes many earthquakes in the region.