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# Earth's Core

## There is an inner and an outer core made of dense metal; it is known from seismic waves and density calculations.

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Discovering the Core

### Discovering the Core

Credit: Washiucho
Source: http://en.wikipedia.org/wiki/File:Earth-crust-cutaway-English-Large_label.PNG

We may know more about the Moon than Earth’s core, but is that surprising? We can see the Moon and have even visited the far side. It’s a lot harder to learn about the center of the Earth. We can’t go there and we can’t drill there. How can we learn about the core?

#### Amazing But True!

• Richard Oldham was the first person to notice the separate arrival times of P-waves, S-waves and surface waves on seismograms.
• From his mathematical analysis he determined that Earth had a core equal to less than 40% its radius.
• Inge Lehman was the first to discover the P wave shadow, indicating the presence of an inner core.
• Lehman later discovered a change in seismic wave velocity in the mantle, which was called the Lehman discontinuity.

#### Explore More

1. Oldham noticed that seismic waves appeared to hit an obstacle, which was the core. What do the P-waves and S-waves do at the core?
2. What did Lehman observe that indicated to her that there were two cores?
3. What are the two main differences between the inner core and the outer core?
4. Why is it important that the outer core is liquid?

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